Add client channel connectivity watcher

6 files changed, 449 insertions, 156 deletions

diff --git a/src/core/ext/filters/client_channel/channel_connectivity.cc b/src/core/ext/filters/client_channel/channel_connectivity.cc
index 31a8fc39ce..33e9deaaf6 100644
--- a/src/core/ext/filters/client_channel/channel_connectivity.cc
+++ b/src/core/ext/filters/client_channel/channel_connectivity.cc
@@ -23,8 +23,8 @@
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
 
+#include "src/core/ext/filters/client_channel/channel_connectivity_internal.h"
 #include "src/core/ext/filters/client_channel/client_channel.h"
-#include "src/core/lib/iomgr/timer.h"
 #include "src/core/lib/surface/api_trace.h"
 #include "src/core/lib/surface/completion_queue.h"
 
@@ -52,125 +52,6 @@ grpc_connectivity_state grpc_channel_check_connectivity_state(
   return GRPC_CHANNEL_SHUTDOWN;
 }
 
-typedef enum {
-  WAITING,
-  READY_TO_CALL_BACK,
-  CALLING_BACK_AND_FINISHED,
-} callback_phase;
-
-typedef struct {
-  gpr_mu mu;
-  callback_phase phase;
-  grpc_closure on_complete;
-  grpc_closure on_timeout;
-  grpc_closure watcher_timer_init;
-  grpc_timer alarm;
-  grpc_connectivity_state state;
-  grpc_completion_queue *cq;
-  grpc_cq_completion completion_storage;
-  grpc_channel *channel;
-  grpc_error *error;
-  void *tag;
-} state_watcher;
-
-static void delete_state_watcher(grpc_exec_ctx *exec_ctx, state_watcher *w) {
-  grpc_channel_element *client_channel_elem = grpc_channel_stack_last_element(
-      grpc_channel_get_channel_stack(w->channel));
-  if (client_channel_elem->filter == &grpc_client_channel_filter) {
-    GRPC_CHANNEL_INTERNAL_UNREF(exec_ctx, w->channel,
-                                "watch_channel_connectivity");
-  } else {
-    abort();
-  }
-  gpr_mu_destroy(&w->mu);
-  gpr_free(w);
-}
-
-static void finished_completion(grpc_exec_ctx *exec_ctx, void *pw,
-                                grpc_cq_completion *ignored) {
-  bool should_delete = false;
-  state_watcher *w = (state_watcher *)pw;
-  gpr_mu_lock(&w->mu);
-  switch (w->phase) {
-    case WAITING:
-    case READY_TO_CALL_BACK:
-      GPR_UNREACHABLE_CODE(return );
-    case CALLING_BACK_AND_FINISHED:
-      should_delete = true;
-      break;
-  }
-  gpr_mu_unlock(&w->mu);
-
-  if (should_delete) {
-    delete_state_watcher(exec_ctx, w);
-  }
-}
-
-static void partly_done(grpc_exec_ctx *exec_ctx, state_watcher *w,
-                        bool due_to_completion, grpc_error *error) {
-  if (due_to_completion) {
-    grpc_timer_cancel(exec_ctx, &w->alarm);
-  } else {
-    grpc_channel_element *client_channel_elem = grpc_channel_stack_last_element(
-        grpc_channel_get_channel_stack(w->channel));
-    grpc_client_channel_watch_connectivity_state(
-        exec_ctx, client_channel_elem,
-        grpc_polling_entity_create_from_pollset(grpc_cq_pollset(w->cq)), NULL,
-        &w->on_complete, NULL);
-  }
-
-  gpr_mu_lock(&w->mu);
-
-  if (due_to_completion) {
-    if (GRPC_TRACER_ON(grpc_trace_operation_failures)) {
-      GRPC_LOG_IF_ERROR("watch_completion_error", GRPC_ERROR_REF(error));
-    }
-    GRPC_ERROR_UNREF(error);
-    error = GRPC_ERROR_NONE;
-  } else {
-    if (error == GRPC_ERROR_NONE) {
-      error = GRPC_ERROR_CREATE_FROM_STATIC_STRING(
-          "Timed out waiting for connection state change");
-    } else if (error == GRPC_ERROR_CANCELLED) {
-      error = GRPC_ERROR_NONE;
-    }
-  }
-  switch (w->phase) {
-    case WAITING:
-      GRPC_ERROR_REF(error);
-      w->error = error;
-      w->phase = READY_TO_CALL_BACK;
-      break;
-    case READY_TO_CALL_BACK:
-      if (error != GRPC_ERROR_NONE) {
-        GPR_ASSERT(!due_to_completion);
-        GRPC_ERROR_UNREF(w->error);
-        GRPC_ERROR_REF(error);
-        w->error = error;
-      }
-      w->phase = CALLING_BACK_AND_FINISHED;
-      grpc_cq_end_op(exec_ctx, w->cq, w->tag, w->error, finished_completion, w,
-                     &w->completion_storage);
-      break;
-    case CALLING_BACK_AND_FINISHED:
-      GPR_UNREACHABLE_CODE(return );
-      break;
-  }
-  gpr_mu_unlock(&w->mu);
-
-  GRPC_ERROR_UNREF(error);
-}
-
-static void watch_complete(grpc_exec_ctx *exec_ctx, void *pw,
-                           grpc_error *error) {
-  partly_done(exec_ctx, (state_watcher *)pw, true, GRPC_ERROR_REF(error));
-}
-
-static void timeout_complete(grpc_exec_ctx *exec_ctx, void *pw,
-                             grpc_error *error) {
-  partly_done(exec_ctx, (state_watcher *)pw, false, GRPC_ERROR_REF(error));
-}
-
 int grpc_channel_num_external_connectivity_watchers(grpc_channel *channel) {
   grpc_channel_element *client_channel_elem =
       grpc_channel_stack_last_element(grpc_channel_get_channel_stack(channel));
@@ -202,10 +83,10 @@ int grpc_channel_support_connectivity_watcher(grpc_channel *channel) {
 void grpc_channel_watch_connectivity_state(
     grpc_channel *channel, grpc_connectivity_state last_observed_state,
     gpr_timespec deadline, grpc_completion_queue *cq, void *tag) {
+  grpc_channel_stack *channel_stack = grpc_channel_get_channel_stack(channel);
   grpc_channel_element *client_channel_elem =
-      grpc_channel_stack_last_element(grpc_channel_get_channel_stack(channel));
+      grpc_channel_stack_last_element(channel_stack);
   grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT;
-  state_watcher *w = (state_watcher *)gpr_malloc(sizeof(*w));
 
   GRPC_API_TRACE(
       "grpc_channel_watch_connectivity_state("
@@ -213,39 +94,11 @@ void grpc_channel_watch_connectivity_state(
       "deadline=gpr_timespec { tv_sec: %" PRId64
       ", tv_nsec: %d, clock_type: %d }, "
       "cq=%p, tag=%p)",
-      7, (channel, (int)last_observed_state, deadline.tv_sec, deadline.tv_nsec,
-          (int)deadline.clock_type, cq, tag));
-
-  GPR_ASSERT(grpc_cq_begin_op(cq, tag));
-
-  gpr_mu_init(&w->mu);
-  GRPC_CLOSURE_INIT(&w->on_complete, watch_complete, w,
-                    grpc_schedule_on_exec_ctx);
-  GRPC_CLOSURE_INIT(&w->on_timeout, timeout_complete, w,
-                    grpc_schedule_on_exec_ctx);
-  w->phase = WAITING;
-  w->state = last_observed_state;
-  w->cq = cq;
-  w->tag = tag;
-  w->channel = channel;
-  w->error = NULL;
-
-  watcher_timer_init_arg *wa =
-      (watcher_timer_init_arg *)gpr_malloc(sizeof(watcher_timer_init_arg));
-  wa->w = w;
-  wa->deadline = deadline;
-  GRPC_CLOSURE_INIT(&w->watcher_timer_init, watcher_timer_init, wa,
-                    grpc_schedule_on_exec_ctx);
-
-  if (client_channel_elem->filter == &grpc_client_channel_filter) {
-    GRPC_CHANNEL_INTERNAL_REF(channel, "watch_channel_connectivity");
-    grpc_client_channel_watch_connectivity_state(
-        &exec_ctx, client_channel_elem,
-        grpc_polling_entity_create_from_pollset(grpc_cq_pollset(cq)), &w->state,
-        &w->on_complete, &w->watcher_timer_init);
-  } else {
-    abort();
-  }
-
+      7,
+      (channel, (int)last_observed_state, deadline.tv_sec, deadline.tv_nsec,
+       (int)deadline.clock_type, cq, tag));
+  grpc_channel_watch_connectivity_state_internal(
+      &exec_ctx, client_channel_elem, channel_stack, last_observed_state,
+      deadline, cq, tag);
   grpc_exec_ctx_finish(&exec_ctx);
 }
diff --git a/src/core/ext/filters/client_channel/channel_connectivity_internal.cc b/src/core/ext/filters/client_channel/channel_connectivity_internal.cc
new file mode 100644
index 0000000000..06bcfe2ffb
--- /dev/null
+++ b/src/core/ext/filters/client_channel/channel_connectivity_internal.cc
@@ -0,0 +1,195 @@
+/*
+ *
+ * Copyright 2015 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#include "src/core/ext/filters/client_channel/channel_connectivity_internal.h"
+
+#include <grpc/support/alloc.h>
+#include <grpc/support/log.h>
+
+#include "src/core/ext/filters/client_channel/client_channel.h"
+#include "src/core/lib/iomgr/timer.h"
+#include "src/core/lib/surface/api_trace.h"
+#include "src/core/lib/surface/completion_queue.h"
+
+typedef enum {
+  WAITING,
+  READY_TO_CALL_BACK,
+  CALLING_BACK_AND_FINISHED,
+} callback_phase;
+
+typedef struct {
+  gpr_mu mu;
+  callback_phase phase;
+  grpc_closure on_complete;
+  grpc_closure on_timeout;
+  grpc_closure watcher_timer_init;
+  grpc_timer alarm;
+  grpc_connectivity_state state;
+  grpc_completion_queue *cq;
+  grpc_cq_completion completion_storage;
+  grpc_channel_element *client_channel_elem;
+  grpc_channel_stack *channel_stack;
+  grpc_error *error;
+  void *tag;
+} state_watcher;
+
+static void delete_state_watcher(grpc_exec_ctx *exec_ctx, state_watcher *w) {
+  GRPC_CHANNEL_STACK_UNREF(exec_ctx, w->channel_stack,
+                           "watch_channel_connectivity");
+  gpr_mu_destroy(&w->mu);
+  gpr_free(w);
+}
+
+static void finished_completion(grpc_exec_ctx *exec_ctx, void *pw,
+                                grpc_cq_completion *ignored) {
+  bool should_delete = false;
+  state_watcher *w = (state_watcher *)pw;
+  gpr_mu_lock(&w->mu);
+  switch (w->phase) {
+    case WAITING:
+    case READY_TO_CALL_BACK:
+      GPR_UNREACHABLE_CODE(return );
+    case CALLING_BACK_AND_FINISHED:
+      should_delete = true;
+      break;
+  }
+  gpr_mu_unlock(&w->mu);
+
+  if (should_delete) {
+    delete_state_watcher(exec_ctx, w);
+  }
+}
+
+static void partly_done(grpc_exec_ctx *exec_ctx, state_watcher *w,
+                        bool due_to_completion, grpc_error *error) {
+  if (due_to_completion) {
+    grpc_timer_cancel(exec_ctx, &w->alarm);
+  } else {
+    grpc_channel_element *client_channel_elem = w->client_channel_elem;
+    grpc_client_channel_watch_connectivity_state(
+        exec_ctx, client_channel_elem,
+        grpc_polling_entity_create_from_pollset(grpc_cq_pollset(w->cq)), NULL,
+        &w->on_complete, NULL);
+  }
+
+  gpr_mu_lock(&w->mu);
+
+  if (due_to_completion) {
+    if (GRPC_TRACER_ON(grpc_trace_operation_failures)) {
+      GRPC_LOG_IF_ERROR("watch_completion_error", GRPC_ERROR_REF(error));
+    }
+    GRPC_ERROR_UNREF(error);
+    error = GRPC_ERROR_NONE;
+  } else {
+    if (error == GRPC_ERROR_NONE) {
+      error = GRPC_ERROR_CREATE_FROM_STATIC_STRING(
+          "Timed out waiting for connection state change");
+    } else if (error == GRPC_ERROR_CANCELLED) {
+      error = GRPC_ERROR_NONE;
+    }
+  }
+  switch (w->phase) {
+    case WAITING:
+      GRPC_ERROR_REF(error);
+      w->error = error;
+      w->phase = READY_TO_CALL_BACK;
+      break;
+    case READY_TO_CALL_BACK:
+      if (error != GRPC_ERROR_NONE) {
+        GPR_ASSERT(!due_to_completion);
+        GRPC_ERROR_UNREF(w->error);
+        GRPC_ERROR_REF(error);
+        w->error = error;
+      }
+      w->phase = CALLING_BACK_AND_FINISHED;
+      grpc_cq_end_op(exec_ctx, w->cq, w->tag, w->error, finished_completion, w,
+                     &w->completion_storage);
+      break;
+    case CALLING_BACK_AND_FINISHED:
+      GPR_UNREACHABLE_CODE(return );
+      break;
+  }
+  gpr_mu_unlock(&w->mu);
+
+  GRPC_ERROR_UNREF(error);
+}
+
+static void watch_complete(grpc_exec_ctx *exec_ctx, void *pw,
+                           grpc_error *error) {
+  partly_done(exec_ctx, (state_watcher *)pw, true, GRPC_ERROR_REF(error));
+}
+
+static void timeout_complete(grpc_exec_ctx *exec_ctx, void *pw,
+                             grpc_error *error) {
+  partly_done(exec_ctx, (state_watcher *)pw, false, GRPC_ERROR_REF(error));
+}
+
+typedef struct watcher_timer_init_arg {
+  state_watcher *w;
+  gpr_timespec deadline;
+} watcher_timer_init_arg;
+
+static void watcher_timer_init(grpc_exec_ctx *exec_ctx, void *arg,
+                               grpc_error *error_ignored) {
+  watcher_timer_init_arg *wa = (watcher_timer_init_arg *)arg;
+
+  grpc_timer_init(exec_ctx, &wa->w->alarm,
+                  gpr_convert_clock_type(wa->deadline, GPR_CLOCK_MONOTONIC),
+                  &wa->w->on_timeout, gpr_now(GPR_CLOCK_MONOTONIC));
+  gpr_free(wa);
+}
+
+void grpc_channel_watch_connectivity_state_internal(
+    grpc_exec_ctx *exec_ctx, grpc_channel_element *client_channel_elem,
+    grpc_channel_stack *channel_stack,
+    grpc_connectivity_state last_observed_state, gpr_timespec deadline,
+    grpc_completion_queue *cq, void *tag) {
+  state_watcher *w = (state_watcher *)gpr_malloc(sizeof(*w));
+
+  GPR_ASSERT(grpc_cq_begin_op(cq, tag));
+
+  gpr_mu_init(&w->mu);
+  GRPC_CLOSURE_INIT(&w->on_complete, watch_complete, w,
+                    grpc_schedule_on_exec_ctx);
+  GRPC_CLOSURE_INIT(&w->on_timeout, timeout_complete, w,
+                    grpc_schedule_on_exec_ctx);
+  w->phase = WAITING;
+  w->state = last_observed_state;
+  w->cq = cq;
+  w->tag = tag;
+  w->client_channel_elem = client_channel_elem;
+  w->channel_stack = channel_stack;
+  w->error = NULL;
+
+  watcher_timer_init_arg *wa =
+      (watcher_timer_init_arg *)gpr_malloc(sizeof(watcher_timer_init_arg));
+  wa->w = w;
+  wa->deadline = deadline;
+  GRPC_CLOSURE_INIT(&w->watcher_timer_init, watcher_timer_init, wa,
+                    grpc_schedule_on_exec_ctx);
+
+  if (client_channel_elem->filter == &grpc_client_channel_filter) {
+    GRPC_CHANNEL_STACK_REF(channel_stack, "watch_channel_connectivity");
+    grpc_client_channel_watch_connectivity_state(
+        exec_ctx, client_channel_elem,
+        grpc_polling_entity_create_from_pollset(grpc_cq_pollset(cq)), &w->state,
+        &w->on_complete, &w->watcher_timer_init);
+  } else {
+    abort();
+  }
+}
diff --git a/src/core/ext/filters/client_channel/channel_connectivity_internal.h b/src/core/ext/filters/client_channel/channel_connectivity_internal.h
new file mode 100644
index 0000000000..d260a20c07
--- /dev/null
+++ b/src/core/ext/filters/client_channel/channel_connectivity_internal.h
@@ -0,0 +1,33 @@
+/*
+ *
+ * Copyright 2015 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#ifndef GRPC_CORE_EXT_FILTERS_CLIENT_CHANNEL_CHANNEL_CONNECTIVITY_INTERNAL_H
+#define GRPC_CORE_EXT_FILTERS_CLIENT_CHANNEL_CHANNEL_CONNECTIVITY_INTERNAL_H
+
+#include <grpc/grpc.h>
+#include "src/core/lib/channel/channel_stack.h"
+#include "src/core/lib/iomgr/exec_ctx.h"
+
+void grpc_channel_watch_connectivity_state_internal(
+    grpc_exec_ctx *exec_ctx, grpc_channel_element *client_channel_elem,
+    grpc_channel_stack *channel_stack,
+    grpc_connectivity_state last_observed_state, gpr_timespec deadline,
+    grpc_completion_queue *cq, void *tag);
+
+#endif /* GRPC_CORE_EXT_FILTERS_CLIENT_CHANNEL_CHANNEL_CONNECTIVITY_INTERNAL_H \
+        */
diff --git a/src/core/ext/filters/client_channel/client_channel.cc b/src/core/ext/filters/client_channel/client_channel.cc
index ea5e076c3b..be8ea81a02 100644
--- a/src/core/ext/filters/client_channel/client_channel.cc
+++ b/src/core/ext/filters/client_channel/client_channel.cc
@@ -31,6 +31,7 @@
 #include <grpc/support/sync.h>
 #include <grpc/support/useful.h>
 
+#include "src/core/ext/filters/client_channel/connectivity_watcher.h"
 #include "src/core/ext/filters/client_channel/http_connect_handshaker.h"
 #include "src/core/ext/filters/client_channel/lb_policy_registry.h"
 #include "src/core/ext/filters/client_channel/proxy_mapper_registry.h"
@@ -753,6 +754,8 @@ static grpc_error *cc_init_channel_elem(grpc_exec_ctx *exec_ctx,
   }
   chand->deadline_checking_enabled =
       grpc_deadline_checking_enabled(args->channel_args);
+  grpc_client_channel_start_watching_connectivity(exec_ctx, elem,
+                                                  chand->owning_stack);
   return GRPC_ERROR_NONE;
 }
 
diff --git a/src/core/ext/filters/client_channel/connectivity_watcher.c b/src/core/ext/filters/client_channel/connectivity_watcher.c
new file mode 100644
index 0000000000..da45929f26
--- /dev/null
+++ b/src/core/ext/filters/client_channel/connectivity_watcher.c
@@ -0,0 +1,179 @@
+/*
+ *
+ * Copyright 2015 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#include "src/core/ext/filters/client_channel/connectivity_watcher.h"
+
+#include <grpc/grpc.h>
+#include <grpc/support/alloc.h>
+#include <grpc/support/log.h>
+#include <grpc/support/sync.h>
+#include "src/core/ext/filters/client_channel/channel_connectivity_internal.h"
+#include "src/core/ext/filters/client_channel/client_channel.h"
+#include "src/core/lib/iomgr/timer.h"
+#include "src/core/lib/support/env.h"
+#include "src/core/lib/support/string.h"
+#include "src/core/lib/surface/channel.h"
+#include "src/core/lib/surface/completion_queue.h"
+
+#define DEFAULT_CONNECTIVITY_CHECK_INTERVAL_MS 500
+
+typedef struct connectivity_watcher {
+  grpc_timer watcher_timer;
+  grpc_closure check_connectivity_closure;
+  grpc_completion_queue* cq;
+  gpr_refcount refs;
+  size_t channel_count;
+  bool shutting_down;
+} connectivity_watcher;
+
+typedef struct channel_state {
+  grpc_channel_element* client_channel_elem;
+  grpc_channel_stack* channel_stack;
+  grpc_connectivity_state state;
+} channel_state;
+
+static gpr_once g_once = GPR_ONCE_INIT;
+static gpr_mu g_watcher_mu;
+static connectivity_watcher* g_watcher = NULL;
+
+static void init_g_watcher_mu() { gpr_mu_init(&g_watcher_mu); }
+
+static void start_watching_locked(grpc_exec_ctx* exec_ctx,
+                                  grpc_channel_element* client_channel_elem,
+                                  grpc_channel_stack* channel_stack) {
+  gpr_ref(&g_watcher->refs);
+  ++g_watcher->channel_count;
+  channel_state* s = (channel_state*)gpr_zalloc(sizeof(channel_state));
+  s->client_channel_elem = client_channel_elem;
+  s->channel_stack = channel_stack;
+  s->state = GRPC_CHANNEL_IDLE;
+  grpc_channel_watch_connectivity_state_internal(
+      exec_ctx, client_channel_elem, channel_stack, s->state,
+      gpr_inf_future(GPR_CLOCK_MONOTONIC), g_watcher->cq, (void*)s);
+}
+
+static bool is_disabled() {
+  char* env = gpr_getenv("GRPC_DISABLE_CHANNEL_CONNECTIVITY_WATCHER");
+  bool disabled = gpr_is_true(env);
+  gpr_free(env);
+  return disabled;
+}
+
+static bool connectivity_watcher_unref(grpc_exec_ctx* exec_ctx) {
+  if (gpr_unref(&g_watcher->refs)) {
+    gpr_mu_lock(&g_watcher_mu);
+    grpc_completion_queue_destroy(g_watcher->cq);
+    gpr_free(g_watcher);
+    g_watcher = NULL;
+    gpr_mu_unlock(&g_watcher_mu);
+    return true;
+  }
+  return false;
+}
+
+static void check_connectivity_state(grpc_exec_ctx* exec_ctx, void* ignored,
+                                     grpc_error* error) {
+  grpc_event ev;
+  while (true) {
+    gpr_mu_lock(&g_watcher_mu);
+    if (g_watcher->shutting_down) {
+      // Drain cq if the watcher is shutting down
+      ev = grpc_completion_queue_next(
+          g_watcher->cq, gpr_inf_future(GPR_CLOCK_MONOTONIC), NULL);
+    } else {
+      ev = grpc_completion_queue_next(g_watcher->cq,
+                                      gpr_inf_past(GPR_CLOCK_MONOTONIC), NULL);
+      // Make sure we've seen 2 TIMEOUTs before going to sleep
+      if (ev.type == GRPC_QUEUE_TIMEOUT) {
+        ev = grpc_completion_queue_next(
+            g_watcher->cq, gpr_inf_past(GPR_CLOCK_MONOTONIC), NULL);
+        if (ev.type == GRPC_QUEUE_TIMEOUT) {
+          gpr_timespec now = gpr_now(GPR_CLOCK_MONOTONIC);
+          grpc_timer_init(
+              exec_ctx, &g_watcher->watcher_timer,
+              gpr_time_add(now, gpr_time_from_millis(
+                                    DEFAULT_CONNECTIVITY_CHECK_INTERVAL_MS,
+                                    GPR_TIMESPAN)),
+              &g_watcher->check_connectivity_closure, now);
+          gpr_mu_unlock(&g_watcher_mu);
+          break;
+        }
+      }
+    }
+    gpr_mu_unlock(&g_watcher_mu);
+    if (ev.type != GRPC_OP_COMPLETE) {
+      break;
+    }
+    channel_state* s = (channel_state*)(ev.tag);
+    s->state = grpc_client_channel_check_connectivity_state(
+        exec_ctx, s->client_channel_elem, false /* try_to_connect */);
+    if (s->state == GRPC_CHANNEL_SHUTDOWN) {
+      GRPC_CHANNEL_STACK_UNREF(exec_ctx, s->channel_stack,
+                               "connectivity_watcher_stop_watching");
+      gpr_free(s);
+      if (connectivity_watcher_unref(exec_ctx)) {
+        break;
+      }
+    } else {
+      grpc_channel_watch_connectivity_state_internal(
+          exec_ctx, s->client_channel_elem, s->channel_stack, s->state,
+          gpr_inf_future(GPR_CLOCK_MONOTONIC), g_watcher->cq, s);
+    }
+  }
+}
+
+void grpc_client_channel_start_watching_connectivity(
+    grpc_exec_ctx* exec_ctx, grpc_channel_element* client_channel_elem,
+    grpc_channel_stack* channel_stack) {
+  if (is_disabled()) return;
+  GRPC_CHANNEL_STACK_REF(channel_stack, "connectivity_watcher_start_watching");
+  gpr_once_init(&g_once, init_g_watcher_mu);
+  gpr_mu_lock(&g_watcher_mu);
+  if (g_watcher == NULL) {
+    g_watcher = (connectivity_watcher*)gpr_zalloc(sizeof(connectivity_watcher));
+    g_watcher->cq = grpc_completion_queue_create_internal(
+        GRPC_CQ_NEXT, GRPC_CQ_DEFAULT_POLLING);
+    gpr_ref_init(&g_watcher->refs, 0);
+    GRPC_CLOSURE_INIT(&g_watcher->check_connectivity_closure,
+                      check_connectivity_state, NULL,
+                      grpc_schedule_on_exec_ctx);
+    gpr_timespec now = gpr_now(GPR_CLOCK_MONOTONIC);
+    grpc_timer_init(
+        exec_ctx, &g_watcher->watcher_timer,
+        gpr_time_add(
+            now, gpr_time_from_millis(DEFAULT_CONNECTIVITY_CHECK_INTERVAL_MS,
+                                      GPR_TIMESPAN)),
+        &g_watcher->check_connectivity_closure, now);
+  }
+  start_watching_locked(exec_ctx, client_channel_elem, channel_stack);
+  gpr_mu_init(&g_watcher_mu);
+}
+
+void grpc_client_channel_stop_watching_connectivity(
+    grpc_exec_ctx* exec_ctx, grpc_channel_element* client_channel_elem,
+    grpc_channel_stack* channel_stack) {
+  if (is_disabled()) return;
+  gpr_once_init(&g_once, init_g_watcher_mu);
+  gpr_mu_lock(&g_watcher_mu);
+  if (--g_watcher->channel_count == 0) {
+    g_watcher->shutting_down = true;
+    grpc_timer_cancel(exec_ctx, &g_watcher->watcher_timer);
+    connectivity_watcher_unref(exec_ctx);
+  }
+  gpr_mu_unlock(&g_watcher_mu);
+}
diff --git a/src/core/ext/filters/client_channel/connectivity_watcher.h b/src/core/ext/filters/client_channel/connectivity_watcher.h
new file mode 100644
index 0000000000..89586dc736
--- /dev/null
+++ b/src/core/ext/filters/client_channel/connectivity_watcher.h
@@ -0,0 +1,30 @@
+/*
+ *
+ * Copyright 2015 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#ifndef GRPC_CORE_EXT_FILTERS_CLIENT_CHANNEL_CONNECTIVITY_WATCHER_H
+#define GRPC_CORE_EXT_FILTERS_CLIENT_CHANNEL_CONNECTIVITY_WATCHER_H
+
+#include <grpc/grpc.h>
+#include "src/core/lib/channel/channel_stack.h"
+#include "src/core/lib/iomgr/exec_ctx.h"
+
+void grpc_client_channel_start_watching_connectivity(
+    grpc_exec_ctx* exec_ctx, grpc_channel_element* client_channel_elem,
+    grpc_channel_stack* channel_stack);
+
+#endif /* GRPC_CORE_EXT_FILTERS_CLIENT_CHANNEL_CONNECTIVITY_WATCHER_H */