stream_op cleanup: channel, client_config changes

19 files changed, 1085 insertions, 1181 deletions

diff --git a/src/core/census/grpc_filter.c b/src/core/census/grpc_filter.c
index 872543057e..daa1a7ac61 100644
--- a/src/core/census/grpc_filter.c
+++ b/src/core/census/grpc_filter.c
@@ -53,28 +53,24 @@ typedef struct call_data {
   int error;
 
   /* recv callback */
-  grpc_stream_op_buffer *recv_ops;
+  grpc_metadata_batch *recv_initial_metadata;
   grpc_closure *on_done_recv;
+  grpc_closure finish_recv;
 } call_data;
 
 typedef struct channel_data {
   grpc_mdstr *path_str; /* pointer to meta data str with key == ":path" */
 } channel_data;
 
-static void extract_and_annotate_method_tag(grpc_stream_op_buffer *sopb,
+static void extract_and_annotate_method_tag(grpc_metadata_batch *md,
                                             call_data *calld,
                                             channel_data *chand) {
   grpc_linked_mdelem *m;
-  size_t i;
-  for (i = 0; i < sopb->nops; i++) {
-    grpc_stream_op *op = &sopb->ops[i];
-    if (op->type != GRPC_OP_METADATA) continue;
-    for (m = op->data.metadata.list.head; m != NULL; m = m->next) {
-      if (m->md->key == chand->path_str) {
-        gpr_log(GPR_DEBUG, "%s",
-                (const char *)GPR_SLICE_START_PTR(m->md->value->slice));
-        /* Add method tag here */
-      }
+  for (m = md->list.head; m != NULL; m = m->next) {
+    if (m->md->key == chand->path_str) {
+      gpr_log(GPR_DEBUG, "%s",
+              (const char *)GPR_SLICE_START_PTR(m->md->value->slice));
+      /* Add method tag here */
     }
   }
 }
@@ -83,8 +79,8 @@ static void client_mutate_op(grpc_call_element *elem,
                              grpc_transport_stream_op *op) {
   call_data *calld = elem->call_data;
   channel_data *chand = elem->channel_data;
-  if (op->send_ops) {
-    extract_and_annotate_method_tag(op->send_ops, calld, chand);
+  if (op->send_initial_metadata) {
+    extract_and_annotate_method_tag(op->send_initial_metadata, calld, chand);
   }
 }
 
@@ -101,7 +97,7 @@ static void server_on_done_recv(grpc_exec_ctx *exec_ctx, void *ptr,
   call_data *calld = elem->call_data;
   channel_data *chand = elem->channel_data;
   if (success) {
-    extract_and_annotate_method_tag(calld->recv_ops, calld, chand);
+    extract_and_annotate_method_tag(calld->recv_initial_metadata, calld, chand);
   }
   calld->on_done_recv->cb(exec_ctx, calld->on_done_recv->cb_arg, success);
 }
@@ -109,11 +105,11 @@ static void server_on_done_recv(grpc_exec_ctx *exec_ctx, void *ptr,
 static void server_mutate_op(grpc_call_element *elem,
                              grpc_transport_stream_op *op) {
   call_data *calld = elem->call_data;
-  if (op->recv_ops) {
+  if (op->recv_initial_metadata) {
     /* substitute our callback for the op callback */
-    calld->recv_ops = op->recv_ops;
-    calld->on_done_recv = op->on_done_recv;
-    op->on_done_recv = calld->on_done_recv;
+    calld->recv_initial_metadata = op->recv_initial_metadata;
+    calld->on_done_recv = op->on_complete;
+    op->on_complete = &calld->finish_recv;
   }
 }
 
@@ -128,13 +124,11 @@ static void server_start_transport_op(grpc_exec_ctx *exec_ctx,
 
 static void client_init_call_elem(grpc_exec_ctx *exec_ctx,
                                   grpc_call_element *elem,
-                                  const void *server_transport_data,
-                                  grpc_transport_stream_op *initial_op) {
+                                  grpc_call_element_args *args) {
   call_data *d = elem->call_data;
   GPR_ASSERT(d != NULL);
   memset(d, 0, sizeof(*d));
   d->start_ts = gpr_now(GPR_CLOCK_REALTIME);
-  if (initial_op) client_mutate_op(elem, initial_op);
 }
 
 static void client_destroy_call_elem(grpc_exec_ctx *exec_ctx,
@@ -146,15 +140,13 @@ static void client_destroy_call_elem(grpc_exec_ctx *exec_ctx,
 
 static void server_init_call_elem(grpc_exec_ctx *exec_ctx,
                                   grpc_call_element *elem,
-                                  const void *server_transport_data,
-                                  grpc_transport_stream_op *initial_op) {
+                                  grpc_call_element_args *args) {
   call_data *d = elem->call_data;
   GPR_ASSERT(d != NULL);
   memset(d, 0, sizeof(*d));
   d->start_ts = gpr_now(GPR_CLOCK_REALTIME);
   /* TODO(hongyu): call census_tracing_start_op here. */
-  grpc_closure_init(d->on_done_recv, server_on_done_recv, elem);
-  if (initial_op) server_mutate_op(elem, initial_op);
+  grpc_closure_init(&d->finish_recv, server_on_done_recv, elem);
 }
 
 static void server_destroy_call_elem(grpc_exec_ctx *exec_ctx,
@@ -165,12 +157,11 @@ static void server_destroy_call_elem(grpc_exec_ctx *exec_ctx,
 }
 
 static void init_channel_elem(grpc_exec_ctx *exec_ctx,
-                              grpc_channel_element *elem, grpc_channel *master,
-                              const grpc_channel_args *args, grpc_mdctx *mdctx,
-                              int is_first, int is_last) {
+                              grpc_channel_element *elem,
+                              grpc_channel_element_args *args) {
   channel_data *chand = elem->channel_data;
   GPR_ASSERT(chand != NULL);
-  chand->path_str = grpc_mdstr_from_string(mdctx, ":path");
+  chand->path_str = grpc_mdstr_from_string(args->metadata_context, ":path");
 }
 
 static void destroy_channel_elem(grpc_exec_ctx *exec_ctx,
@@ -183,15 +174,13 @@ static void destroy_channel_elem(grpc_exec_ctx *exec_ctx,
 }
 
 const grpc_channel_filter grpc_client_census_filter = {
-    client_start_transport_op, grpc_channel_next_op,
-    sizeof(call_data),         client_init_call_elem,
-    client_destroy_call_elem,  sizeof(channel_data),
-    init_channel_elem,         destroy_channel_elem,
-    grpc_call_next_get_peer,   "census-client"};
+    client_start_transport_op, grpc_channel_next_op, sizeof(call_data),
+    client_init_call_elem, grpc_call_stack_ignore_set_pollset,
+    client_destroy_call_elem, sizeof(channel_data), init_channel_elem,
+    destroy_channel_elem, grpc_call_next_get_peer, "census-client"};
 
 const grpc_channel_filter grpc_server_census_filter = {
-    server_start_transport_op, grpc_channel_next_op,
-    sizeof(call_data),         server_init_call_elem,
-    server_destroy_call_elem,  sizeof(channel_data),
-    init_channel_elem,         destroy_channel_elem,
-    grpc_call_next_get_peer,   "census-server"};
+    server_start_transport_op, grpc_channel_next_op, sizeof(call_data),
+    server_init_call_elem, grpc_call_stack_ignore_set_pollset,
+    server_destroy_call_elem, sizeof(channel_data), init_channel_elem,
+    destroy_channel_elem, grpc_call_next_get_peer, "census-server"};
diff --git a/src/core/channel/channel_stack.c b/src/core/channel/channel_stack.c
index abd7f719e7..02e33a09ab 100644
--- a/src/core/channel/channel_stack.c
+++ b/src/core/channel/channel_stack.c
@@ -104,13 +104,14 @@ grpc_call_element *grpc_call_stack_element(grpc_call_stack *call_stack,
 void grpc_channel_stack_init(grpc_exec_ctx *exec_ctx,
                              const grpc_channel_filter **filters,
                              size_t filter_count, grpc_channel *master,
-                             const grpc_channel_args *args,
+                             const grpc_channel_args *channel_args,
                              grpc_mdctx *metadata_context,
                              grpc_channel_stack *stack) {
   size_t call_size =
       ROUND_UP_TO_ALIGNMENT_SIZE(sizeof(grpc_call_stack)) +
       ROUND_UP_TO_ALIGNMENT_SIZE(filter_count * sizeof(grpc_call_element));
   grpc_channel_element *elems;
+  grpc_channel_element_args args;
   char *user_data;
   size_t i;
 
@@ -122,11 +123,14 @@ void grpc_channel_stack_init(grpc_exec_ctx *exec_ctx,
 
   /* init per-filter data */
   for (i = 0; i < filter_count; i++) {
+    args.master = master;
+    args.channel_args = channel_args;
+    args.metadata_context = metadata_context;
+    args.is_first = i == 0;
+    args.is_last = i == (filter_count - 1);
     elems[i].filter = filters[i];
     elems[i].channel_data = user_data;
-    elems[i].filter->init_channel_elem(exec_ctx, &elems[i], master, args,
-                                       metadata_context, i == 0,
-                                       i == (filter_count - 1));
+    elems[i].filter->init_channel_elem(exec_ctx, &elems[i], &args);
     user_data += ROUND_UP_TO_ALIGNMENT_SIZE(filters[i]->sizeof_channel_data);
     call_size += ROUND_UP_TO_ALIGNMENT_SIZE(filters[i]->sizeof_call_data);
   }
@@ -151,33 +155,63 @@ void grpc_channel_stack_destroy(grpc_exec_ctx *exec_ctx,
 }
 
 void grpc_call_stack_init(grpc_exec_ctx *exec_ctx,
-                          grpc_channel_stack *channel_stack,
+                          grpc_channel_stack *channel_stack, int initial_refs,
+                          grpc_iomgr_cb_func destroy, void *destroy_arg,
+                          grpc_call_context_element *context,
                           const void *transport_server_data,
-                          grpc_transport_stream_op *initial_op,
                           grpc_call_stack *call_stack) {
   grpc_channel_element *channel_elems = CHANNEL_ELEMS_FROM_STACK(channel_stack);
+  grpc_call_element_args args;
   size_t count = channel_stack->count;
   grpc_call_element *call_elems;
   char *user_data;
   size_t i;
 
   call_stack->count = count;
+  gpr_ref_init(&call_stack->refcount.refs, initial_refs);
+  grpc_closure_init(&call_stack->refcount.destroy, destroy, destroy_arg);
   call_elems = CALL_ELEMS_FROM_STACK(call_stack);
   user_data = ((char *)call_elems) +
               ROUND_UP_TO_ALIGNMENT_SIZE(count * sizeof(grpc_call_element));
 
   /* init per-filter data */
   for (i = 0; i < count; i++) {
+    args.refcount = &call_stack->refcount;
+    args.server_transport_data = transport_server_data;
+    args.context = context;
     call_elems[i].filter = channel_elems[i].filter;
     call_elems[i].channel_data = channel_elems[i].channel_data;
     call_elems[i].call_data = user_data;
-    call_elems[i].filter->init_call_elem(exec_ctx, &call_elems[i],
-                                         transport_server_data, initial_op);
+    call_elems[i].filter->init_call_elem(exec_ctx, &call_elems[i], &args);
     user_data +=
         ROUND_UP_TO_ALIGNMENT_SIZE(call_elems[i].filter->sizeof_call_data);
   }
 }
 
+void grpc_call_stack_set_pollset(grpc_exec_ctx *exec_ctx,
+                                 grpc_call_stack *call_stack,
+                                 grpc_pollset *pollset) {
+  size_t count = call_stack->count;
+  grpc_call_element *call_elems;
+  char *user_data;
+  size_t i;
+
+  call_elems = CALL_ELEMS_FROM_STACK(call_stack);
+  user_data = ((char *)call_elems) +
+              ROUND_UP_TO_ALIGNMENT_SIZE(count * sizeof(grpc_call_element));
+
+  /* init per-filter data */
+  for (i = 0; i < count; i++) {
+    call_elems[i].filter->set_pollset(exec_ctx, &call_elems[i], pollset);
+    user_data +=
+        ROUND_UP_TO_ALIGNMENT_SIZE(call_elems[i].filter->sizeof_call_data);
+  }
+}
+
+void grpc_call_stack_ignore_set_pollset(grpc_exec_ctx *exec_ctx,
+                                        grpc_call_element *elem,
+                                        grpc_pollset *pollset) {}
+
 void grpc_call_stack_destroy(grpc_exec_ctx *exec_ctx, grpc_call_stack *stack) {
   grpc_call_element *elems = CALL_ELEMS_FROM_STACK(stack);
   size_t count = stack->count;
diff --git a/src/core/channel/channel_stack.h b/src/core/channel/channel_stack.h
index 6732cc3018..1279fec080 100644
--- a/src/core/channel/channel_stack.h
+++ b/src/core/channel/channel_stack.h
@@ -51,6 +51,20 @@
 typedef struct grpc_channel_element grpc_channel_element;
 typedef struct grpc_call_element grpc_call_element;
 
+typedef struct {
+  grpc_channel *master;
+  const grpc_channel_args *channel_args;
+  grpc_mdctx *metadata_context;
+  int is_first;
+  int is_last;
+} grpc_channel_element_args;
+
+typedef struct {
+  grpc_stream_refcount *refcount;
+  const void *server_transport_data;
+  grpc_call_context_element *context;
+} grpc_call_element_args;
+
 /* Channel filters specify:
    1. the amount of memory needed in the channel & call (via the sizeof_XXX
       members)
@@ -84,8 +98,9 @@ typedef struct {
      transport and is on the server. Most filters want to ignore this
      argument. */
   void (*init_call_elem)(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
-                         const void *server_transport_data,
-                         grpc_transport_stream_op *initial_op);
+                         grpc_call_element_args *args);
+  void (*set_pollset)(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
+                      grpc_pollset *pollset);
   /* Destroy per call data.
      The filter does not need to do any chaining */
   void (*destroy_call_elem)(grpc_exec_ctx *exec_ctx, grpc_call_element *elem);
@@ -99,9 +114,7 @@ typedef struct {
      useful for asserting correct configuration by upper layer code.
      The filter does not need to do any chaining */
   void (*init_channel_elem)(grpc_exec_ctx *exec_ctx, grpc_channel_element *elem,
-                            grpc_channel *master, const grpc_channel_args *args,
-                            grpc_mdctx *metadata_context, int is_first,
-                            int is_last);
+                            grpc_channel_element_args *args);
   /* Destroy per channel data.
      The filter does not need to do any chaining */
   void (*destroy_channel_elem)(grpc_exec_ctx *exec_ctx,
@@ -141,7 +154,14 @@ typedef struct {
 
 /* A call stack tracks a set of related filters for one call, and guarantees
    they live within a single malloc() allocation */
-typedef struct { size_t count; } grpc_call_stack;
+typedef struct {
+  /* shared refcount for this channel stack.
+     MUST be the first element: the underlying code calls destroy
+     with the address of the refcount, but higher layers prefer to think
+     about the address of the call stack itself. */
+  grpc_stream_refcount refcount;
+  size_t count;
+} grpc_call_stack;
 
 /* Get a channel element given a channel stack and its index */
 grpc_channel_element *grpc_channel_stack_element(grpc_channel_stack *stack,
@@ -170,13 +190,34 @@ void grpc_channel_stack_destroy(grpc_exec_ctx *exec_ctx,
    expected to be NULL on a client, or an opaque transport owned pointer on the
    server. */
 void grpc_call_stack_init(grpc_exec_ctx *exec_ctx,
-                          grpc_channel_stack *channel_stack,
+                          grpc_channel_stack *channel_stack, int initial_refs,
+                          grpc_iomgr_cb_func destroy, void *destroy_arg,
+                          grpc_call_context_element *context,
                           const void *transport_server_data,
-                          grpc_transport_stream_op *initial_op,
                           grpc_call_stack *call_stack);
+/* Set a pollset for a call stack: must occur before the first op is started */
+void grpc_call_stack_set_pollset(grpc_exec_ctx *exec_ctx,
+                                 grpc_call_stack *call_stack,
+                                 grpc_pollset *pollset);
+
+#ifdef GRPC_STREAM_REFCOUNT_DEBUG
+#define grpc_call_stack_ref(call_stack, reason) \
+  grpc_stream_ref(&(call_stack)->refcount, reason)
+#define grpc_call_stack_unref(exec_ctx, call_stack, reason) \
+  grpc_stream_unref(exec_ctx, &(call_stack)->refcount, reason)
+#else
+#define grpc_call_stack_ref(call_stack) grpc_stream_ref(&(call_stack)->refcount)
+#define grpc_call_stack_unref(exec_ctx, call_stack) \
+  grpc_stream_unref(exec_ctx, &(call_stack)->refcount)
+#endif
+
 /* Destroy a call stack */
 void grpc_call_stack_destroy(grpc_exec_ctx *exec_ctx, grpc_call_stack *stack);
 
+/* Ignore set pollset */
+void grpc_call_stack_ignore_set_pollset(grpc_exec_ctx *exec_ctx,
+                                        grpc_call_element *elem,
+                                        grpc_pollset *pollset);
 /* Call the next operation in a call stack */
 void grpc_call_next_op(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
                        grpc_transport_stream_op *op);
diff --git a/src/core/channel/client_channel.c b/src/core/channel/client_channel.c
index 9f85557ea1..16d91d4277 100644
--- a/src/core/channel/client_channel.c
+++ b/src/core/channel/client_channel.c
@@ -43,6 +43,7 @@
 
 #include "src/core/channel/channel_args.h"
 #include "src/core/channel/connected_channel.h"
+#include "src/core/channel/subchannel_call_holder.h"
 #include "src/core/iomgr/iomgr.h"
 #include "src/core/profiling/timers.h"
 #include "src/core/support/string.h"
@@ -51,7 +52,7 @@
 
 /* Client channel implementation */
 
-typedef struct call_data call_data;
+typedef grpc_subchannel_call_holder call_data;
 
 typedef struct client_channel_channel_data {
   /** metadata context for this channel */
@@ -98,360 +99,22 @@ typedef struct {
   grpc_lb_policy *lb_policy;
 } lb_policy_connectivity_watcher;
 
-typedef enum {
-  CALL_CREATED,
-  CALL_WAITING_FOR_SEND,
-  CALL_WAITING_FOR_CONFIG,
-  CALL_WAITING_FOR_PICK,
-  CALL_WAITING_FOR_CALL,
-  CALL_ACTIVE,
-  CALL_CANCELLED
-} call_state;
-
-struct call_data {
-  /* owning element */
-  grpc_call_element *elem;
-
-  gpr_mu mu_state;
-
-  call_state state;
-  gpr_timespec deadline;
-  grpc_subchannel *picked_channel;
-  grpc_closure async_setup_task;
-  grpc_transport_stream_op waiting_op;
-  /* our child call stack */
-  grpc_subchannel_call *subchannel_call;
-  grpc_linked_mdelem status;
-  grpc_linked_mdelem details;
-};
-
-static grpc_closure *merge_into_waiting_op(grpc_call_element *elem,
-                                           grpc_transport_stream_op *new_op)
-    GRPC_MUST_USE_RESULT;
-
-static void handle_op_after_cancellation(grpc_exec_ctx *exec_ctx,
-                                         grpc_call_element *elem,
-                                         grpc_transport_stream_op *op) {
-  call_data *calld = elem->call_data;
-  channel_data *chand = elem->channel_data;
-  if (op->send_ops) {
-    grpc_stream_ops_unref_owned_objects(op->send_ops->ops, op->send_ops->nops);
-    op->on_done_send->cb(exec_ctx, op->on_done_send->cb_arg, 0);
-  }
-  if (op->recv_ops) {
-    char status[GPR_LTOA_MIN_BUFSIZE];
-    grpc_metadata_batch mdb;
-    gpr_ltoa(GRPC_STATUS_CANCELLED, status);
-    calld->status.md =
-        grpc_mdelem_from_strings(chand->mdctx, "grpc-status", status);
-    calld->details.md =
-        grpc_mdelem_from_strings(chand->mdctx, "grpc-message", "Cancelled");
-    calld->status.prev = calld->details.next = NULL;
-    calld->status.next = &calld->details;
-    calld->details.prev = &calld->status;
-    mdb.list.head = &calld->status;
-    mdb.list.tail = &calld->details;
-    mdb.garbage.head = mdb.garbage.tail = NULL;
-    mdb.deadline = gpr_inf_future(GPR_CLOCK_REALTIME);
-    grpc_sopb_add_metadata(op->recv_ops, mdb);
-    *op->recv_state = GRPC_STREAM_CLOSED;
-    op->on_done_recv->cb(exec_ctx, op->on_done_recv->cb_arg, 1);
-  }
-  if (op->on_consumed) {
-    op->on_consumed->cb(exec_ctx, op->on_consumed->cb_arg, 0);
-  }
-}
-
 typedef struct {
   grpc_closure closure;
   grpc_call_element *elem;
 } waiting_call;
 
-static void perform_transport_stream_op(grpc_exec_ctx *exec_ctx,
-                                        grpc_call_element *elem,
-                                        grpc_transport_stream_op *op,
-                                        int continuation);
-
-static void continue_with_pick(grpc_exec_ctx *exec_ctx, void *arg,
-                               int iomgr_success) {
-  waiting_call *wc = arg;
-  call_data *calld = wc->elem->call_data;
-  perform_transport_stream_op(exec_ctx, wc->elem, &calld->waiting_op, 1);
-  gpr_free(wc);
-}
-
-static void add_to_lb_policy_wait_queue_locked_state_config(
-    grpc_call_element *elem) {
-  channel_data *chand = elem->channel_data;
-  waiting_call *wc = gpr_malloc(sizeof(*wc));
-  grpc_closure_init(&wc->closure, continue_with_pick, wc);
-  wc->elem = elem;
-  grpc_closure_list_add(&chand->waiting_for_config_closures, &wc->closure, 1);
-}
-
-static int is_empty(void *p, int len) {
-  char *ptr = p;
-  int i;
-  for (i = 0; i < len; i++) {
-    if (ptr[i] != 0) return 0;
-  }
-  return 1;
-}
-
-static void started_call_locked(grpc_exec_ctx *exec_ctx, void *arg,
-                                int iomgr_success) {
-  call_data *calld = arg;
-  grpc_transport_stream_op op;
-  int have_waiting;
-
-  if (calld->state == CALL_CANCELLED && calld->subchannel_call != NULL) {
-    memset(&op, 0, sizeof(op));
-    op.cancel_with_status = GRPC_STATUS_CANCELLED;
-    gpr_mu_unlock(&calld->mu_state);
-    grpc_subchannel_call_process_op(exec_ctx, calld->subchannel_call, &op);
-  } else if (calld->state == CALL_WAITING_FOR_CALL) {
-    have_waiting = !is_empty(&calld->waiting_op, sizeof(calld->waiting_op));
-    if (calld->subchannel_call != NULL) {
-      calld->state = CALL_ACTIVE;
-      gpr_mu_unlock(&calld->mu_state);
-      if (have_waiting) {
-        grpc_subchannel_call_process_op(exec_ctx, calld->subchannel_call,
-                                        &calld->waiting_op);
-      }
-    } else {
-      calld->state = CALL_CANCELLED;
-      gpr_mu_unlock(&calld->mu_state);
-      if (have_waiting) {
-        handle_op_after_cancellation(exec_ctx, calld->elem, &calld->waiting_op);
-      }
-    }
-  } else {
-    GPR_ASSERT(calld->state == CALL_CANCELLED);
-    gpr_mu_unlock(&calld->mu_state);
-  }
-}
-
-static void started_call(grpc_exec_ctx *exec_ctx, void *arg,
-                         int iomgr_success) {
-  call_data *calld = arg;
-  gpr_mu_lock(&calld->mu_state);
-  started_call_locked(exec_ctx, arg, iomgr_success);
-}
-
-static void picked_target(grpc_exec_ctx *exec_ctx, void *arg,
-                          int iomgr_success) {
-  call_data *calld = arg;
-  grpc_pollset *pollset;
-  grpc_subchannel_call_create_status call_creation_status;
-
-  GPR_TIMER_BEGIN("picked_target", 0);
-
-  if (calld->picked_channel == NULL) {
-    /* treat this like a cancellation */
-    calld->waiting_op.cancel_with_status = GRPC_STATUS_UNAVAILABLE;
-    perform_transport_stream_op(exec_ctx, calld->elem, &calld->waiting_op, 1);
-  } else {
-    gpr_mu_lock(&calld->mu_state);
-    if (calld->state == CALL_CANCELLED) {
-      gpr_mu_unlock(&calld->mu_state);
-      handle_op_after_cancellation(exec_ctx, calld->elem, &calld->waiting_op);
-    } else {
-      GPR_ASSERT(calld->state == CALL_WAITING_FOR_PICK);
-      calld->state = CALL_WAITING_FOR_CALL;
-      pollset = calld->waiting_op.bind_pollset;
-      grpc_closure_init(&calld->async_setup_task, started_call, calld);
-      call_creation_status = grpc_subchannel_create_call(
-          exec_ctx, calld->picked_channel, pollset, &calld->subchannel_call,
-          &calld->async_setup_task);
-      if (call_creation_status == GRPC_SUBCHANNEL_CALL_CREATE_READY) {
-        started_call_locked(exec_ctx, calld, iomgr_success);
-      } else {
-        gpr_mu_unlock(&calld->mu_state);
-      }
-    }
-  }
-
-  GPR_TIMER_END("picked_target", 0);
-}
-
-static grpc_closure *merge_into_waiting_op(grpc_call_element *elem,
-                                           grpc_transport_stream_op *new_op) {
-  call_data *calld = elem->call_data;
-  grpc_closure *consumed_op = NULL;
-  grpc_transport_stream_op *waiting_op = &calld->waiting_op;
-  GPR_ASSERT((waiting_op->send_ops != NULL) + (new_op->send_ops != NULL) <= 1);
-  GPR_ASSERT((waiting_op->recv_ops != NULL) + (new_op->recv_ops != NULL) <= 1);
-  if (new_op->send_ops != NULL) {
-    waiting_op->send_ops = new_op->send_ops;
-    waiting_op->is_last_send = new_op->is_last_send;
-    waiting_op->on_done_send = new_op->on_done_send;
-  }
-  if (new_op->recv_ops != NULL) {
-    waiting_op->recv_ops = new_op->recv_ops;
-    waiting_op->recv_state = new_op->recv_state;
-    waiting_op->on_done_recv = new_op->on_done_recv;
-  }
-  if (new_op->on_consumed != NULL) {
-    if (waiting_op->on_consumed != NULL) {
-      consumed_op = waiting_op->on_consumed;
-    }
-    waiting_op->on_consumed = new_op->on_consumed;
-  }
-  if (new_op->cancel_with_status != GRPC_STATUS_OK) {
-    waiting_op->cancel_with_status = new_op->cancel_with_status;
-  }
-  return consumed_op;
-}
-
 static char *cc_get_peer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem) {
-  call_data *calld = elem->call_data;
   channel_data *chand = elem->channel_data;
-  grpc_subchannel_call *subchannel_call;
-  char *result;
-
-  gpr_mu_lock(&calld->mu_state);
-  if (calld->state == CALL_ACTIVE) {
-    subchannel_call = calld->subchannel_call;
-    GRPC_SUBCHANNEL_CALL_REF(subchannel_call, "get_peer");
-    gpr_mu_unlock(&calld->mu_state);
-    result = grpc_subchannel_call_get_peer(exec_ctx, subchannel_call);
-    GRPC_SUBCHANNEL_CALL_UNREF(exec_ctx, subchannel_call, "get_peer");
-    return result;
-  } else {
-    gpr_mu_unlock(&calld->mu_state);
-    return grpc_channel_get_target(chand->master);
-  }
-}
-
-static void perform_transport_stream_op(grpc_exec_ctx *exec_ctx,
-                                        grpc_call_element *elem,
-                                        grpc_transport_stream_op *op,
-                                        int continuation) {
-  call_data *calld = elem->call_data;
-  channel_data *chand = elem->channel_data;
-  grpc_subchannel_call *subchannel_call;
-  grpc_lb_policy *lb_policy;
-  grpc_transport_stream_op op2;
-  GPR_TIMER_BEGIN("perform_transport_stream_op", 0);
-  GPR_ASSERT(elem->filter == &grpc_client_channel_filter);
-  GRPC_CALL_LOG_OP(GPR_INFO, elem, op);
-
-  gpr_mu_lock(&calld->mu_state);
-  switch (calld->state) {
-    case CALL_ACTIVE:
-      GPR_ASSERT(!continuation);
-      subchannel_call = calld->subchannel_call;
-      gpr_mu_unlock(&calld->mu_state);
-      grpc_subchannel_call_process_op(exec_ctx, subchannel_call, op);
-      break;
-    case CALL_CANCELLED:
-      gpr_mu_unlock(&calld->mu_state);
-      handle_op_after_cancellation(exec_ctx, elem, op);
-      break;
-    case CALL_WAITING_FOR_SEND:
-      GPR_ASSERT(!continuation);
-      grpc_exec_ctx_enqueue(exec_ctx, merge_into_waiting_op(elem, op), 1);
-      if (!calld->waiting_op.send_ops &&
-          calld->waiting_op.cancel_with_status == GRPC_STATUS_OK) {
-        gpr_mu_unlock(&calld->mu_state);
-        break;
-      }
-      *op = calld->waiting_op;
-      memset(&calld->waiting_op, 0, sizeof(calld->waiting_op));
-      continuation = 1;
-    /* fall through */
-    case CALL_WAITING_FOR_CONFIG:
-    case CALL_WAITING_FOR_PICK:
-    case CALL_WAITING_FOR_CALL:
-      if (!continuation) {
-        if (op->cancel_with_status != GRPC_STATUS_OK) {
-          calld->state = CALL_CANCELLED;
-          op2 = calld->waiting_op;
-          memset(&calld->waiting_op, 0, sizeof(calld->waiting_op));
-          if (op->on_consumed) {
-            calld->waiting_op.on_consumed = op->on_consumed;
-            op->on_consumed = NULL;
-          } else if (op2.on_consumed) {
-            calld->waiting_op.on_consumed = op2.on_consumed;
-            op2.on_consumed = NULL;
-          }
-          gpr_mu_unlock(&calld->mu_state);
-          handle_op_after_cancellation(exec_ctx, elem, op);
-          handle_op_after_cancellation(exec_ctx, elem, &op2);
-        } else {
-          grpc_exec_ctx_enqueue(exec_ctx, merge_into_waiting_op(elem, op), 1);
-          gpr_mu_unlock(&calld->mu_state);
-        }
-        break;
-      }
-    /* fall through */
-    case CALL_CREATED:
-      if (op->cancel_with_status != GRPC_STATUS_OK) {
-        calld->state = CALL_CANCELLED;
-        gpr_mu_unlock(&calld->mu_state);
-        handle_op_after_cancellation(exec_ctx, elem, op);
-      } else {
-        calld->waiting_op = *op;
-
-        if (op->send_ops == NULL) {
-          /* need to have some send ops before we can select the
-             lb target */
-          calld->state = CALL_WAITING_FOR_SEND;
-          gpr_mu_unlock(&calld->mu_state);
-        } else {
-          gpr_mu_lock(&chand->mu_config);
-          lb_policy = chand->lb_policy;
-          if (lb_policy) {
-            grpc_transport_stream_op *waiting_op = &calld->waiting_op;
-            grpc_pollset *bind_pollset = waiting_op->bind_pollset;
-            grpc_metadata_batch *initial_metadata =
-                &waiting_op->send_ops->ops[0].data.metadata;
-            GRPC_LB_POLICY_REF(lb_policy, "pick");
-            gpr_mu_unlock(&chand->mu_config);
-            calld->state = CALL_WAITING_FOR_PICK;
-
-            GPR_ASSERT(waiting_op->bind_pollset);
-            GPR_ASSERT(waiting_op->send_ops);
-            GPR_ASSERT(waiting_op->send_ops->nops >= 1);
-            GPR_ASSERT(waiting_op->send_ops->ops[0].type == GRPC_OP_METADATA);
-            gpr_mu_unlock(&calld->mu_state);
-
-            grpc_closure_init(&calld->async_setup_task, picked_target, calld);
-            grpc_lb_policy_pick(exec_ctx, lb_policy, bind_pollset,
-                                initial_metadata, &calld->picked_channel,
-                                &calld->async_setup_task);
-
-            GRPC_LB_POLICY_UNREF(exec_ctx, lb_policy, "pick");
-          } else if (chand->resolver != NULL) {
-            calld->state = CALL_WAITING_FOR_CONFIG;
-            add_to_lb_policy_wait_queue_locked_state_config(elem);
-            if (!chand->started_resolving && chand->resolver != NULL) {
-              GRPC_CHANNEL_INTERNAL_REF(chand->master, "resolver");
-              chand->started_resolving = 1;
-              grpc_resolver_next(exec_ctx, chand->resolver,
-                                 &chand->incoming_configuration,
-                                 &chand->on_config_changed);
-            }
-            gpr_mu_unlock(&chand->mu_config);
-            gpr_mu_unlock(&calld->mu_state);
-          } else {
-            calld->state = CALL_CANCELLED;
-            gpr_mu_unlock(&chand->mu_config);
-            gpr_mu_unlock(&calld->mu_state);
-            handle_op_after_cancellation(exec_ctx, elem, op);
-          }
-        }
-      }
-      break;
-  }
-
-  GPR_TIMER_END("perform_transport_stream_op", 0);
+  return grpc_subchannel_call_holder_get_peer(exec_ctx, elem->call_data,
+                                              chand->master);
 }
 
 static void cc_start_transport_stream_op(grpc_exec_ctx *exec_ctx,
                                          grpc_call_element *elem,
                                          grpc_transport_stream_op *op) {
-  perform_transport_stream_op(exec_ctx, elem, op, 0);
+  GRPC_CALL_LOG_OP(GPR_INFO, elem, op);
+  grpc_subchannel_call_holder_perform_op(exec_ctx, elem->call_data, op);
 }
 
 static void watch_lb_policy(grpc_exec_ctx *exec_ctx, channel_data *chand,
@@ -593,11 +256,9 @@ static void cc_start_transport_op(grpc_exec_ctx *exec_ctx,
     op->connectivity_state = NULL;
   }
 
-  if (!is_empty(op, sizeof(*op))) {
-    lb_policy = chand->lb_policy;
-    if (lb_policy) {
-      GRPC_LB_POLICY_REF(lb_policy, "broadcast");
-    }
+  lb_policy = chand->lb_policy;
+  if (lb_policy) {
+    GRPC_LB_POLICY_REF(lb_policy, "broadcast");
   }
 
   if (op->disconnect && chand->resolver != NULL) {
@@ -624,67 +285,110 @@ static void cc_start_transport_op(grpc_exec_ctx *exec_ctx,
   }
 }
 
-/* Constructor for call_data */
-static void init_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
-                           const void *server_transport_data,
-                           grpc_transport_stream_op *initial_op) {
+typedef struct {
+  grpc_metadata_batch *initial_metadata;
+  grpc_subchannel **subchannel;
+  grpc_closure *on_ready;
+  grpc_call_element *elem;
+  grpc_closure closure;
+} continue_picking_args;
+
+static int cc_pick_subchannel(grpc_exec_ctx *exec_ctx, void *arg,
+                              grpc_metadata_batch *initial_metadata,
+                              grpc_subchannel **subchannel,
+                              grpc_closure *on_ready);
+
+static void continue_picking(grpc_exec_ctx *exec_ctx, void *arg, int success) {
+  continue_picking_args *cpa = arg;
+  if (!success) {
+    grpc_exec_ctx_enqueue(exec_ctx, cpa->on_ready, 0);
+  } else if (cpa->subchannel == NULL) {
+    /* cancelled, do nothing */
+  } else if (cc_pick_subchannel(exec_ctx, cpa->elem, cpa->initial_metadata,
+                                cpa->subchannel, cpa->on_ready)) {
+    grpc_exec_ctx_enqueue(exec_ctx, cpa->on_ready, 1);
+  }
+  gpr_free(cpa);
+}
+
+static int cc_pick_subchannel(grpc_exec_ctx *exec_ctx, void *elemp,
+                              grpc_metadata_batch *initial_metadata,
+                              grpc_subchannel **subchannel,
+                              grpc_closure *on_ready) {
+  grpc_call_element *elem = elemp;
+  channel_data *chand = elem->channel_data;
   call_data *calld = elem->call_data;
+  continue_picking_args *cpa;
+  grpc_closure *closure;
 
-  /* TODO(ctiller): is there something useful we can do here? */
-  GPR_ASSERT(initial_op == NULL);
+  GPR_ASSERT(subchannel);
 
-  GPR_ASSERT(elem->filter == &grpc_client_channel_filter);
-  GPR_ASSERT(server_transport_data == NULL);
-  gpr_mu_init(&calld->mu_state);
-  calld->elem = elem;
-  calld->state = CALL_CREATED;
-  calld->deadline = gpr_inf_future(GPR_CLOCK_REALTIME);
+  gpr_mu_lock(&chand->mu_config);
+  if (initial_metadata == NULL) {
+    if (chand->lb_policy != NULL) {
+      grpc_lb_policy_cancel_pick(exec_ctx, chand->lb_policy, subchannel);
+    }
+    for (closure = chand->waiting_for_config_closures.head; closure != NULL;
+         closure = grpc_closure_next(closure)) {
+      cpa = closure->cb_arg;
+      if (cpa->subchannel == subchannel) {
+        cpa->subchannel = NULL;
+        grpc_exec_ctx_enqueue(exec_ctx, cpa->on_ready, 0);
+      }
+    }
+    gpr_mu_unlock(&chand->mu_config);
+    return 1;
+  }
+  if (chand->lb_policy != NULL) {
+    int r = grpc_lb_policy_pick(exec_ctx, chand->lb_policy, calld->pollset,
+                                initial_metadata, subchannel, on_ready);
+    gpr_mu_unlock(&chand->mu_config);
+    return r;
+  }
+  if (chand->resolver != NULL && !chand->started_resolving) {
+    chand->started_resolving = 1;
+    GRPC_CHANNEL_INTERNAL_REF(chand->master, "resolver");
+    grpc_resolver_next(exec_ctx, chand->resolver,
+                       &chand->incoming_configuration,
+                       &chand->on_config_changed);
+  }
+  cpa = gpr_malloc(sizeof(*cpa));
+  cpa->initial_metadata = initial_metadata;
+  cpa->subchannel = subchannel;
+  cpa->on_ready = on_ready;
+  cpa->elem = elem;
+  grpc_closure_init(&cpa->closure, continue_picking, cpa);
+  grpc_closure_list_add(&chand->waiting_for_config_closures, &cpa->closure, 1);
+  gpr_mu_unlock(&chand->mu_config);
+  return 0;
+}
+
+/* Constructor for call_data */
+static void init_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
+                           grpc_call_element_args *args) {
+  grpc_subchannel_call_holder_init(elem->call_data, cc_pick_subchannel, elem);
 }
 
 /* Destructor for call_data */
 static void destroy_call_elem(grpc_exec_ctx *exec_ctx,
                               grpc_call_element *elem) {
-  call_data *calld = elem->call_data;
-  grpc_subchannel_call *subchannel_call;
-
-  /* if the call got activated, we need to destroy the child stack also, and
-     remove it from the in-flight requests tracked by the child_entry we
-     picked */
-  gpr_mu_lock(&calld->mu_state);
-  switch (calld->state) {
-    case CALL_ACTIVE:
-      subchannel_call = calld->subchannel_call;
-      gpr_mu_unlock(&calld->mu_state);
-      GRPC_SUBCHANNEL_CALL_UNREF(exec_ctx, subchannel_call, "client_channel");
-      break;
-    case CALL_CREATED:
-    case CALL_CANCELLED:
-      gpr_mu_unlock(&calld->mu_state);
-      break;
-    case CALL_WAITING_FOR_PICK:
-    case CALL_WAITING_FOR_CONFIG:
-    case CALL_WAITING_FOR_CALL:
-    case CALL_WAITING_FOR_SEND:
-      GPR_UNREACHABLE_CODE(return );
-  }
+  grpc_subchannel_call_holder_destroy(exec_ctx, elem->call_data);
 }
 
 /* Constructor for channel_data */
 static void init_channel_elem(grpc_exec_ctx *exec_ctx,
-                              grpc_channel_element *elem, grpc_channel *master,
-                              const grpc_channel_args *args,
-                              grpc_mdctx *metadata_context, int is_first,
-                              int is_last) {
+                              grpc_channel_element *elem,
+                              grpc_channel_element_args *args) {
   channel_data *chand = elem->channel_data;
 
   memset(chand, 0, sizeof(*chand));
 
-  GPR_ASSERT(is_last);
+  GPR_ASSERT(args->is_last);
   GPR_ASSERT(elem->filter == &grpc_client_channel_filter);
 
   gpr_mu_init(&chand->mu_config);
-  chand->mdctx = metadata_context;
-  chand->master = master;
+  chand->mdctx = args->metadata_context;
+  chand->master = args->master;
   grpc_pollset_set_init(&chand->pollset_set);
   grpc_closure_init(&chand->on_config_changed, cc_on_config_changed, chand);
 
@@ -709,10 +413,16 @@ static void destroy_channel_elem(grpc_exec_ctx *exec_ctx,
   gpr_mu_destroy(&chand->mu_config);
 }
 
+static void cc_set_pollset(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
+                           grpc_pollset *pollset) {
+  call_data *calld = elem->call_data;
+  calld->pollset = pollset;
+}
+
 const grpc_channel_filter grpc_client_channel_filter = {
     cc_start_transport_stream_op, cc_start_transport_op, sizeof(call_data),
-    init_call_elem, destroy_call_elem, sizeof(channel_data), init_channel_elem,
-    destroy_channel_elem, cc_get_peer, "client-channel",
+    init_call_elem, cc_set_pollset, destroy_call_elem, sizeof(channel_data),
+    init_channel_elem, destroy_channel_elem, cc_get_peer, "client-channel",
 };
 
 void grpc_client_channel_set_resolver(grpc_exec_ctx *exec_ctx,
diff --git a/src/core/channel/client_uchannel.c b/src/core/channel/client_uchannel.c
index 510677a844..ec6b02381a 100644
--- a/src/core/channel/client_uchannel.c
+++ b/src/core/channel/client_uchannel.c
@@ -39,6 +39,7 @@
 #include "src/core/channel/channel_args.h"
 #include "src/core/channel/client_channel.h"
 #include "src/core/channel/compress_filter.h"
+#include "src/core/channel/subchannel_call_holder.h"
 #include "src/core/iomgr/iomgr.h"
 #include "src/core/support/string.h"
 #include "src/core/surface/channel.h"
@@ -52,8 +53,6 @@
 /** Microchannel (uchannel) implementation: a lightweight channel without any
  * load-balancing mechanisms meant for communication from within the core. */
 
-typedef struct call_data call_data;
-
 typedef struct client_uchannel_channel_data {
   /** metadata context for this channel */
   grpc_mdctx *mdctx;
@@ -80,85 +79,7 @@ typedef struct client_uchannel_channel_data {
   gpr_mu mu_state;
 } channel_data;
 
-typedef enum {
-  CALL_CREATED,
-  CALL_WAITING_FOR_SEND,
-  CALL_WAITING_FOR_CALL,
-  CALL_ACTIVE,
-  CALL_CANCELLED
-} call_state;
-
-struct call_data {
-  /* owning element */
-  grpc_call_element *elem;
-
-  gpr_mu mu_state;
-
-  call_state state;
-  gpr_timespec deadline;
-  grpc_closure async_setup_task;
-  grpc_transport_stream_op waiting_op;
-  /* our child call stack */
-  grpc_subchannel_call *subchannel_call;
-  grpc_linked_mdelem status;
-  grpc_linked_mdelem details;
-};
-
-static grpc_closure *merge_into_waiting_op(grpc_call_element *elem,
-                                           grpc_transport_stream_op *new_op)
-    GRPC_MUST_USE_RESULT;
-
-static void handle_op_after_cancellation(grpc_exec_ctx *exec_ctx,
-                                         grpc_call_element *elem,
-                                         grpc_transport_stream_op *op) {
-  call_data *calld = elem->call_data;
-  channel_data *chand = elem->channel_data;
-  if (op->send_ops) {
-    grpc_stream_ops_unref_owned_objects(op->send_ops->ops, op->send_ops->nops);
-    op->on_done_send->cb(exec_ctx, op->on_done_send->cb_arg, 0);
-  }
-  if (op->recv_ops) {
-    char status[GPR_LTOA_MIN_BUFSIZE];
-    grpc_metadata_batch mdb;
-    gpr_ltoa(GRPC_STATUS_CANCELLED, status);
-    calld->status.md =
-        grpc_mdelem_from_strings(chand->mdctx, "grpc-status", status);
-    calld->details.md =
-        grpc_mdelem_from_strings(chand->mdctx, "grpc-message", "Cancelled");
-    calld->status.prev = calld->details.next = NULL;
-    calld->status.next = &calld->details;
-    calld->details.prev = &calld->status;
-    mdb.list.head = &calld->status;
-    mdb.list.tail = &calld->details;
-    mdb.garbage.head = mdb.garbage.tail = NULL;
-    mdb.deadline = gpr_inf_future(GPR_CLOCK_REALTIME);
-    grpc_sopb_add_metadata(op->recv_ops, mdb);
-    *op->recv_state = GRPC_STREAM_CLOSED;
-    op->on_done_recv->cb(exec_ctx, op->on_done_recv->cb_arg, 1);
-  }
-  if (op->on_consumed) {
-    op->on_consumed->cb(exec_ctx, op->on_consumed->cb_arg, 0);
-  }
-}
-
-typedef struct {
-  grpc_closure closure;
-  grpc_call_element *elem;
-} waiting_call;
-
-static void perform_transport_stream_op(grpc_exec_ctx *exec_ctx,
-                                        grpc_call_element *elem,
-                                        grpc_transport_stream_op *op,
-                                        int continuation);
-
-static int is_empty(void *p, int len) {
-  char *ptr = p;
-  int i;
-  for (i = 0; i < len; i++) {
-    if (ptr[i] != 0) return 0;
-  }
-  return 1;
-}
+typedef grpc_subchannel_call_holder call_data;
 
 static void monitor_subchannel(grpc_exec_ctx *exec_ctx, void *arg,
                                int iomgr_success) {
@@ -171,201 +92,17 @@ static void monitor_subchannel(grpc_exec_ctx *exec_ctx, void *arg,
                                          &chand->connectivity_cb);
 }
 
-static void started_call_locked(grpc_exec_ctx *exec_ctx, void *arg,
-                         int iomgr_success) {
-  call_data *calld = arg;
-  grpc_transport_stream_op op;
-  int have_waiting;
-
-  if (calld->state == CALL_CANCELLED && iomgr_success == 0) {
-    have_waiting = !is_empty(&calld->waiting_op, sizeof(calld->waiting_op));
-    gpr_mu_unlock(&calld->mu_state);
-    if (have_waiting) {
-      handle_op_after_cancellation(exec_ctx, calld->elem, &calld->waiting_op);
-    }
-  } else if (calld->state == CALL_CANCELLED && calld->subchannel_call != NULL) {
-    memset(&op, 0, sizeof(op));
-    op.cancel_with_status = GRPC_STATUS_CANCELLED;
-    gpr_mu_unlock(&calld->mu_state);
-    grpc_subchannel_call_process_op(exec_ctx, calld->subchannel_call, &op);
-  } else if (calld->state == CALL_WAITING_FOR_CALL) {
-    have_waiting = !is_empty(&calld->waiting_op, sizeof(calld->waiting_op));
-    if (calld->subchannel_call != NULL) {
-      calld->state = CALL_ACTIVE;
-      gpr_mu_unlock(&calld->mu_state);
-      if (have_waiting) {
-        grpc_subchannel_call_process_op(exec_ctx, calld->subchannel_call,
-                                        &calld->waiting_op);
-      }
-    } else {
-      calld->state = CALL_CANCELLED;
-      gpr_mu_unlock(&calld->mu_state);
-      if (have_waiting) {
-        handle_op_after_cancellation(exec_ctx, calld->elem, &calld->waiting_op);
-      }
-    }
-  } else {
-    GPR_ASSERT(calld->state == CALL_CANCELLED);
-    gpr_mu_unlock(&calld->mu_state);
-    have_waiting = !is_empty(&calld->waiting_op, sizeof(calld->waiting_op));
-    if (have_waiting) {
-      handle_op_after_cancellation(exec_ctx, calld->elem, &calld->waiting_op);
-    }
-  }
-}
-
-static void started_call(grpc_exec_ctx *exec_ctx, void *arg,
-                         int iomgr_success) {
-  call_data *calld = arg;
-  gpr_mu_lock(&calld->mu_state);
-  started_call_locked(exec_ctx, arg, iomgr_success);
-}
-
-static grpc_closure *merge_into_waiting_op(grpc_call_element *elem,
-                                           grpc_transport_stream_op *new_op) {
-  call_data *calld = elem->call_data;
-  grpc_closure *consumed_op = NULL;
-  grpc_transport_stream_op *waiting_op = &calld->waiting_op;
-  GPR_ASSERT((waiting_op->send_ops != NULL) + (new_op->send_ops != NULL) <= 1);
-  GPR_ASSERT((waiting_op->recv_ops != NULL) + (new_op->recv_ops != NULL) <= 1);
-  if (new_op->send_ops != NULL) {
-    waiting_op->send_ops = new_op->send_ops;
-    waiting_op->is_last_send = new_op->is_last_send;
-    waiting_op->on_done_send = new_op->on_done_send;
-  }
-  if (new_op->recv_ops != NULL) {
-    waiting_op->recv_ops = new_op->recv_ops;
-    waiting_op->recv_state = new_op->recv_state;
-    waiting_op->on_done_recv = new_op->on_done_recv;
-  }
-  if (new_op->on_consumed != NULL) {
-    if (waiting_op->on_consumed != NULL) {
-      consumed_op = waiting_op->on_consumed;
-    }
-    waiting_op->on_consumed = new_op->on_consumed;
-  }
-  if (new_op->cancel_with_status != GRPC_STATUS_OK) {
-    waiting_op->cancel_with_status = new_op->cancel_with_status;
-  }
-  return consumed_op;
-}
-
 static char *cuc_get_peer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem) {
-  call_data *calld = elem->call_data;
-  channel_data *chand = elem->channel_data;
-  grpc_subchannel_call *subchannel_call;
-  char *result;
-
-  gpr_mu_lock(&calld->mu_state);
-  if (calld->state == CALL_ACTIVE) {
-    subchannel_call = calld->subchannel_call;
-    GRPC_SUBCHANNEL_CALL_REF(subchannel_call, "get_peer");
-    gpr_mu_unlock(&calld->mu_state);
-    result = grpc_subchannel_call_get_peer(exec_ctx, subchannel_call);
-    GRPC_SUBCHANNEL_CALL_UNREF(exec_ctx, subchannel_call, "get_peer");
-    return result;
-  } else {
-    gpr_mu_unlock(&calld->mu_state);
-    return grpc_channel_get_target(chand->master);
-  }
-}
-
-static void perform_transport_stream_op(grpc_exec_ctx *exec_ctx,
-                                        grpc_call_element *elem,
-                                        grpc_transport_stream_op *op,
-                                        int continuation) {
-  call_data *calld = elem->call_data;
   channel_data *chand = elem->channel_data;
-  grpc_subchannel_call *subchannel_call;
-  grpc_transport_stream_op op2;
-  GPR_ASSERT(elem->filter == &grpc_client_uchannel_filter);
-  GRPC_CALL_LOG_OP(GPR_INFO, elem, op);
-
-  gpr_mu_lock(&calld->mu_state);
-  /* make sure the wrapped subchannel has been set (see
-   * grpc_client_uchannel_set_subchannel) */
-  GPR_ASSERT(chand->subchannel != NULL);
-
-  switch (calld->state) {
-    case CALL_ACTIVE:
-      GPR_ASSERT(!continuation);
-      subchannel_call = calld->subchannel_call;
-      gpr_mu_unlock(&calld->mu_state);
-      grpc_subchannel_call_process_op(exec_ctx, subchannel_call, op);
-      break;
-    case CALL_CANCELLED:
-      gpr_mu_unlock(&calld->mu_state);
-      handle_op_after_cancellation(exec_ctx, elem, op);
-      break;
-    case CALL_WAITING_FOR_SEND:
-      GPR_ASSERT(!continuation);
-      grpc_exec_ctx_enqueue(exec_ctx, merge_into_waiting_op(elem, op), 1);
-      if (!calld->waiting_op.send_ops &&
-          calld->waiting_op.cancel_with_status == GRPC_STATUS_OK) {
-        gpr_mu_unlock(&calld->mu_state);
-        break;
-      }
-      *op = calld->waiting_op;
-      memset(&calld->waiting_op, 0, sizeof(calld->waiting_op));
-      continuation = 1;
-    /* fall through */
-    case CALL_WAITING_FOR_CALL:
-      if (!continuation) {
-        if (op->cancel_with_status != GRPC_STATUS_OK) {
-          calld->state = CALL_CANCELLED;
-          op2 = calld->waiting_op;
-          memset(&calld->waiting_op, 0, sizeof(calld->waiting_op));
-          if (op->on_consumed) {
-            calld->waiting_op.on_consumed = op->on_consumed;
-            op->on_consumed = NULL;
-          } else if (op2.on_consumed) {
-            calld->waiting_op.on_consumed = op2.on_consumed;
-            op2.on_consumed = NULL;
-          }
-          gpr_mu_unlock(&calld->mu_state);
-          handle_op_after_cancellation(exec_ctx, elem, op);
-          handle_op_after_cancellation(exec_ctx, elem, &op2);
-          grpc_subchannel_cancel_waiting_call(exec_ctx, chand->subchannel, 1);
-        } else {
-          grpc_exec_ctx_enqueue(exec_ctx, merge_into_waiting_op(elem, op), 1);
-          gpr_mu_unlock(&calld->mu_state);
-        }
-        break;
-      }
-    /* fall through */
-    case CALL_CREATED:
-      if (op->cancel_with_status != GRPC_STATUS_OK) {
-        calld->state = CALL_CANCELLED;
-        gpr_mu_unlock(&calld->mu_state);
-        handle_op_after_cancellation(exec_ctx, elem, op);
-      } else {
-        calld->waiting_op = *op;
-        if (op->send_ops == NULL) {
-          calld->state = CALL_WAITING_FOR_SEND;
-          gpr_mu_unlock(&calld->mu_state);
-        } else {
-          grpc_subchannel_call_create_status call_creation_status;
-          grpc_pollset *pollset = calld->waiting_op.bind_pollset;
-          calld->state = CALL_WAITING_FOR_CALL;
-          grpc_closure_init(&calld->async_setup_task, started_call, calld);
-          call_creation_status = grpc_subchannel_create_call(
-              exec_ctx, chand->subchannel, pollset, &calld->subchannel_call,
-              &calld->async_setup_task);
-          if (call_creation_status == GRPC_SUBCHANNEL_CALL_CREATE_READY) {
-            started_call_locked(exec_ctx, calld, 1);
-          } else {
-            gpr_mu_unlock(&calld->mu_state);
-          }
-        }
-      }
-      break;
-  }
+  return grpc_subchannel_call_holder_get_peer(exec_ctx, elem->call_data,
+                                              chand->master);
 }
 
 static void cuc_start_transport_stream_op(grpc_exec_ctx *exec_ctx,
                                           grpc_call_element *elem,
                                           grpc_transport_stream_op *op) {
-  perform_transport_stream_op(exec_ctx, elem, op, 0);
+  GRPC_CALL_LOG_OP(GPR_INFO, elem, op);
+  grpc_subchannel_call_holder_perform_op(exec_ctx, elem->call_data, op);
 }
 
 static void cuc_start_transport_op(grpc_exec_ctx *exec_ctx,
@@ -392,64 +129,40 @@ static void cuc_start_transport_op(grpc_exec_ctx *exec_ctx,
   }
 }
 
+static int cuc_pick_subchannel(grpc_exec_ctx *exec_ctx, void *arg,
+                               grpc_metadata_batch *initial_metadata,
+                               grpc_subchannel **subchannel,
+                               grpc_closure *on_ready) {
+  channel_data *chand = arg;
+  GPR_ASSERT(initial_metadata != NULL);
+  *subchannel = chand->subchannel;
+  return 1;
+}
+
 /* Constructor for call_data */
 static void cuc_init_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
-                               const void *server_transport_data,
-                               grpc_transport_stream_op *initial_op) {
-  call_data *calld = elem->call_data;
-  memset(calld, 0, sizeof(call_data));
-
-  /* TODO(ctiller): is there something useful we can do here? */
-  GPR_ASSERT(initial_op == NULL);
-
-  GPR_ASSERT(elem->filter == &grpc_client_uchannel_filter);
-  GPR_ASSERT(server_transport_data == NULL);
-  gpr_mu_init(&calld->mu_state);
-  calld->elem = elem;
-  calld->state = CALL_CREATED;
-  calld->deadline = gpr_inf_future(GPR_CLOCK_REALTIME);
+                               grpc_call_element_args *args) {
+  grpc_subchannel_call_holder_init(elem->call_data, cuc_pick_subchannel,
+                                   elem->channel_data);
 }
 
 /* Destructor for call_data */
 static void cuc_destroy_call_elem(grpc_exec_ctx *exec_ctx,
                                   grpc_call_element *elem) {
-  call_data *calld = elem->call_data;
-  grpc_subchannel_call *subchannel_call;
-
-  /* if the call got activated, we need to destroy the child stack also, and
-     remove it from the in-flight requests tracked by the child_entry we
-     picked */
-  gpr_mu_lock(&calld->mu_state);
-  switch (calld->state) {
-    case CALL_ACTIVE:
-      subchannel_call = calld->subchannel_call;
-      gpr_mu_unlock(&calld->mu_state);
-      GRPC_SUBCHANNEL_CALL_UNREF(exec_ctx, subchannel_call, "client_uchannel");
-      break;
-    case CALL_CREATED:
-    case CALL_CANCELLED:
-      gpr_mu_unlock(&calld->mu_state);
-      break;
-    case CALL_WAITING_FOR_CALL:
-    case CALL_WAITING_FOR_SEND:
-      GPR_UNREACHABLE_CODE(return );
-  }
+  grpc_subchannel_call_holder_destroy(exec_ctx, elem->call_data);
 }
 
 /* Constructor for channel_data */
 static void cuc_init_channel_elem(grpc_exec_ctx *exec_ctx,
                                   grpc_channel_element *elem,
-                                  grpc_channel *master,
-                                  const grpc_channel_args *args,
-                                  grpc_mdctx *metadata_context, int is_first,
-                                  int is_last) {
+                                  grpc_channel_element_args *args) {
   channel_data *chand = elem->channel_data;
   memset(chand, 0, sizeof(*chand));
   grpc_closure_init(&chand->connectivity_cb, monitor_subchannel, chand);
-  GPR_ASSERT(is_last);
+  GPR_ASSERT(args->is_last);
   GPR_ASSERT(elem->filter == &grpc_client_uchannel_filter);
-  chand->mdctx = metadata_context;
-  chand->master = master;
+  chand->mdctx = args->metadata_context;
+  chand->master = args->master;
   grpc_connectivity_state_init(&chand->state_tracker, GRPC_CHANNEL_IDLE,
                                "client_uchannel");
   gpr_mu_init(&chand->mu_state);
@@ -465,17 +178,17 @@ static void cuc_destroy_channel_elem(grpc_exec_ctx *exec_ctx,
   gpr_mu_destroy(&chand->mu_state);
 }
 
+static void cuc_set_pollset(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
+                            grpc_pollset *pollset) {
+  call_data *calld = elem->call_data;
+  calld->pollset = pollset;
+}
+
 const grpc_channel_filter grpc_client_uchannel_filter = {
-    cuc_start_transport_stream_op,
-    cuc_start_transport_op,
-    sizeof(call_data),
-    cuc_init_call_elem,
-    cuc_destroy_call_elem,
-    sizeof(channel_data),
-    cuc_init_channel_elem,
-    cuc_destroy_channel_elem,
-    cuc_get_peer,
-    "client-uchannel",
+    cuc_start_transport_stream_op, cuc_start_transport_op, sizeof(call_data),
+    cuc_init_call_elem, cuc_set_pollset, cuc_destroy_call_elem,
+    sizeof(channel_data), cuc_init_channel_elem, cuc_destroy_channel_elem,
+    cuc_get_peer, "client-uchannel",
 };
 
 grpc_connectivity_state grpc_client_uchannel_check_connectivity_state(
diff --git a/src/core/channel/compress_filter.c b/src/core/channel/compress_filter.c
index 20b5084044..f23d8052f3 100644
--- a/src/core/channel/compress_filter.c
+++ b/src/core/channel/compress_filter.c
@@ -50,13 +50,20 @@ typedef struct call_data {
   grpc_linked_mdelem compression_algorithm_storage;
   grpc_linked_mdelem accept_encoding_storage;
   gpr_uint32 remaining_slice_bytes;
-  /**< Input data to be read, as per BEGIN_MESSAGE */
-  int written_initial_metadata; /**< Already processed initial md? */
   /** Compression algorithm we'll try to use. It may be given by incoming
    * metadata, or by the channel's default compression settings. */
   grpc_compression_algorithm compression_algorithm;
   /** If true, contents of \a compression_algorithm are authoritative */
   int has_compression_algorithm;
+
+  grpc_transport_stream_op send_op;
+  gpr_uint32 send_length;
+  gpr_uint32 send_flags;
+  gpr_slice incoming_slice;
+  grpc_slice_buffer_stream replacement_stream;
+  grpc_closure *post_send;
+  grpc_closure send_done;
+  grpc_closure got_slice;
 } call_data;
 
 typedef struct channel_data {
@@ -76,24 +83,6 @@ typedef struct channel_data {
   grpc_compression_options compression_options;
 } channel_data;
 
-/** Compress \a slices in place using \a algorithm. Returns 1 if compression did
- * actually happen, 0 otherwise (for example if the compressed output size was
- * larger than the raw input).
- *
- * Returns 1 if the data was actually compress and 0 otherwise. */
-static int compress_send_sb(grpc_compression_algorithm algorithm,
-                            gpr_slice_buffer *slices) {
-  int did_compress;
-  gpr_slice_buffer tmp;
-  gpr_slice_buffer_init(&tmp);
-  did_compress = grpc_msg_compress(algorithm, slices, &tmp);
-  if (did_compress) {
-    gpr_slice_buffer_swap(slices, &tmp);
-  }
-  gpr_slice_buffer_destroy(&tmp);
-  return did_compress;
-}
-
 /** For each \a md element from the incoming metadata, filter out the entry for
  * "grpc-encoding", using its value to populate the call data's
  * compression_algorithm field. */
@@ -127,7 +116,9 @@ static grpc_mdelem *compression_md_filter(void *user_data, grpc_mdelem *md) {
   return md;
 }
 
-static int skip_compression(channel_data *channeld, call_data *calld) {
+static int skip_compression(grpc_call_element *elem) {
+  call_data *calld = elem->call_data;
+  channel_data *channeld = elem->channel_data;
   if (calld->has_compression_algorithm) {
     if (calld->compression_algorithm == GRPC_COMPRESS_NONE) {
       return 1;
@@ -138,169 +129,127 @@ static int skip_compression(channel_data *channeld, call_data *calld) {
   return channeld->default_compression_algorithm == GRPC_COMPRESS_NONE;
 }
 
-/** Assembles a new grpc_stream_op_buffer with the compressed slices, modifying
- * the associated GRPC_OP_BEGIN_MESSAGE accordingly (new compressed length,
- * flags indicating compression is in effect) and replaces \a send_ops with it.
- * */
-static void finish_compressed_sopb(grpc_stream_op_buffer *send_ops,
-                                   grpc_call_element *elem) {
-  size_t i;
+/** Filter initial metadata */
+static void process_send_initial_metadata(
+    grpc_call_element *elem, grpc_metadata_batch *initial_metadata) {
   call_data *calld = elem->call_data;
-  int new_slices_added = 0; /* GPR_FALSE */
-  grpc_metadata_batch metadata;
-  grpc_stream_op_buffer new_send_ops;
-  grpc_sopb_init(&new_send_ops);
-
-  for (i = 0; i < send_ops->nops; i++) {
-    grpc_stream_op *sop = &send_ops->ops[i];
-    switch (sop->type) {
-      case GRPC_OP_BEGIN_MESSAGE:
-        GPR_ASSERT(calld->slices.length <= GPR_UINT32_MAX);
-        grpc_sopb_add_begin_message(
-            &new_send_ops, (gpr_uint32)calld->slices.length,
-            sop->data.begin_message.flags | GRPC_WRITE_INTERNAL_COMPRESS);
-        break;
-      case GRPC_OP_SLICE:
-        /* Once we reach the slices section of the original buffer, simply add
-         * all the new (compressed) slices. We obviously want to do this only
-         * once, hence the "new_slices_added" guard. */
-        if (!new_slices_added) {
-          size_t j;
-          for (j = 0; j < calld->slices.count; ++j) {
-            grpc_sopb_add_slice(&new_send_ops,
-                                gpr_slice_ref(calld->slices.slices[j]));
-          }
-          new_slices_added = 1; /* GPR_TRUE */
-        }
-        break;
-      case GRPC_OP_METADATA:
-        /* move the metadata to the new buffer. */
-        grpc_metadata_batch_move(&metadata, &sop->data.metadata);
-        grpc_sopb_add_metadata(&new_send_ops, metadata);
-        break;
-      case GRPC_NO_OP:
-        break;
-    }
+  channel_data *channeld = elem->channel_data;
+  /* Parse incoming request for compression. If any, it'll be available
+   * at calld->compression_algorithm */
+  grpc_metadata_batch_filter(initial_metadata, compression_md_filter, elem);
+  if (!calld->has_compression_algorithm) {
+    /* If no algorithm was found in the metadata and we aren't
+     * exceptionally skipping compression, fall back to the channel
+     * default */
+    calld->compression_algorithm = channeld->default_compression_algorithm;
+    calld->has_compression_algorithm = 1; /* GPR_TRUE */
   }
-  grpc_sopb_swap(send_ops, &new_send_ops);
-  grpc_sopb_destroy(&new_send_ops);
+  /* hint compression algorithm */
+  grpc_metadata_batch_add_tail(
+      initial_metadata, &calld->compression_algorithm_storage,
+      GRPC_MDELEM_REF(
+          channeld
+              ->mdelem_compression_algorithms[calld->compression_algorithm]));
+
+  /* convey supported compression algorithms */
+  grpc_metadata_batch_add_tail(
+      initial_metadata, &calld->accept_encoding_storage,
+      GRPC_MDELEM_REF(channeld->mdelem_accept_encoding));
 }
 
-/** Filter's "main" function, called for any incoming grpc_transport_stream_op
- * instance that holds a non-zero number of send operations, accesible to this
- * function in \a send_ops.  */
-static void process_send_ops(grpc_call_element *elem,
-                             grpc_stream_op_buffer *send_ops) {
-  call_data *calld = elem->call_data;
-  channel_data *channeld = elem->channel_data;
-  size_t i;
-  int did_compress = 0;
+static void continue_send_message(grpc_exec_ctx *exec_ctx,
+                                  grpc_call_element *elem);
 
-  /* In streaming calls, we need to reset the previously accumulated slices */
+static void send_done(grpc_exec_ctx *exec_ctx, void *elemp, int success) {
+  grpc_call_element *elem = elemp;
+  call_data *calld = elem->call_data;
   gpr_slice_buffer_reset_and_unref(&calld->slices);
-  for (i = 0; i < send_ops->nops; ++i) {
-    grpc_stream_op *sop = &send_ops->ops[i];
-    switch (sop->type) {
-      case GRPC_OP_BEGIN_MESSAGE:
-        /* buffer up slices until we've processed all the expected ones (as
-         * given by GRPC_OP_BEGIN_MESSAGE) */
-        calld->remaining_slice_bytes = sop->data.begin_message.length;
-        if (sop->data.begin_message.flags & GRPC_WRITE_NO_COMPRESS) {
-          calld->has_compression_algorithm = 1; /* GPR_TRUE */
-          calld->compression_algorithm = GRPC_COMPRESS_NONE;
-        }
-        break;
-      case GRPC_OP_METADATA:
-        if (!calld->written_initial_metadata) {
-          /* Parse incoming request for compression. If any, it'll be available
-           * at calld->compression_algorithm */
-          grpc_metadata_batch_filter(&(sop->data.metadata),
-                                     compression_md_filter, elem);
-          if (!calld->has_compression_algorithm) {
-            /* If no algorithm was found in the metadata and we aren't
-             * exceptionally skipping compression, fall back to the channel
-             * default */
-            calld->compression_algorithm =
-                channeld->default_compression_algorithm;
-            calld->has_compression_algorithm = 1; /* GPR_TRUE */
-          }
-          /* hint compression algorithm */
-          grpc_metadata_batch_add_tail(
-              &(sop->data.metadata), &calld->compression_algorithm_storage,
-              GRPC_MDELEM_REF(channeld->mdelem_compression_algorithms
-                                  [calld->compression_algorithm]));
-
-          /* convey supported compression algorithms */
-          grpc_metadata_batch_add_tail(
-              &(sop->data.metadata), &calld->accept_encoding_storage,
-              GRPC_MDELEM_REF(channeld->mdelem_accept_encoding));
-
-          calld->written_initial_metadata = 1; /* GPR_TRUE */
-        }
-        break;
-      case GRPC_OP_SLICE:
-        if (skip_compression(channeld, calld)) continue;
-        GPR_ASSERT(calld->remaining_slice_bytes > 0);
-        /* Increase input ref count, gpr_slice_buffer_add takes ownership.  */
-        gpr_slice_buffer_add(&calld->slices, gpr_slice_ref(sop->data.slice));
-        GPR_ASSERT(GPR_SLICE_LENGTH(sop->data.slice) <=
-                   calld->remaining_slice_bytes);
-        calld->remaining_slice_bytes -=
-            (gpr_uint32)GPR_SLICE_LENGTH(sop->data.slice);
-        if (calld->remaining_slice_bytes == 0) {
-          did_compress =
-              compress_send_sb(calld->compression_algorithm, &calld->slices);
-        }
-        break;
-      case GRPC_NO_OP:
-        break;
-    }
-  }
+  calld->post_send->cb(exec_ctx, calld->post_send->cb_arg, success);
+}
 
-  /* Modify the send_ops stream_op_buffer depending on whether compression was
-   * carried out */
+static void finish_send_message(grpc_exec_ctx *exec_ctx,
+                                grpc_call_element *elem) {
+  call_data *calld = elem->call_data;
+  int did_compress;
+  gpr_slice_buffer tmp;
+  gpr_slice_buffer_init(&tmp);
+  did_compress =
+      grpc_msg_compress(calld->compression_algorithm, &calld->slices, &tmp);
   if (did_compress) {
-    finish_compressed_sopb(send_ops, elem);
+    gpr_slice_buffer_swap(&calld->slices, &tmp);
+    calld->send_flags |= GRPC_WRITE_INTERNAL_COMPRESS;
+  }
+  gpr_slice_buffer_destroy(&tmp);
+
+  grpc_slice_buffer_stream_init(&calld->replacement_stream, &calld->slices,
+                                calld->send_flags);
+  calld->send_op.send_message = &calld->replacement_stream.base;
+  calld->post_send = calld->send_op.on_complete;
+  calld->send_op.on_complete = &calld->send_done;
+
+  grpc_call_next_op(exec_ctx, elem, &calld->send_op);
+}
+
+static void got_slice(grpc_exec_ctx *exec_ctx, void *elemp, int success) {
+  grpc_call_element *elem = elemp;
+  call_data *calld = elem->call_data;
+  gpr_slice_buffer_add(&calld->slices, calld->incoming_slice);
+  if (calld->send_length == calld->slices.length) {
+    finish_send_message(exec_ctx, elem);
+  } else {
+    continue_send_message(exec_ctx, elem);
+  }
+}
+
+static void continue_send_message(grpc_exec_ctx *exec_ctx,
+                                  grpc_call_element *elem) {
+  call_data *calld = elem->call_data;
+  while (grpc_byte_stream_next(exec_ctx, calld->send_op.send_message,
+                               &calld->incoming_slice, ~(size_t)0,
+                               &calld->got_slice)) {
+    gpr_slice_buffer_add(&calld->slices, calld->incoming_slice);
+    if (calld->send_length == calld->slices.length) {
+      finish_send_message(exec_ctx, elem);
+      break;
+    }
   }
 }
 
-/* Called either:
-     - in response to an API call (or similar) from above, to send something
-     - a network event (or similar) from below, to receive something
-   op contains type and call direction information, in addition to the data
-   that is being sent or received. */
 static void compress_start_transport_stream_op(grpc_exec_ctx *exec_ctx,
                                                grpc_call_element *elem,
                                                grpc_transport_stream_op *op) {
+  call_data *calld = elem->call_data;
+
   GPR_TIMER_BEGIN("compress_start_transport_stream_op", 0);
 
-  if (op->send_ops && op->send_ops->nops > 0) {
-    process_send_ops(elem, op->send_ops);
+  if (op->send_initial_metadata) {
+    process_send_initial_metadata(elem, op->send_initial_metadata);
+  }
+  if (op->send_message != NULL && !skip_compression(elem) &&
+      0 == (op->send_message->flags & GRPC_WRITE_NO_COMPRESS)) {
+    calld->send_op = *op;
+    calld->send_length = op->send_message->length;
+    calld->send_flags = op->send_message->flags;
+    continue_send_message(exec_ctx, elem);
+  } else {
+    /* pass control down the stack */
+    grpc_call_next_op(exec_ctx, elem, op);
   }
 
   GPR_TIMER_END("compress_start_transport_stream_op", 0);
-
-  /* pass control down the stack */
-  grpc_call_next_op(exec_ctx, elem, op);
 }
 
 /* Constructor for call_data */
 static void init_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
-                           const void *server_transport_data,
-                           grpc_transport_stream_op *initial_op) {
+                           grpc_call_element_args *args) {
   /* grab pointers to our data from the call element */
   call_data *calld = elem->call_data;
 
   /* initialize members */
   gpr_slice_buffer_init(&calld->slices);
   calld->has_compression_algorithm = 0;
-  calld->written_initial_metadata = 0; /* GPR_FALSE */
-
-  if (initial_op) {
-    if (initial_op->send_ops && initial_op->send_ops->nops > 0) {
-      process_send_ops(elem, initial_op->send_ops);
-    }
-  }
+  grpc_closure_init(&calld->got_slice, got_slice, elem);
+  grpc_closure_init(&calld->send_done, send_done, elem);
 }
 
 /* Destructor for call_data */
@@ -313,9 +262,8 @@ static void destroy_call_elem(grpc_exec_ctx *exec_ctx,
 
 /* Constructor for channel_data */
 static void init_channel_elem(grpc_exec_ctx *exec_ctx,
-                              grpc_channel_element *elem, grpc_channel *master,
-                              const grpc_channel_args *args, grpc_mdctx *mdctx,
-                              int is_first, int is_last) {
+                              grpc_channel_element *elem,
+                              grpc_channel_element_args *args) {
   channel_data *channeld = elem->channel_data;
   grpc_compression_algorithm algo_idx;
   const char *supported_algorithms_names[GRPC_COMPRESS_ALGORITHMS_COUNT - 1];
@@ -325,24 +273,25 @@ static void init_channel_elem(grpc_exec_ctx *exec_ctx,
 
   grpc_compression_options_init(&channeld->compression_options);
   channeld->compression_options.enabled_algorithms_bitset =
-      (gpr_uint32)grpc_channel_args_compression_algorithm_get_states(args);
+      (gpr_uint32)grpc_channel_args_compression_algorithm_get_states(
+          args->channel_args);
 
   channeld->default_compression_algorithm =
-      grpc_channel_args_get_compression_algorithm(args);
+      grpc_channel_args_get_compression_algorithm(args->channel_args);
   /* Make sure the default isn't disabled. */
   GPR_ASSERT(grpc_compression_options_is_algorithm_enabled(
       &channeld->compression_options, channeld->default_compression_algorithm));
   channeld->compression_options.default_compression_algorithm =
       channeld->default_compression_algorithm;
 
-  channeld->mdstr_request_compression_algorithm_key =
-      grpc_mdstr_from_string(mdctx, GRPC_COMPRESS_REQUEST_ALGORITHM_KEY);
+  channeld->mdstr_request_compression_algorithm_key = grpc_mdstr_from_string(
+      args->metadata_context, GRPC_COMPRESS_REQUEST_ALGORITHM_KEY);
 
   channeld->mdstr_outgoing_compression_algorithm_key =
-      grpc_mdstr_from_string(mdctx, "grpc-encoding");
+      grpc_mdstr_from_string(args->metadata_context, "grpc-encoding");
 
   channeld->mdstr_compression_capabilities_key =
-      grpc_mdstr_from_string(mdctx, "grpc-accept-encoding");
+      grpc_mdstr_from_string(args->metadata_context, "grpc-accept-encoding");
 
   for (algo_idx = 0; algo_idx < GRPC_COMPRESS_ALGORITHMS_COUNT; ++algo_idx) {
     char *algorithm_name;
@@ -354,9 +303,9 @@ static void init_channel_elem(grpc_exec_ctx *exec_ctx,
     GPR_ASSERT(grpc_compression_algorithm_name(algo_idx, &algorithm_name) != 0);
     channeld->mdelem_compression_algorithms[algo_idx] =
         grpc_mdelem_from_metadata_strings(
-            mdctx,
+            args->metadata_context,
             GRPC_MDSTR_REF(channeld->mdstr_outgoing_compression_algorithm_key),
-            grpc_mdstr_from_string(mdctx, algorithm_name));
+            grpc_mdstr_from_string(args->metadata_context, algorithm_name));
     if (algo_idx > 0) {
       supported_algorithms_names[supported_algorithms_idx++] = algorithm_name;
     }
@@ -369,11 +318,12 @@ static void init_channel_elem(grpc_exec_ctx *exec_ctx,
                       &accept_encoding_str_len);
 
   channeld->mdelem_accept_encoding = grpc_mdelem_from_metadata_strings(
-      mdctx, GRPC_MDSTR_REF(channeld->mdstr_compression_capabilities_key),
-      grpc_mdstr_from_string(mdctx, accept_encoding_str));
+      args->metadata_context,
+      GRPC_MDSTR_REF(channeld->mdstr_compression_capabilities_key),
+      grpc_mdstr_from_string(args->metadata_context, accept_encoding_str));
   gpr_free(accept_encoding_str);
 
-  GPR_ASSERT(!is_last);
+  GPR_ASSERT(!args->is_last);
 }
 
 /* Destructor for channel data */
@@ -393,5 +343,6 @@ static void destroy_channel_elem(grpc_exec_ctx *exec_ctx,
 
 const grpc_channel_filter grpc_compress_filter = {
     compress_start_transport_stream_op, grpc_channel_next_op, sizeof(call_data),
-    init_call_elem, destroy_call_elem, sizeof(channel_data), init_channel_elem,
-    destroy_channel_elem, grpc_call_next_get_peer, "compress"};
+    init_call_elem, grpc_call_stack_ignore_set_pollset, destroy_call_elem,
+    sizeof(channel_data), init_channel_elem, destroy_channel_elem,
+    grpc_call_next_get_peer, "compress"};
diff --git a/src/core/channel/connected_channel.c b/src/core/channel/connected_channel.c
index 6d4d7be632..0e1efd965a 100644
--- a/src/core/channel/connected_channel.c
+++ b/src/core/channel/connected_channel.c
@@ -83,8 +83,7 @@ static void con_start_transport_op(grpc_exec_ctx *exec_ctx,
 
 /* Constructor for call_data */
 static void init_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
-                           const void *server_transport_data,
-                           grpc_transport_stream_op *initial_op) {
+                           grpc_call_element_args *args) {
   call_data *calld = elem->call_data;
   channel_data *chand = elem->channel_data;
   int r;
@@ -92,10 +91,18 @@ static void init_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
   GPR_ASSERT(elem->filter == &grpc_connected_channel_filter);
   r = grpc_transport_init_stream(exec_ctx, chand->transport,
                                  TRANSPORT_STREAM_FROM_CALL_DATA(calld),
-                                 server_transport_data, initial_op);
+                                 args->refcount, args->server_transport_data);
   GPR_ASSERT(r == 0);
 }
 
+static void set_pollset(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
+                        grpc_pollset *pollset) {
+  call_data *calld = elem->call_data;
+  channel_data *chand = elem->channel_data;
+  grpc_transport_set_pollset(exec_ctx, chand->transport,
+                             TRANSPORT_STREAM_FROM_CALL_DATA(calld), pollset);
+}
+
 /* Destructor for call_data */
 static void destroy_call_elem(grpc_exec_ctx *exec_ctx,
                               grpc_call_element *elem) {
@@ -108,11 +115,10 @@ static void destroy_call_elem(grpc_exec_ctx *exec_ctx,
 
 /* Constructor for channel_data */
 static void init_channel_elem(grpc_exec_ctx *exec_ctx,
-                              grpc_channel_element *elem, grpc_channel *master,
-                              const grpc_channel_args *args, grpc_mdctx *mdctx,
-                              int is_first, int is_last) {
+                              grpc_channel_element *elem,
+                              grpc_channel_element_args *args) {
   channel_data *cd = (channel_data *)elem->channel_data;
-  GPR_ASSERT(is_last);
+  GPR_ASSERT(args->is_last);
   GPR_ASSERT(elem->filter == &grpc_connected_channel_filter);
   cd->transport = NULL;
 }
@@ -132,8 +138,8 @@ static char *con_get_peer(grpc_exec_ctx *exec_ctx, grpc_call_element *elem) {
 
 const grpc_channel_filter grpc_connected_channel_filter = {
     con_start_transport_stream_op, con_start_transport_op, sizeof(call_data),
-    init_call_elem, destroy_call_elem, sizeof(channel_data), init_channel_elem,
-    destroy_channel_elem, con_get_peer, "connected",
+    init_call_elem, set_pollset, destroy_call_elem, sizeof(channel_data),
+    init_channel_elem, destroy_channel_elem, con_get_peer, "connected",
 };
 
 void grpc_connected_channel_bind_transport(grpc_channel_stack *channel_stack,
@@ -154,3 +160,8 @@ void grpc_connected_channel_bind_transport(grpc_channel_stack *channel_stack,
      channel. */
   channel_stack->call_stack_size += grpc_transport_stream_size(transport);
 }
+
+grpc_stream *grpc_connected_channel_get_stream(grpc_call_element *elem) {
+  call_data *calld = elem->call_data;
+  return TRANSPORT_STREAM_FROM_CALL_DATA(calld);
+}
diff --git a/src/core/channel/connected_channel.h b/src/core/channel/connected_channel.h
index eac6eb7ebe..95c1834bfa 100644
--- a/src/core/channel/connected_channel.h
+++ b/src/core/channel/connected_channel.h
@@ -46,4 +46,6 @@ extern const grpc_channel_filter grpc_connected_channel_filter;
 void grpc_connected_channel_bind_transport(grpc_channel_stack* channel_stack,
                                            grpc_transport* transport);
 
+grpc_stream* grpc_connected_channel_get_stream(grpc_call_element* elem);
+
 #endif /* GRPC_INTERNAL_CORE_CHANNEL_CONNECTED_CHANNEL_H */
diff --git a/src/core/channel/http_client_filter.c b/src/core/channel/http_client_filter.c
index f78a5cc315..3a0f68f30f 100644
--- a/src/core/channel/http_client_filter.c
+++ b/src/core/channel/http_client_filter.c
@@ -45,10 +45,8 @@ typedef struct call_data {
   grpc_linked_mdelem te_trailers;
   grpc_linked_mdelem content_type;
   grpc_linked_mdelem user_agent;
-  int sent_initial_metadata;
 
-  int got_initial_metadata;
-  grpc_stream_op_buffer *recv_ops;
+  grpc_metadata_batch *recv_initial_metadata;
 
   /** Closure to call when finished with the hc_on_recv hook */
   grpc_closure *on_done_recv;
@@ -91,18 +89,11 @@ static grpc_mdelem *client_recv_filter(void *user_data, grpc_mdelem *md) {
 static void hc_on_recv(grpc_exec_ctx *exec_ctx, void *user_data, int success) {
   grpc_call_element *elem = user_data;
   call_data *calld = elem->call_data;
-  size_t i;
-  size_t nops = calld->recv_ops->nops;
-  grpc_stream_op *ops = calld->recv_ops->ops;
-  for (i = 0; i < nops; i++) {
-    grpc_stream_op *op = &ops[i];
-    client_recv_filter_args a;
-    if (op->type != GRPC_OP_METADATA) continue;
-    calld->got_initial_metadata = 1;
-    a.elem = elem;
-    a.exec_ctx = exec_ctx;
-    grpc_metadata_batch_filter(&op->data.metadata, client_recv_filter, &a);
-  }
+  client_recv_filter_args a;
+  a.elem = elem;
+  a.exec_ctx = exec_ctx;
+  grpc_metadata_batch_filter(calld->recv_initial_metadata, client_recv_filter,
+                             &a);
   calld->on_done_recv->cb(exec_ctx, calld->on_done_recv->cb_arg, success);
 }
 
@@ -123,40 +114,29 @@ static void hc_mutate_op(grpc_call_element *elem,
   /* grab pointers to our data from the call element */
   call_data *calld = elem->call_data;
   channel_data *channeld = elem->channel_data;
-  size_t i;
-  if (op->send_ops && !calld->sent_initial_metadata) {
-    size_t nops = op->send_ops->nops;
-    grpc_stream_op *ops = op->send_ops->ops;
-    for (i = 0; i < nops; i++) {
-      grpc_stream_op *stream_op = &ops[i];
-      if (stream_op->type != GRPC_OP_METADATA) continue;
-      calld->sent_initial_metadata = 1;
-      grpc_metadata_batch_filter(&stream_op->data.metadata, client_strip_filter,
-                                 elem);
-      /* Send : prefixed headers, which have to be before any application
-         layer headers. */
-      grpc_metadata_batch_add_head(&stream_op->data.metadata, &calld->method,
-                                   GRPC_MDELEM_REF(channeld->method));
-      grpc_metadata_batch_add_head(&stream_op->data.metadata, &calld->scheme,
-                                   GRPC_MDELEM_REF(channeld->scheme));
-      grpc_metadata_batch_add_tail(&stream_op->data.metadata,
-                                   &calld->te_trailers,
-                                   GRPC_MDELEM_REF(channeld->te_trailers));
-      grpc_metadata_batch_add_tail(&stream_op->data.metadata,
-                                   &calld->content_type,
-                                   GRPC_MDELEM_REF(channeld->content_type));
-      grpc_metadata_batch_add_tail(&stream_op->data.metadata,
-                                   &calld->user_agent,
-                                   GRPC_MDELEM_REF(channeld->user_agent));
-      break;
-    }
+  if (op->send_initial_metadata != NULL) {
+    grpc_metadata_batch_filter(op->send_initial_metadata, client_strip_filter,
+                               elem);
+    /* Send : prefixed headers, which have to be before any application
+       layer headers. */
+    grpc_metadata_batch_add_head(op->send_initial_metadata, &calld->method,
+                                 GRPC_MDELEM_REF(channeld->method));
+    grpc_metadata_batch_add_head(op->send_initial_metadata, &calld->scheme,
+                                 GRPC_MDELEM_REF(channeld->scheme));
+    grpc_metadata_batch_add_tail(op->send_initial_metadata, &calld->te_trailers,
+                                 GRPC_MDELEM_REF(channeld->te_trailers));
+    grpc_metadata_batch_add_tail(op->send_initial_metadata,
+                                 &calld->content_type,
+                                 GRPC_MDELEM_REF(channeld->content_type));
+    grpc_metadata_batch_add_tail(op->send_initial_metadata, &calld->user_agent,
+                                 GRPC_MDELEM_REF(channeld->user_agent));
   }
 
-  if (op->recv_ops && !calld->got_initial_metadata) {
+  if (op->recv_initial_metadata != NULL) {
     /* substitute our callback for the higher callback */
-    calld->recv_ops = op->recv_ops;
-    calld->on_done_recv = op->on_done_recv;
-    op->on_done_recv = &calld->hc_on_recv;
+    calld->recv_initial_metadata = op->recv_initial_metadata;
+    calld->on_done_recv = op->on_complete;
+    op->on_complete = &calld->hc_on_recv;
   }
 }
 
@@ -172,14 +152,10 @@ static void hc_start_transport_op(grpc_exec_ctx *exec_ctx,
 
 /* Constructor for call_data */
 static void init_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
-                           const void *server_transport_data,
-                           grpc_transport_stream_op *initial_op) {
+                           grpc_call_element_args *args) {
   call_data *calld = elem->call_data;
-  calld->sent_initial_metadata = 0;
-  calld->got_initial_metadata = 0;
   calld->on_done_recv = NULL;
   grpc_closure_init(&calld->hc_on_recv, hc_on_recv, elem);
-  if (initial_op) hc_mutate_op(elem, initial_op);
 }
 
 /* Destructor for call_data */
@@ -250,28 +226,31 @@ static grpc_mdstr *user_agent_from_args(grpc_mdctx *mdctx,
 
 /* Constructor for channel_data */
 static void init_channel_elem(grpc_exec_ctx *exec_ctx,
-                              grpc_channel_element *elem, grpc_channel *master,
-                              const grpc_channel_args *channel_args,
-                              grpc_mdctx *mdctx, int is_first, int is_last) {
+                              grpc_channel_element *elem,
+                              grpc_channel_element_args *args) {
   /* grab pointers to our data from the channel element */
   channel_data *channeld = elem->channel_data;
 
   /* The first and the last filters tend to be implemented differently to
      handle the case that there's no 'next' filter to call on the up or down
      path */
-  GPR_ASSERT(!is_last);
+  GPR_ASSERT(!args->is_last);
 
   /* initialize members */
-  channeld->te_trailers = grpc_mdelem_from_strings(mdctx, "te", "trailers");
-  channeld->method = grpc_mdelem_from_strings(mdctx, ":method", "POST");
-  channeld->scheme = grpc_mdelem_from_strings(mdctx, ":scheme",
-                                              scheme_from_args(channel_args));
-  channeld->content_type =
-      grpc_mdelem_from_strings(mdctx, "content-type", "application/grpc");
-  channeld->status = grpc_mdelem_from_strings(mdctx, ":status", "200");
+  channeld->te_trailers =
+      grpc_mdelem_from_strings(args->metadata_context, "te", "trailers");
+  channeld->method =
+      grpc_mdelem_from_strings(args->metadata_context, ":method", "POST");
+  channeld->scheme = grpc_mdelem_from_strings(
+      args->metadata_context, ":scheme", scheme_from_args(args->channel_args));
+  channeld->content_type = grpc_mdelem_from_strings(
+      args->metadata_context, "content-type", "application/grpc");
+  channeld->status =
+      grpc_mdelem_from_strings(args->metadata_context, ":status", "200");
   channeld->user_agent = grpc_mdelem_from_metadata_strings(
-      mdctx, grpc_mdstr_from_string(mdctx, "user-agent"),
-      user_agent_from_args(mdctx, channel_args));
+      args->metadata_context,
+      grpc_mdstr_from_string(args->metadata_context, "user-agent"),
+      user_agent_from_args(args->metadata_context, args->channel_args));
 }
 
 /* Destructor for channel data */
@@ -290,6 +269,6 @@ static void destroy_channel_elem(grpc_exec_ctx *exec_ctx,
 
 const grpc_channel_filter grpc_http_client_filter = {
     hc_start_transport_op, grpc_channel_next_op, sizeof(call_data),
-    init_call_elem,        destroy_call_elem,    sizeof(channel_data),
-    init_channel_elem,     destroy_channel_elem, grpc_call_next_get_peer,
-    "http-client"};
+    init_call_elem, grpc_call_stack_ignore_set_pollset, destroy_call_elem,
+    sizeof(channel_data), init_channel_elem, destroy_channel_elem,
+    grpc_call_next_get_peer, "http-client"};
diff --git a/src/core/channel/http_server_filter.c b/src/core/channel/http_server_filter.c
index 99e5066a4e..2adfe2bb61 100644
--- a/src/core/channel/http_server_filter.c
+++ b/src/core/channel/http_server_filter.c
@@ -39,7 +39,6 @@
 #include "src/core/profiling/timers.h"
 
 typedef struct call_data {
-  gpr_uint8 got_initial_metadata;
   gpr_uint8 seen_path;
   gpr_uint8 seen_post;
   gpr_uint8 sent_status;
@@ -49,7 +48,7 @@ typedef struct call_data {
   grpc_linked_mdelem status;
   grpc_linked_mdelem content_type;
 
-  grpc_stream_op_buffer *recv_ops;
+  grpc_metadata_batch *recv_initial_metadata;
   /** Closure to call when finished with the hs_on_recv hook */
   grpc_closure *on_done_recv;
   /** Receive closures are chained: we inject this closure as the on_done_recv
@@ -154,43 +153,35 @@ static void hs_on_recv(grpc_exec_ctx *exec_ctx, void *user_data, int success) {
   grpc_call_element *elem = user_data;
   call_data *calld = elem->call_data;
   if (success) {
-    size_t i;
-    size_t nops = calld->recv_ops->nops;
-    grpc_stream_op *ops = calld->recv_ops->ops;
-    for (i = 0; i < nops; i++) {
-      grpc_stream_op *op = &ops[i];
-      server_filter_args a;
-      if (op->type != GRPC_OP_METADATA) continue;
-      calld->got_initial_metadata = 1;
-      a.elem = elem;
-      a.exec_ctx = exec_ctx;
-      grpc_metadata_batch_filter(&op->data.metadata, server_filter, &a);
-      /* Have we seen the required http2 transport headers?
-         (:method, :scheme, content-type, with :path and :authority covered
-         at the channel level right now) */
-      if (calld->seen_post && calld->seen_scheme && calld->seen_te_trailers &&
-          calld->seen_path && calld->seen_authority) {
-        /* do nothing */
-      } else {
-        if (!calld->seen_path) {
-          gpr_log(GPR_ERROR, "Missing :path header");
-        }
-        if (!calld->seen_authority) {
-          gpr_log(GPR_ERROR, "Missing :authority header");
-        }
-        if (!calld->seen_post) {
-          gpr_log(GPR_ERROR, "Missing :method header");
-        }
-        if (!calld->seen_scheme) {
-          gpr_log(GPR_ERROR, "Missing :scheme header");
-        }
-        if (!calld->seen_te_trailers) {
-          gpr_log(GPR_ERROR, "Missing te trailers header");
-        }
-        /* Error this call out */
-        success = 0;
-        grpc_call_element_send_cancel(exec_ctx, elem);
+    server_filter_args a;
+    a.elem = elem;
+    a.exec_ctx = exec_ctx;
+    grpc_metadata_batch_filter(calld->recv_initial_metadata, server_filter, &a);
+    /* Have we seen the required http2 transport headers?
+       (:method, :scheme, content-type, with :path and :authority covered
+       at the channel level right now) */
+    if (calld->seen_post && calld->seen_scheme && calld->seen_te_trailers &&
+        calld->seen_path && calld->seen_authority) {
+      /* do nothing */
+    } else {
+      if (!calld->seen_path) {
+        gpr_log(GPR_ERROR, "Missing :path header");
+      }
+      if (!calld->seen_authority) {
+        gpr_log(GPR_ERROR, "Missing :authority header");
+      }
+      if (!calld->seen_post) {
+        gpr_log(GPR_ERROR, "Missing :method header");
       }
+      if (!calld->seen_scheme) {
+        gpr_log(GPR_ERROR, "Missing :scheme header");
+      }
+      if (!calld->seen_te_trailers) {
+        gpr_log(GPR_ERROR, "Missing te trailers header");
+      }
+      /* Error this call out */
+      success = 0;
+      grpc_call_element_send_cancel(exec_ctx, elem);
     }
   }
   calld->on_done_recv->cb(exec_ctx, calld->on_done_recv->cb_arg, success);
@@ -201,29 +192,21 @@ static void hs_mutate_op(grpc_call_element *elem,
   /* grab pointers to our data from the call element */
   call_data *calld = elem->call_data;
   channel_data *channeld = elem->channel_data;
-  size_t i;
 
-  if (op->send_ops && !calld->sent_status) {
-    size_t nops = op->send_ops->nops;
-    grpc_stream_op *ops = op->send_ops->ops;
-    for (i = 0; i < nops; i++) {
-      grpc_stream_op *stream_op = &ops[i];
-      if (stream_op->type != GRPC_OP_METADATA) continue;
-      calld->sent_status = 1;
-      grpc_metadata_batch_add_head(&stream_op->data.metadata, &calld->status,
-                                   GRPC_MDELEM_REF(channeld->status_ok));
-      grpc_metadata_batch_add_tail(&stream_op->data.metadata,
-                                   &calld->content_type,
-                                   GRPC_MDELEM_REF(channeld->content_type));
-      break;
-    }
+  if (op->send_initial_metadata != NULL && !calld->sent_status) {
+    calld->sent_status = 1;
+    grpc_metadata_batch_add_head(op->send_initial_metadata, &calld->status,
+                                 GRPC_MDELEM_REF(channeld->status_ok));
+    grpc_metadata_batch_add_tail(op->send_initial_metadata,
+                                 &calld->content_type,
+                                 GRPC_MDELEM_REF(channeld->content_type));
   }
 
-  if (op->recv_ops && !calld->got_initial_metadata) {
+  if (op->recv_initial_metadata) {
     /* substitute our callback for the higher callback */
-    calld->recv_ops = op->recv_ops;
-    calld->on_done_recv = op->on_done_recv;
-    op->on_done_recv = &calld->hs_on_recv;
+    calld->recv_initial_metadata = op->recv_initial_metadata;
+    calld->on_done_recv = op->on_complete;
+    op->on_complete = &calld->hs_on_recv;
   }
 }
 
@@ -239,14 +222,12 @@ static void hs_start_transport_op(grpc_exec_ctx *exec_ctx,
 
 /* Constructor for call_data */
 static void init_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
-                           const void *server_transport_data,
-                           grpc_transport_stream_op *initial_op) {
+                           grpc_call_element_args *args) {
   /* grab pointers to our data from the call element */
   call_data *calld = elem->call_data;
   /* initialize members */
   memset(calld, 0, sizeof(*calld));
   grpc_closure_init(&calld->hs_on_recv, hs_on_recv, elem);
-  if (initial_op) hs_mutate_op(elem, initial_op);
 }
 
 /* Destructor for call_data */
@@ -255,34 +236,39 @@ static void destroy_call_elem(grpc_exec_ctx *exec_ctx,
 
 /* Constructor for channel_data */
 static void init_channel_elem(grpc_exec_ctx *exec_ctx,
-                              grpc_channel_element *elem, grpc_channel *master,
-                              const grpc_channel_args *args, grpc_mdctx *mdctx,
-                              int is_first, int is_last) {
+                              grpc_channel_element *elem,
+                              grpc_channel_element_args *args) {
   /* grab pointers to our data from the channel element */
   channel_data *channeld = elem->channel_data;
 
   /* The first and the last filters tend to be implemented differently to
      handle the case that there's no 'next' filter to call on the up or down
      path */
-  GPR_ASSERT(!is_first);
-  GPR_ASSERT(!is_last);
+  GPR_ASSERT(!args->is_last);
 
   /* initialize members */
-  channeld->te_trailers = grpc_mdelem_from_strings(mdctx, "te", "trailers");
-  channeld->status_ok = grpc_mdelem_from_strings(mdctx, ":status", "200");
+  channeld->te_trailers =
+      grpc_mdelem_from_strings(args->metadata_context, "te", "trailers");
+  channeld->status_ok =
+      grpc_mdelem_from_strings(args->metadata_context, ":status", "200");
   channeld->status_not_found =
-      grpc_mdelem_from_strings(mdctx, ":status", "404");
-  channeld->method_post = grpc_mdelem_from_strings(mdctx, ":method", "POST");
-  channeld->http_scheme = grpc_mdelem_from_strings(mdctx, ":scheme", "http");
-  channeld->https_scheme = grpc_mdelem_from_strings(mdctx, ":scheme", "https");
-  channeld->grpc_scheme = grpc_mdelem_from_strings(mdctx, ":scheme", "grpc");
-  channeld->path_key = grpc_mdstr_from_string(mdctx, ":path");
-  channeld->authority_key = grpc_mdstr_from_string(mdctx, ":authority");
-  channeld->host_key = grpc_mdstr_from_string(mdctx, "host");
-  channeld->content_type =
-      grpc_mdelem_from_strings(mdctx, "content-type", "application/grpc");
+      grpc_mdelem_from_strings(args->metadata_context, ":status", "404");
+  channeld->method_post =
+      grpc_mdelem_from_strings(args->metadata_context, ":method", "POST");
+  channeld->http_scheme =
+      grpc_mdelem_from_strings(args->metadata_context, ":scheme", "http");
+  channeld->https_scheme =
+      grpc_mdelem_from_strings(args->metadata_context, ":scheme", "https");
+  channeld->grpc_scheme =
+      grpc_mdelem_from_strings(args->metadata_context, ":scheme", "grpc");
+  channeld->path_key = grpc_mdstr_from_string(args->metadata_context, ":path");
+  channeld->authority_key =
+      grpc_mdstr_from_string(args->metadata_context, ":authority");
+  channeld->host_key = grpc_mdstr_from_string(args->metadata_context, "host");
+  channeld->content_type = grpc_mdelem_from_strings(
+      args->metadata_context, "content-type", "application/grpc");
 
-  channeld->mdctx = mdctx;
+  channeld->mdctx = args->metadata_context;
 }
 
 /* Destructor for channel data */
@@ -306,6 +292,6 @@ static void destroy_channel_elem(grpc_exec_ctx *exec_ctx,
 
 const grpc_channel_filter grpc_http_server_filter = {
     hs_start_transport_op, grpc_channel_next_op, sizeof(call_data),
-    init_call_elem,        destroy_call_elem,    sizeof(channel_data),
-    init_channel_elem,     destroy_channel_elem, grpc_call_next_get_peer,
-    "http-server"};
+    init_call_elem, grpc_call_stack_ignore_set_pollset, destroy_call_elem,
+    sizeof(channel_data), init_channel_elem, destroy_channel_elem,
+    grpc_call_next_get_peer, "http-server"};
diff --git a/src/core/channel/noop_filter.c b/src/core/channel/noop_filter.c
index 48f6b1c650..2fbf1c06bb 100644
--- a/src/core/channel/noop_filter.c
+++ b/src/core/channel/noop_filter.c
@@ -73,16 +73,13 @@ static void noop_start_transport_stream_op(grpc_exec_ctx *exec_ctx,
 
 /* Constructor for call_data */
 static void init_call_elem(grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
-                           const void *server_transport_data,
-                           grpc_transport_stream_op *initial_op) {
+                           grpc_call_element_args *args) {
   /* grab pointers to our data from the call element */
   call_data *calld = elem->call_data;
   channel_data *channeld = elem->channel_data;
 
   /* initialize members */
   calld->unused = channeld->unused;
-
-  if (initial_op) noop_mutate_op(elem, initial_op);
 }
 
 /* Destructor for call_data */
@@ -91,17 +88,15 @@ static void destroy_call_elem(grpc_exec_ctx *exec_ctx,
 
 /* Constructor for channel_data */
 static void init_channel_elem(grpc_exec_ctx *exec_ctx,
-                              grpc_channel_element *elem, grpc_channel *master,
-                              const grpc_channel_args *args, grpc_mdctx *mdctx,
-                              int is_first, int is_last) {
+                              grpc_channel_element *elem,
+                              grpc_channel_element_args *args) {
   /* grab pointers to our data from the channel element */
   channel_data *channeld = elem->channel_data;
 
-  /* The first and the last filters tend to be implemented differently to
-     handle the case that there's no 'next' filter to call on the up or down
+  /* The last filter tends to be implemented differently to
+     handle the case that there's no 'next' filter to call on the down
      path */
-  GPR_ASSERT(!is_first);
-  GPR_ASSERT(!is_last);
+  GPR_ASSERT(!args->is_last);
 
   /* initialize members */
   channeld->unused = 0;
@@ -118,5 +113,6 @@ static void destroy_channel_elem(grpc_exec_ctx *exec_ctx,
 
 const grpc_channel_filter grpc_no_op_filter = {
     noop_start_transport_stream_op, grpc_channel_next_op, sizeof(call_data),
-    init_call_elem, destroy_call_elem, sizeof(channel_data), init_channel_elem,
-    destroy_channel_elem, grpc_call_next_get_peer, "no-op"};
+    init_call_elem, grpc_call_stack_ignore_set_pollset, destroy_call_elem,
+    sizeof(channel_data), init_channel_elem, destroy_channel_elem,
+    grpc_call_next_get_peer, "no-op"};
diff --git a/src/core/channel/subchannel_call_holder.c b/src/core/channel/subchannel_call_holder.c
new file mode 100644
index 0000000000..d39a099234
--- /dev/null
+++ b/src/core/channel/subchannel_call_holder.c
@@ -0,0 +1,282 @@
+/*
+ *
+ * Copyright 2015, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include "src/core/channel/subchannel_call_holder.h"
+
+#include <grpc/support/alloc.h>
+
+#include "src/core/profiling/timers.h"
+
+#define GET_CALL(holder) \
+  ((grpc_subchannel_call *)(gpr_atm_acq_load(&(holder)->subchannel_call)))
+
+#define CANCELLED_CALL ((grpc_subchannel_call *)1)
+
+static void subchannel_ready(grpc_exec_ctx *exec_ctx, void *holder,
+                             int success);
+static void call_ready(grpc_exec_ctx *exec_ctx, void *holder, int success);
+static void retry_ops(grpc_exec_ctx *exec_ctx, void *retry_ops_args,
+                      int success);
+
+static void add_waiting_locked(grpc_subchannel_call_holder *holder,
+                               grpc_transport_stream_op *op);
+static void fail_locked(grpc_exec_ctx *exec_ctx,
+                        grpc_subchannel_call_holder *holder);
+static void retry_waiting_locked(grpc_exec_ctx *exec_ctx,
+                                 grpc_subchannel_call_holder *holder);
+
+void grpc_subchannel_call_holder_init(
+    grpc_subchannel_call_holder *holder,
+    grpc_subchannel_call_holder_pick_subchannel pick_subchannel,
+    void *pick_subchannel_arg) {
+  gpr_atm_rel_store(&holder->subchannel_call, 0);
+  holder->pick_subchannel = pick_subchannel;
+  holder->pick_subchannel_arg = pick_subchannel_arg;
+  gpr_mu_init(&holder->mu);
+  holder->subchannel = NULL;
+  holder->waiting_ops = NULL;
+  holder->waiting_ops_count = 0;
+  holder->waiting_ops_capacity = 0;
+  holder->creation_phase = GRPC_SUBCHANNEL_CALL_HOLDER_NOT_CREATING;
+}
+
+void grpc_subchannel_call_holder_destroy(grpc_exec_ctx *exec_ctx,
+                                         grpc_subchannel_call_holder *holder) {
+  grpc_subchannel_call *call = GET_CALL(holder);
+  if (call != NULL && call != CANCELLED_CALL) {
+    GRPC_SUBCHANNEL_CALL_UNREF(exec_ctx, call, "holder");
+  }
+  GPR_ASSERT(holder->creation_phase ==
+             GRPC_SUBCHANNEL_CALL_HOLDER_NOT_CREATING);
+  gpr_mu_destroy(&holder->mu);
+  GPR_ASSERT(holder->waiting_ops_count == 0);
+  gpr_free(holder->waiting_ops);
+}
+
+void grpc_subchannel_call_holder_perform_op(grpc_exec_ctx *exec_ctx,
+                                            grpc_subchannel_call_holder *holder,
+                                            grpc_transport_stream_op *op) {
+  /* try to (atomically) get the call */
+  grpc_subchannel_call *call = GET_CALL(holder);
+  GPR_TIMER_BEGIN("grpc_subchannel_call_holder_perform_op", 0);
+  if (call == CANCELLED_CALL) {
+    grpc_transport_stream_op_finish_with_failure(exec_ctx, op);
+    GPR_TIMER_END("grpc_subchannel_call_holder_perform_op", 0);
+    return;
+  }
+  if (call != NULL) {
+    grpc_subchannel_call_process_op(exec_ctx, call, op);
+    GPR_TIMER_END("grpc_subchannel_call_holder_perform_op", 0);
+    return;
+  }
+  /* we failed; lock and figure out what to do */
+  gpr_mu_lock(&holder->mu);
+retry:
+  /* need to recheck that another thread hasn't set the call */
+  call = GET_CALL(holder);
+  if (call == CANCELLED_CALL) {
+    gpr_mu_unlock(&holder->mu);
+    grpc_transport_stream_op_finish_with_failure(exec_ctx, op);
+    GPR_TIMER_END("grpc_subchannel_call_holder_perform_op", 0);
+    return;
+  }
+  if (call != NULL) {
+    gpr_mu_unlock(&holder->mu);
+    grpc_subchannel_call_process_op(exec_ctx, call, op);
+    GPR_TIMER_END("grpc_subchannel_call_holder_perform_op", 0);
+    return;
+  }
+  /* if this is a cancellation, then we can raise our cancelled flag */
+  if (op->cancel_with_status != GRPC_STATUS_OK) {
+    if (!gpr_atm_rel_cas(&holder->subchannel_call, 0, 1)) {
+      goto retry;
+    } else {
+      switch (holder->creation_phase) {
+        case GRPC_SUBCHANNEL_CALL_HOLDER_NOT_CREATING:
+          break;
+        case GRPC_SUBCHANNEL_CALL_HOLDER_CREATING_CALL:
+          grpc_subchannel_cancel_create_call(exec_ctx, holder->subchannel,
+                                             &holder->subchannel_call);
+          break;
+        case GRPC_SUBCHANNEL_CALL_HOLDER_PICKING_SUBCHANNEL:
+          holder->pick_subchannel(exec_ctx, holder->pick_subchannel_arg, NULL,
+                                  &holder->subchannel, NULL);
+          break;
+      }
+      gpr_mu_unlock(&holder->mu);
+      grpc_transport_stream_op_finish_with_failure(exec_ctx, op);
+      GPR_TIMER_END("grpc_subchannel_call_holder_perform_op", 0);
+      return;
+    }
+  }
+  /* if we don't have a subchannel, try to get one */
+  if (holder->creation_phase == GRPC_SUBCHANNEL_CALL_HOLDER_NOT_CREATING &&
+      holder->subchannel == NULL && op->send_initial_metadata != NULL) {
+    holder->creation_phase = GRPC_SUBCHANNEL_CALL_HOLDER_PICKING_SUBCHANNEL;
+    grpc_closure_init(&holder->next_step, subchannel_ready, holder);
+    if (holder->pick_subchannel(exec_ctx, holder->pick_subchannel_arg,
+                                op->send_initial_metadata, &holder->subchannel,
+                                &holder->next_step)) {
+      holder->creation_phase = GRPC_SUBCHANNEL_CALL_HOLDER_NOT_CREATING;
+    }
+  }
+  /* if we've got a subchannel, then let's ask it to create a call */
+  if (holder->creation_phase == GRPC_SUBCHANNEL_CALL_HOLDER_NOT_CREATING &&
+      holder->subchannel != NULL) {
+    holder->creation_phase = GRPC_SUBCHANNEL_CALL_HOLDER_CREATING_CALL;
+    grpc_closure_init(&holder->next_step, call_ready, holder);
+    if (grpc_subchannel_create_call(exec_ctx, holder->subchannel,
+                                    holder->pollset, &holder->subchannel_call,
+                                    &holder->next_step)) {
+      /* got one immediately - continue the op (and any waiting ops) */
+      holder->creation_phase = GRPC_SUBCHANNEL_CALL_HOLDER_NOT_CREATING;
+      retry_waiting_locked(exec_ctx, holder);
+      goto retry;
+    }
+  }
+  /* nothing to be done but wait */
+  add_waiting_locked(holder, op);
+  gpr_mu_unlock(&holder->mu);
+  GPR_TIMER_END("grpc_subchannel_call_holder_perform_op", 0);
+}
+
+static void subchannel_ready(grpc_exec_ctx *exec_ctx, void *arg, int success) {
+  grpc_subchannel_call_holder *holder = arg;
+  grpc_subchannel_call *call;
+  gpr_mu_lock(&holder->mu);
+  GPR_ASSERT(holder->creation_phase ==
+             GRPC_SUBCHANNEL_CALL_HOLDER_PICKING_SUBCHANNEL);
+  call = GET_CALL(holder);
+  GPR_ASSERT(call == NULL || call == CANCELLED_CALL);
+  if (holder->subchannel == NULL) {
+    holder->creation_phase = GRPC_SUBCHANNEL_CALL_HOLDER_NOT_CREATING;
+    fail_locked(exec_ctx, holder);
+  } else {
+    grpc_closure_init(&holder->next_step, call_ready, holder);
+    if (grpc_subchannel_create_call(exec_ctx, holder->subchannel,
+                                    holder->pollset, &holder->subchannel_call,
+                                    &holder->next_step)) {
+      holder->creation_phase = GRPC_SUBCHANNEL_CALL_HOLDER_NOT_CREATING;
+      /* got one immediately - continue the op (and any waiting ops) */
+      retry_waiting_locked(exec_ctx, holder);
+    }
+  }
+  gpr_mu_unlock(&holder->mu);
+}
+
+static void call_ready(grpc_exec_ctx *exec_ctx, void *arg, int success) {
+  grpc_subchannel_call_holder *holder = arg;
+  GPR_TIMER_BEGIN("call_ready", 0);
+  gpr_mu_lock(&holder->mu);
+  GPR_ASSERT(holder->creation_phase ==
+             GRPC_SUBCHANNEL_CALL_HOLDER_CREATING_CALL);
+  holder->creation_phase = GRPC_SUBCHANNEL_CALL_HOLDER_NOT_CREATING;
+  if (GET_CALL(holder) != NULL) {
+    retry_waiting_locked(exec_ctx, holder);
+  } else {
+    fail_locked(exec_ctx, holder);
+  }
+  gpr_mu_unlock(&holder->mu);
+  GPR_TIMER_END("call_ready", 0);
+}
+
+typedef struct {
+  grpc_transport_stream_op *ops;
+  size_t nops;
+  grpc_subchannel_call *call;
+} retry_ops_args;
+
+static void retry_waiting_locked(grpc_exec_ctx *exec_ctx,
+                                 grpc_subchannel_call_holder *holder) {
+  retry_ops_args *a = gpr_malloc(sizeof(*a));
+  a->ops = holder->waiting_ops;
+  a->nops = holder->waiting_ops_count;
+  a->call = GET_CALL(holder);
+  if (a->call == CANCELLED_CALL) {
+    gpr_free(a);
+    fail_locked(exec_ctx, holder);
+    return;
+  }
+  holder->waiting_ops = NULL;
+  holder->waiting_ops_count = 0;
+  holder->waiting_ops_capacity = 0;
+  GRPC_SUBCHANNEL_CALL_REF(a->call, "retry_ops");
+  grpc_exec_ctx_enqueue(exec_ctx, grpc_closure_create(retry_ops, a), 1);
+}
+
+static void retry_ops(grpc_exec_ctx *exec_ctx, void *args, int success) {
+  retry_ops_args *a = args;
+  size_t i;
+  for (i = 0; i < a->nops; i++) {
+    grpc_subchannel_call_process_op(exec_ctx, a->call, &a->ops[i]);
+  }
+  GRPC_SUBCHANNEL_CALL_UNREF(exec_ctx, a->call, "retry_ops");
+  gpr_free(a->ops);
+  gpr_free(a);
+}
+
+static void add_waiting_locked(grpc_subchannel_call_holder *holder,
+                               grpc_transport_stream_op *op) {
+  GPR_TIMER_BEGIN("add_waiting_locked", 0);
+  if (holder->waiting_ops_count == holder->waiting_ops_capacity) {
+    holder->waiting_ops_capacity = GPR_MAX(3, 2 * holder->waiting_ops_capacity);
+    holder->waiting_ops =
+        gpr_realloc(holder->waiting_ops, holder->waiting_ops_capacity *
+                                             sizeof(*holder->waiting_ops));
+  }
+  holder->waiting_ops[holder->waiting_ops_count++] = *op;
+  GPR_TIMER_END("add_waiting_locked", 0);
+}
+
+static void fail_locked(grpc_exec_ctx *exec_ctx,
+                        grpc_subchannel_call_holder *holder) {
+  size_t i;
+  for (i = 0; i < holder->waiting_ops_count; i++) {
+    grpc_exec_ctx_enqueue(exec_ctx, holder->waiting_ops[i].on_complete, 0);
+    grpc_exec_ctx_enqueue(exec_ctx, holder->waiting_ops[i].recv_message_ready,
+                          0);
+  }
+  holder->waiting_ops_count = 0;
+}
+
+char *grpc_subchannel_call_holder_get_peer(grpc_exec_ctx *exec_ctx,
+                                           grpc_subchannel_call_holder *holder,
+                                           grpc_channel *master) {
+  grpc_subchannel_call *subchannel_call = GET_CALL(holder);
+
+  if (subchannel_call) {
+    return grpc_subchannel_call_get_peer(exec_ctx, subchannel_call);
+  } else {
+    return grpc_channel_get_target(master);
+  }
+}
diff --git a/src/core/channel/subchannel_call_holder.h b/src/core/channel/subchannel_call_holder.h
new file mode 100644
index 0000000000..3dd43c9c3f
--- /dev/null
+++ b/src/core/channel/subchannel_call_holder.h
@@ -0,0 +1,83 @@
+/*
+ *
+ * Copyright 2015, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#ifndef GRPC_INTERNAL_CORE_CHANNEL_SUBCHANNEL_CALL_HOLDER_H
+#define GRPC_INTERNAL_CORE_CHANNEL_SUBCHANNEL_CALL_HOLDER_H
+
+#include "src/core/client_config/subchannel.h"
+
+typedef int (*grpc_subchannel_call_holder_pick_subchannel)(
+    grpc_exec_ctx *exec_ctx, void *arg, grpc_metadata_batch *initial_metadata,
+    grpc_subchannel **subchannel, grpc_closure *on_ready);
+
+typedef enum {
+  GRPC_SUBCHANNEL_CALL_HOLDER_NOT_CREATING,
+  GRPC_SUBCHANNEL_CALL_HOLDER_PICKING_SUBCHANNEL,
+  GRPC_SUBCHANNEL_CALL_HOLDER_CREATING_CALL
+} grpc_subchannel_call_holder_creation_phase;
+
+typedef struct grpc_subchannel_call_holder {
+  /* either 0 for no call, 1 for cancelled, or a pointer to a
+     grpc_subchannel_call */
+  gpr_atm subchannel_call;
+  grpc_subchannel_call_holder_pick_subchannel pick_subchannel;
+  void *pick_subchannel_arg;
+
+  gpr_mu mu;
+
+  grpc_subchannel_call_holder_creation_phase creation_phase;
+  grpc_subchannel *subchannel;
+  grpc_pollset *pollset;
+
+  grpc_transport_stream_op *waiting_ops;
+  size_t waiting_ops_count;
+  size_t waiting_ops_capacity;
+
+  grpc_closure next_step;
+} grpc_subchannel_call_holder;
+
+void grpc_subchannel_call_holder_init(
+    grpc_subchannel_call_holder *holder,
+    grpc_subchannel_call_holder_pick_subchannel pick_subchannel,
+    void *pick_subchannel_arg);
+void grpc_subchannel_call_holder_destroy(grpc_exec_ctx *exec_ctx,
+                                         grpc_subchannel_call_holder *holder);
+
+void grpc_subchannel_call_holder_perform_op(grpc_exec_ctx *exec_ctx,
+                                            grpc_subchannel_call_holder *holder,
+                                            grpc_transport_stream_op *op);
+char *grpc_subchannel_call_holder_get_peer(grpc_exec_ctx *exec_ctx,
+                                           grpc_subchannel_call_holder *holder,
+                                           grpc_channel *master);
+
+#endif
diff --git a/src/core/client_config/lb_policies/pick_first.c b/src/core/client_config/lb_policies/pick_first.c
index e5bf0680ff..93312abb00 100644
--- a/src/core/client_config/lb_policies/pick_first.c
+++ b/src/core/client_config/lb_policies/pick_first.c
@@ -130,6 +130,30 @@ void pf_shutdown(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol) {
   }
 }
 
+static void pf_cancel_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
+                           grpc_subchannel **target) {
+  pick_first_lb_policy *p = (pick_first_lb_policy *)pol;
+  pending_pick *pp;
+  gpr_mu_lock(&p->mu);
+  pp = p->pending_picks;
+  p->pending_picks = NULL;
+  while (pp != NULL) {
+    pending_pick *next = pp->next;
+    if (pp->target == target) {
+      grpc_subchannel_del_interested_party(
+          exec_ctx, p->subchannels[p->checking_subchannel], pp->pollset);
+      *target = NULL;
+      grpc_exec_ctx_enqueue(exec_ctx, pp->on_complete, 0);
+      gpr_free(pp);
+    } else {
+      pp->next = p->pending_picks;
+      p->pending_picks = pp;
+    }
+    pp = next;
+  }
+  gpr_mu_unlock(&p->mu);
+}
+
 static void start_picking(grpc_exec_ctx *exec_ctx, pick_first_lb_policy *p) {
   p->started_picking = 1;
   p->checking_subchannel = 0;
@@ -149,16 +173,16 @@ void pf_exit_idle(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol) {
   gpr_mu_unlock(&p->mu);
 }
 
-void pf_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
-             grpc_pollset *pollset, grpc_metadata_batch *initial_metadata,
-             grpc_subchannel **target, grpc_closure *on_complete) {
+int pf_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol, grpc_pollset *pollset,
+            grpc_metadata_batch *initial_metadata, grpc_subchannel **target,
+            grpc_closure *on_complete) {
   pick_first_lb_policy *p = (pick_first_lb_policy *)pol;
   pending_pick *pp;
   gpr_mu_lock(&p->mu);
   if (p->selected) {
     gpr_mu_unlock(&p->mu);
     *target = p->selected;
-    grpc_exec_ctx_enqueue(exec_ctx, on_complete, 1);
+    return 1;
   } else {
     if (!p->started_picking) {
       start_picking(exec_ctx, p);
@@ -172,6 +196,7 @@ void pf_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
     pp->on_complete = on_complete;
     p->pending_picks = pp;
     gpr_mu_unlock(&p->mu);
+    return 0;
   }
 }
 
@@ -365,8 +390,8 @@ void pf_notify_on_state_change(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
 }
 
 static const grpc_lb_policy_vtable pick_first_lb_policy_vtable = {
-    pf_destroy, pf_shutdown, pf_pick, pf_exit_idle, pf_broadcast,
-    pf_check_connectivity, pf_notify_on_state_change};
+    pf_destroy, pf_shutdown, pf_pick, pf_cancel_pick, pf_exit_idle,
+    pf_broadcast, pf_check_connectivity, pf_notify_on_state_change};
 
 static void pick_first_factory_ref(grpc_lb_policy_factory *factory) {}
 
diff --git a/src/core/client_config/lb_policies/round_robin.c b/src/core/client_config/lb_policies/round_robin.c
index d0b60a0df2..1ffe32fff2 100644
--- a/src/core/client_config/lb_policies/round_robin.c
+++ b/src/core/client_config/lb_policies/round_robin.c
@@ -264,6 +264,33 @@ void rr_shutdown(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol) {
   gpr_mu_unlock(&p->mu);
 }
 
+static void rr_cancel_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
+                           grpc_subchannel **target) {
+  round_robin_lb_policy *p = (round_robin_lb_policy *)pol;
+  pending_pick *pp;
+  size_t i;
+  gpr_mu_lock(&p->mu);
+  pp = p->pending_picks;
+  p->pending_picks = NULL;
+  while (pp != NULL) {
+    pending_pick *next = pp->next;
+    if (pp->target == target) {
+      for (i = 0; i < p->num_subchannels; i++) {
+        grpc_subchannel_add_interested_party(exec_ctx, p->subchannels[i],
+                                             pp->pollset);
+      }
+      *target = NULL;
+      grpc_exec_ctx_enqueue(exec_ctx, pp->on_complete, 0);
+      gpr_free(pp);
+    } else {
+      pp->next = p->pending_picks;
+      p->pending_picks = pp;
+    }
+    pp = next;
+  }
+  gpr_mu_unlock(&p->mu);
+}
+
 static void start_picking(grpc_exec_ctx *exec_ctx, round_robin_lb_policy *p) {
   size_t i;
   p->started_picking = 1;
@@ -286,9 +313,9 @@ void rr_exit_idle(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol) {
   gpr_mu_unlock(&p->mu);
 }
 
-void rr_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
-             grpc_pollset *pollset, grpc_metadata_batch *initial_metadata,
-             grpc_subchannel **target, grpc_closure *on_complete) {
+int rr_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol, grpc_pollset *pollset,
+            grpc_metadata_batch *initial_metadata, grpc_subchannel **target,
+            grpc_closure *on_complete) {
   size_t i;
   round_robin_lb_policy *p = (round_robin_lb_policy *)pol;
   pending_pick *pp;
@@ -303,7 +330,7 @@ void rr_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
     }
     /* only advance the last picked pointer if the selection was used */
     advance_last_picked_locked(p);
-    on_complete->cb(exec_ctx, on_complete->cb_arg, 1);
+    return 1;
   } else {
     if (!p->started_picking) {
       start_picking(exec_ctx, p);
@@ -319,6 +346,7 @@ void rr_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
     pp->on_complete = on_complete;
     p->pending_picks = pp;
     gpr_mu_unlock(&p->mu);
+    return 0;
   }
 }
 
@@ -487,8 +515,8 @@ static void rr_notify_on_state_change(grpc_exec_ctx *exec_ctx,
 }
 
 static const grpc_lb_policy_vtable round_robin_lb_policy_vtable = {
-    rr_destroy, rr_shutdown, rr_pick, rr_exit_idle, rr_broadcast,
-    rr_check_connectivity, rr_notify_on_state_change};
+    rr_destroy, rr_shutdown, rr_pick, rr_cancel_pick, rr_exit_idle,
+    rr_broadcast, rr_check_connectivity, rr_notify_on_state_change};
 
 static void round_robin_factory_ref(grpc_lb_policy_factory *factory) {}
 
diff --git a/src/core/client_config/lb_policy.c b/src/core/client_config/lb_policy.c
index c955186f7f..36a2454309 100644
--- a/src/core/client_config/lb_policy.c
+++ b/src/core/client_config/lb_policy.c
@@ -68,12 +68,17 @@ void grpc_lb_policy_shutdown(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy) {
   policy->vtable->shutdown(exec_ctx, policy);
 }
 
-void grpc_lb_policy_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
-                         grpc_pollset *pollset,
-                         grpc_metadata_batch *initial_metadata,
-                         grpc_subchannel **target, grpc_closure *on_complete) {
-  policy->vtable->pick(exec_ctx, policy, pollset, initial_metadata, target,
-                       on_complete);
+int grpc_lb_policy_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
+                        grpc_pollset *pollset,
+                        grpc_metadata_batch *initial_metadata,
+                        grpc_subchannel **target, grpc_closure *on_complete) {
+  return policy->vtable->pick(exec_ctx, policy, pollset, initial_metadata,
+                              target, on_complete);
+}
+
+void grpc_lb_policy_cancel_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
+                                grpc_subchannel **target) {
+  policy->vtable->cancel_pick(exec_ctx, policy, target);
 }
 
 void grpc_lb_policy_broadcast(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
diff --git a/src/core/client_config/lb_policy.h b/src/core/client_config/lb_policy.h
index 0eefe64991..a696c3ce64 100644
--- a/src/core/client_config/lb_policy.h
+++ b/src/core/client_config/lb_policy.h
@@ -56,9 +56,11 @@ struct grpc_lb_policy_vtable {
   void (*shutdown)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy);
 
   /** implement grpc_lb_policy_pick */
-  void (*pick)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
-               grpc_pollset *pollset, grpc_metadata_batch *initial_metadata,
-               grpc_subchannel **target, grpc_closure *on_complete);
+  int (*pick)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
+              grpc_pollset *pollset, grpc_metadata_batch *initial_metadata,
+              grpc_subchannel **target, grpc_closure *on_complete);
+  void (*cancel_pick)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
+                      grpc_subchannel **target);
 
   /** try to enter a READY connectivity state */
   void (*exit_idle)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy);
@@ -106,10 +108,13 @@ void grpc_lb_policy_shutdown(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy);
     target for this rpc, and 'return' it by calling \a on_complete after setting
     \a target.
     Picking can be asynchronous. Any IO should be done under \a pollset. */
-void grpc_lb_policy_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
-                         grpc_pollset *pollset,
-                         grpc_metadata_batch *initial_metadata,
-                         grpc_subchannel **target, grpc_closure *on_complete);
+int grpc_lb_policy_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
+                        grpc_pollset *pollset,
+                        grpc_metadata_batch *initial_metadata,
+                        grpc_subchannel **target, grpc_closure *on_complete);
+
+void grpc_lb_policy_cancel_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
+                                grpc_subchannel **target);
 
 void grpc_lb_policy_broadcast(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                               grpc_transport_op *op);
diff --git a/src/core/client_config/subchannel.c b/src/core/client_config/subchannel.c
index 0401dd3868..e911a46faf 100644
--- a/src/core/client_config/subchannel.c
+++ b/src/core/client_config/subchannel.c
@@ -22,7 +22,7 @@
  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
- * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMA`S (INCLUDING, BUT NOT
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
@@ -41,8 +41,10 @@
 #include "src/core/channel/client_channel.h"
 #include "src/core/channel/connected_channel.h"
 #include "src/core/iomgr/timer.h"
-#include "src/core/transport/connectivity_state.h"
+#include "src/core/profiling/timers.h"
 #include "src/core/surface/channel.h"
+#include "src/core/transport/connectivity_state.h"
+#include "src/core/transport/connectivity_state.h"
 
 #define GRPC_SUBCHANNEL_MIN_CONNECT_TIMEOUT_SECONDS 20
 #define GRPC_SUBCHANNEL_INITIAL_CONNECT_BACKOFF_SECONDS 1
@@ -69,7 +71,7 @@ typedef struct waiting_for_connect {
   struct waiting_for_connect *next;
   grpc_closure *notify;
   grpc_pollset *pollset;
-  grpc_subchannel_call **target;
+  gpr_atm *target;
   grpc_subchannel *subchannel;
   grpc_closure continuation;
 } waiting_for_connect;
@@ -137,14 +139,16 @@ struct grpc_subchannel {
 
 struct grpc_subchannel_call {
   connection *connection;
-  gpr_refcount refs;
 };
 
 #define SUBCHANNEL_CALL_TO_CALL_STACK(call) ((grpc_call_stack *)((call) + 1))
 #define CHANNEL_STACK_FROM_CONNECTION(con) ((grpc_channel_stack *)((con) + 1))
+#define CALLSTACK_TO_SUBCHANNEL_CALL(callstack) \
+  (((grpc_subchannel_call *)(callstack)) - 1)
 
 static grpc_subchannel_call *create_call(grpc_exec_ctx *exec_ctx,
-                                         connection *con);
+                                         connection *con,
+                                         grpc_pollset *pollset);
 static void connectivity_state_changed_locked(grpc_exec_ctx *exec_ctx,
                                               grpc_subchannel *c,
                                               const char *reason);
@@ -163,7 +167,7 @@ static grpc_subchannel *connection_unref_locked(
     connection *c GRPC_SUBCHANNEL_REF_EXTRA_ARGS) GRPC_MUST_USE_RESULT;
 static void subchannel_destroy(grpc_exec_ctx *exec_ctx, grpc_subchannel *c);
 
-#ifdef GRPC_SUBCHANNEL_REFCOUNT_DEBUG
+#ifdef GRPC_STREAM_REFCOUNT_DEBUG
 #define SUBCHANNEL_REF_LOCKED(p, r) \
   subchannel_ref_locked((p), __FILE__, __LINE__, (r))
 #define SUBCHANNEL_UNREF_LOCKED(p, r) \
@@ -173,6 +177,7 @@ static void subchannel_destroy(grpc_exec_ctx *exec_ctx, grpc_subchannel *c);
 #define CONNECTION_UNREF_LOCKED(cl, p, r) \
   connection_unref_locked((cl), (p), __FILE__, __LINE__, (r))
 #define REF_PASS_ARGS , file, line, reason
+#define REF_PASS_REASON , reason
 #define REF_LOG(name, p)                                                  \
   gpr_log(file, line, GPR_LOG_SEVERITY_DEBUG, "%s: %p   ref %d -> %d %s", \
           (name), (p), (p)->refs, (p)->refs + 1, reason)
@@ -185,6 +190,7 @@ static void subchannel_destroy(grpc_exec_ctx *exec_ctx, grpc_subchannel *c);
 #define CONNECTION_REF_LOCKED(p, r) connection_ref_locked((p))
 #define CONNECTION_UNREF_LOCKED(cl, p, r) connection_unref_locked((cl), (p))
 #define REF_PASS_ARGS
+#define REF_PASS_REASON
 #define REF_LOG(name, p) \
   do {                   \
   } while (0)
@@ -312,9 +318,9 @@ grpc_subchannel *grpc_subchannel_create(grpc_connector *connector,
   return c;
 }
 
-void grpc_subchannel_cancel_waiting_call(grpc_exec_ctx *exec_ctx,
-                                         grpc_subchannel *subchannel,
-                                         int iomgr_success) {
+static void cancel_waiting_calls(grpc_exec_ctx *exec_ctx,
+                                 grpc_subchannel *subchannel,
+                                 int iomgr_success) {
   waiting_for_connect *w4c;
   gpr_mu_lock(&subchannel->mu);
   w4c = subchannel->waiting;
@@ -335,6 +341,37 @@ void grpc_subchannel_cancel_waiting_call(grpc_exec_ctx *exec_ctx,
   }
 }
 
+void grpc_subchannel_cancel_create_call(grpc_exec_ctx *exec_ctx,
+                                        grpc_subchannel *subchannel,
+                                        gpr_atm *target) {
+  waiting_for_connect *w4c;
+  int unref_count = 0;
+  gpr_mu_lock(&subchannel->mu);
+  w4c = subchannel->waiting;
+  subchannel->waiting = NULL;
+  while (w4c != NULL) {
+    waiting_for_connect *next = w4c->next;
+    if (w4c->target == target) {
+      grpc_subchannel_del_interested_party(exec_ctx, w4c->subchannel,
+                                           w4c->pollset);
+      grpc_exec_ctx_enqueue(exec_ctx, w4c->notify, 0);
+
+      unref_count++;
+      gpr_free(w4c);
+    } else {
+      w4c->next = subchannel->waiting;
+      subchannel->waiting = w4c;
+    }
+
+    w4c = next;
+  }
+  gpr_mu_unlock(&subchannel->mu);
+
+  while (unref_count-- > 0) {
+    GRPC_SUBCHANNEL_UNREF(exec_ctx, subchannel, "waiting_for_connect");
+  }
+}
+
 static void continue_connect(grpc_exec_ctx *exec_ctx, grpc_subchannel *c) {
   grpc_connect_in_args args;
 
@@ -358,29 +395,35 @@ static void start_connect(grpc_exec_ctx *exec_ctx, grpc_subchannel *c) {
 
 static void continue_creating_call(grpc_exec_ctx *exec_ctx, void *arg,
                                    int iomgr_success) {
-  grpc_subchannel_call_create_status call_creation_status;
+  int call_creation_status;
   waiting_for_connect *w4c = arg;
   grpc_subchannel_del_interested_party(exec_ctx, w4c->subchannel, w4c->pollset);
   call_creation_status = grpc_subchannel_create_call(
       exec_ctx, w4c->subchannel, w4c->pollset, w4c->target, w4c->notify);
-  GPR_ASSERT(call_creation_status == GRPC_SUBCHANNEL_CALL_CREATE_READY);
+  GPR_ASSERT(call_creation_status == 1);
   w4c->notify->cb(exec_ctx, w4c->notify->cb_arg, iomgr_success);
   GRPC_SUBCHANNEL_UNREF(exec_ctx, w4c->subchannel, "waiting_for_connect");
   gpr_free(w4c);
 }
 
-grpc_subchannel_call_create_status grpc_subchannel_create_call(
-    grpc_exec_ctx *exec_ctx, grpc_subchannel *c, grpc_pollset *pollset,
-    grpc_subchannel_call **target, grpc_closure *notify) {
+int grpc_subchannel_create_call(grpc_exec_ctx *exec_ctx, grpc_subchannel *c,
+                                grpc_pollset *pollset, gpr_atm *target,
+                                grpc_closure *notify) {
   connection *con;
+  grpc_subchannel_call *call;
+  GPR_TIMER_BEGIN("grpc_subchannel_create_call", 0);
   gpr_mu_lock(&c->mu);
   if (c->active != NULL) {
     con = c->active;
     CONNECTION_REF_LOCKED(con, "call");
     gpr_mu_unlock(&c->mu);
 
-    *target = create_call(exec_ctx, con);
-    return GRPC_SUBCHANNEL_CALL_CREATE_READY;
+    call = create_call(exec_ctx, con, pollset);
+    if (!gpr_atm_rel_cas(target, 0, (gpr_atm)(gpr_uintptr)call)) {
+      GRPC_SUBCHANNEL_CALL_UNREF(exec_ctx, call, "failed to set");
+    }
+    GPR_TIMER_END("grpc_subchannel_create_call", 0);
+    return 1;
   } else {
     waiting_for_connect *w4c = gpr_malloc(sizeof(*w4c));
     w4c->next = c->waiting;
@@ -405,7 +448,8 @@ grpc_subchannel_call_create_status grpc_subchannel_create_call(
     } else {
       gpr_mu_unlock(&c->mu);
     }
-    return GRPC_SUBCHANNEL_CALL_CREATE_PENDING;
+    GPR_TIMER_END("grpc_subchannel_create_call", 0);
+    return 0;
   }
 }
 
@@ -687,7 +731,7 @@ static void on_alarm(grpc_exec_ctx *exec_ctx, void *arg, int iomgr_success) {
     update_reconnect_parameters(c);
     continue_connect(exec_ctx, c);
   } else {
-    grpc_subchannel_cancel_waiting_call(exec_ctx, c, iomgr_success);
+    cancel_waiting_calls(exec_ctx, c, iomgr_success);
     GRPC_CHANNEL_INTERNAL_UNREF(exec_ctx, c->master, "connecting");
     GRPC_SUBCHANNEL_UNREF(exec_ctx, c, "connecting");
   }
@@ -747,26 +791,40 @@ static void connectivity_state_changed_locked(grpc_exec_ctx *exec_ctx,
  * grpc_subchannel_call implementation
  */
 
+static void subchannel_call_destroy(grpc_exec_ctx *exec_ctx, void *call,
+                                    int success) {
+  grpc_subchannel_call *c = call;
+  gpr_mu *mu = &c->connection->subchannel->mu;
+  grpc_subchannel *destroy;
+  GPR_TIMER_BEGIN("grpc_subchannel_call_unref.destroy", 0);
+  grpc_call_stack_destroy(exec_ctx, SUBCHANNEL_CALL_TO_CALL_STACK(c));
+  gpr_mu_lock(mu);
+  destroy = CONNECTION_UNREF_LOCKED(exec_ctx, c->connection, "call");
+  gpr_mu_unlock(mu);
+  gpr_free(c);
+  if (destroy != NULL) {
+    subchannel_destroy(exec_ctx, destroy);
+  }
+  GPR_TIMER_END("grpc_subchannel_call_unref.destroy", 0);
+}
+
 void grpc_subchannel_call_ref(grpc_subchannel_call *c
                                   GRPC_SUBCHANNEL_REF_EXTRA_ARGS) {
-  gpr_ref(&c->refs);
+#ifdef GRPC_STREAM_REFCOUNT_DEBUG
+  grpc_call_stack_ref(SUBCHANNEL_CALL_TO_CALL_STACK(c), reason);
+#else
+  grpc_call_stack_ref(SUBCHANNEL_CALL_TO_CALL_STACK(c));
+#endif
 }
 
 void grpc_subchannel_call_unref(grpc_exec_ctx *exec_ctx,
                                 grpc_subchannel_call *c
                                     GRPC_SUBCHANNEL_REF_EXTRA_ARGS) {
-  if (gpr_unref(&c->refs)) {
-    gpr_mu *mu = &c->connection->subchannel->mu;
-    grpc_subchannel *destroy;
-    grpc_call_stack_destroy(exec_ctx, SUBCHANNEL_CALL_TO_CALL_STACK(c));
-    gpr_mu_lock(mu);
-    destroy = CONNECTION_UNREF_LOCKED(exec_ctx, c->connection, "call");
-    gpr_mu_unlock(mu);
-    gpr_free(c);
-    if (destroy != NULL) {
-      subchannel_destroy(exec_ctx, destroy);
-    }
-  }
+#ifdef GRPC_STREAM_REFCOUNT_DEBUG
+  grpc_call_stack_unref(exec_ctx, SUBCHANNEL_CALL_TO_CALL_STACK(c), reason);
+#else
+  grpc_call_stack_unref(exec_ctx, SUBCHANNEL_CALL_TO_CALL_STACK(c));
+#endif
 }
 
 char *grpc_subchannel_call_get_peer(grpc_exec_ctx *exec_ctx,
@@ -785,14 +843,16 @@ void grpc_subchannel_call_process_op(grpc_exec_ctx *exec_ctx,
 }
 
 static grpc_subchannel_call *create_call(grpc_exec_ctx *exec_ctx,
-                                         connection *con) {
+                                         connection *con,
+                                         grpc_pollset *pollset) {
   grpc_channel_stack *chanstk = CHANNEL_STACK_FROM_CONNECTION(con);
   grpc_subchannel_call *call =
       gpr_malloc(sizeof(grpc_subchannel_call) + chanstk->call_stack_size);
   grpc_call_stack *callstk = SUBCHANNEL_CALL_TO_CALL_STACK(call);
   call->connection = con;
-  gpr_ref_init(&call->refs, 1);
-  grpc_call_stack_init(exec_ctx, chanstk, NULL, NULL, callstk);
+  grpc_call_stack_init(exec_ctx, chanstk, 1, subchannel_call_destroy, call,
+                       NULL, NULL, callstk);
+  grpc_call_stack_set_pollset(exec_ctx, callstk, pollset);
   return call;
 }
 
@@ -803,3 +863,8 @@ grpc_mdctx *grpc_subchannel_get_mdctx(grpc_subchannel *subchannel) {
 grpc_channel *grpc_subchannel_get_master(grpc_subchannel *subchannel) {
   return subchannel->master;
 }
+
+grpc_call_stack *grpc_subchannel_call_get_call_stack(
+    grpc_subchannel_call *subchannel_call) {
+  return SUBCHANNEL_CALL_TO_CALL_STACK(subchannel_call);
+}
diff --git a/src/core/client_config/subchannel.h b/src/core/client_config/subchannel.h
index ec1cc7cc69..4769a6de68 100644
--- a/src/core/client_config/subchannel.h
+++ b/src/core/client_config/subchannel.h
@@ -44,7 +44,7 @@ typedef struct grpc_subchannel grpc_subchannel;
 typedef struct grpc_subchannel_call grpc_subchannel_call;
 typedef struct grpc_subchannel_args grpc_subchannel_args;
 
-#ifdef GRPC_SUBCHANNEL_REFCOUNT_DEBUG
+#ifdef GRPC_STREAM_REFCOUNT_DEBUG
 #define GRPC_SUBCHANNEL_REF(p, r) \
   grpc_subchannel_ref((p), __FILE__, __LINE__, (r))
 #define GRPC_SUBCHANNEL_UNREF(cl, p, r) \
@@ -75,11 +75,6 @@ void grpc_subchannel_call_unref(grpc_exec_ctx *exec_ctx,
                                 grpc_subchannel_call *call
                                     GRPC_SUBCHANNEL_REF_EXTRA_ARGS);
 
-typedef enum {
-  GRPC_SUBCHANNEL_CALL_CREATE_READY,
-  GRPC_SUBCHANNEL_CALL_CREATE_PENDING
-} grpc_subchannel_call_create_status;
-
 /** construct a subchannel call (possibly asynchronously).
  *
  * If the returned status is \a GRPC_SUBCHANNEL_CALL_CREATE_READY, the call will
@@ -88,14 +83,15 @@ typedef enum {
  * Otherwise, if the returned status is GRPC_SUBCHANNEL_CALL_CREATE_PENDING, the
  * subchannel call will be created asynchronously, invoking the \a notify
  * callback upon completion. */
-grpc_subchannel_call_create_status grpc_subchannel_create_call(
-    grpc_exec_ctx *exec_ctx, grpc_subchannel *subchannel, grpc_pollset *pollset,
-    grpc_subchannel_call **target, grpc_closure *notify);
+int grpc_subchannel_create_call(grpc_exec_ctx *exec_ctx,
+                                grpc_subchannel *subchannel,
+                                grpc_pollset *pollset, gpr_atm *target,
+                                grpc_closure *notify);
 
 /** cancel \a call in the waiting state. */
-void grpc_subchannel_cancel_waiting_call(grpc_exec_ctx *exec_ctx,
-                                         grpc_subchannel *subchannel,
-                                         int iomgr_success);
+void grpc_subchannel_cancel_create_call(grpc_exec_ctx *exec_ctx,
+                                        grpc_subchannel *subchannel,
+                                        gpr_atm *target);
 
 /** process a transport level op */
 void grpc_subchannel_process_transport_op(grpc_exec_ctx *exec_ctx,
@@ -138,6 +134,9 @@ void grpc_subchannel_call_process_op(grpc_exec_ctx *exec_ctx,
 char *grpc_subchannel_call_get_peer(grpc_exec_ctx *exec_ctx,
                                     grpc_subchannel_call *subchannel_call);
 
+grpc_call_stack *grpc_subchannel_call_get_call_stack(
+    grpc_subchannel_call *subchannel_call);
+
 struct grpc_subchannel_args {
   /** Channel filters for this channel - wrapped factories will likely
       want to mutate this */