Add channelz subchannel support

13 files changed, 354 insertions, 103 deletions

diff --git a/include/grpc/grpc.h b/include/grpc/grpc.h
index f0eb2c0121..942c83bcde 100644
--- a/include/grpc/grpc.h
+++ b/include/grpc/grpc.h
@@ -477,6 +477,10 @@ GRPCAPI char* grpc_channelz_get_top_channels(intptr_t start_channel_id);
    is allocated and must be freed by the application. */
 GRPCAPI char* grpc_channelz_get_channel(intptr_t channel_id);
 
+/* Returns a single Subchannel, or else a NOT_FOUND code. The returned string
+   is allocated and must be freed by the application. */
+GRPCAPI char* grpc_channelz_get_subchannel(intptr_t subchannel_id);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/src/core/ext/filters/client_channel/client_channel_channelz.cc b/src/core/ext/filters/client_channel/client_channel_channelz.cc
index d43e9ea67a..5a0eec5d15 100644
--- a/src/core/ext/filters/client_channel/client_channel_channelz.cc
+++ b/src/core/ext/filters/client_channel/client_channel_channelz.cc
@@ -24,6 +24,8 @@
 #include "src/core/lib/surface/channel.h"
 #include "src/core/lib/transport/connectivity_state.h"
 
+#include <grpc/support/string_util.h>
+
 namespace grpc_core {
 namespace channelz {
 namespace {
@@ -109,5 +111,32 @@ RefCountedPtr<ChannelNode> ClientChannelNode::MakeClientChannelNode(
       channel, channel_tracer_max_nodes, is_top_level_channel);
 }
 
+ClientChannelSubchannelNode::ClientChannelSubchannelNode(
+    size_t channel_tracer_max_nodes, grpc_subchannel* subchannel)
+    : SubchannelNode(channel_tracer_max_nodes), subchannel_(subchannel) {
+  target_ =
+      UniquePtr<char>(gpr_strdup(grpc_subchannel_get_target(subchannel_)));
+}
+
+void ClientChannelSubchannelNode::PopulateTarget(grpc_json* json) {
+  GPR_ASSERT(target_.get() != nullptr);
+  grpc_json_create_child(nullptr, json, "target", target_.get(),
+                         GRPC_JSON_STRING, false);
+}
+
+void ClientChannelSubchannelNode::PopulateConnectivityState(grpc_json* json) {
+  grpc_connectivity_state state;
+  if (subchannel_ == nullptr) {
+    state = GRPC_CHANNEL_SHUTDOWN;
+  } else {
+    state = grpc_subchannel_check_connectivity(subchannel_, nullptr);
+  }
+  json = grpc_json_create_child(nullptr, json, "state", nullptr,
+                                GRPC_JSON_OBJECT, false);
+  grpc_json_create_child(nullptr, json, "state",
+                         grpc_connectivity_state_name(state), GRPC_JSON_STRING,
+                         false);
+}
+
 }  // namespace channelz
 }  // namespace grpc_core
diff --git a/src/core/ext/filters/client_channel/client_channel_channelz.h b/src/core/ext/filters/client_channel/client_channel_channelz.h
index 6f27b5c8b7..b1bd8db89e 100644
--- a/src/core/ext/filters/client_channel/client_channel_channelz.h
+++ b/src/core/ext/filters/client_channel/client_channel_channelz.h
@@ -26,6 +26,8 @@
 #include "src/core/lib/channel/channelz.h"
 #include "src/core/lib/gprpp/inlined_vector.h"
 
+typedef struct grpc_subchannel grpc_subchannel;
+
 namespace grpc_core {
 
 // TODO(ncteisen), this only contains the uuids of the children for now,
@@ -55,16 +57,45 @@ class ClientChannelNode : public ChannelNode {
   static grpc_arg CreateChannelArg();
 
  protected:
-  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_DELETE
-  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_NEW
   ClientChannelNode(grpc_channel* channel, size_t channel_tracer_max_nodes,
                     bool is_top_level_channel);
   virtual ~ClientChannelNode() {}
 
  private:
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_DELETE
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_NEW
   grpc_channel_element* client_channel_;
 };
 
+// Subtype of SubchannelNode that overrides and provides client_channel
+// specific functionality like querying for connectivity_state and
+// subchannel target.
+class ClientChannelSubchannelNode : public SubchannelNode {
+ public:
+  ClientChannelSubchannelNode(size_t channel_tracer_max_nodes,
+                              grpc_subchannel* subchannel);
+  ~ClientChannelSubchannelNode() override {}
+
+  // Override this functionality since subchannels have a notion of
+  // channel connectivity.
+  void PopulateConnectivityState(grpc_json* json) override;
+
+  // Override this functionality since client_channels subchannels hold
+  // their own target.
+  void PopulateTarget(grpc_json* json) override;
+
+  void MarkSubchannelDestroyed() {
+    GPR_ASSERT(subchannel_ != nullptr);
+    subchannel_ = nullptr;
+  }
+
+ private:
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_DELETE
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_NEW
+  grpc_subchannel* subchannel_;
+  UniquePtr<char> target_;
+};
+
 }  // namespace channelz
 }  // namespace grpc_core
 
diff --git a/src/core/ext/filters/client_channel/subchannel.cc b/src/core/ext/filters/client_channel/subchannel.cc
index 93df2aff70..0c3313c5de 100644
--- a/src/core/ext/filters/client_channel/subchannel.cc
+++ b/src/core/ext/filters/client_channel/subchannel.cc
@@ -135,7 +135,7 @@ struct grpc_subchannel {
   /** our alarm */
   grpc_timer alarm;
 
-  grpc_core::RefCountedPtr<grpc_core::channelz::SubchannelNode>
+  grpc_core::RefCountedPtr<grpc_core::channelz::ClientChannelSubchannelNode>
       channelz_subchannel;
 };
 
@@ -181,7 +181,13 @@ static void connection_destroy(void* arg, grpc_error* error) {
 
 static void subchannel_destroy(void* arg, grpc_error* error) {
   grpc_subchannel* c = static_cast<grpc_subchannel*>(arg);
-  c->channelz_subchannel.reset();
+  if (c->channelz_subchannel != nullptr) {
+    c->channelz_subchannel->trace()->AddTraceEvent(
+        grpc_core::channelz::ChannelTrace::Severity::Info,
+        grpc_slice_from_static_string("Subchannel destroyed"));
+    c->channelz_subchannel->MarkSubchannelDestroyed();
+    c->channelz_subchannel.reset();
+  }
   gpr_free((void*)c->filters);
   grpc_channel_args_destroy(c->args);
   grpc_connectivity_state_destroy(&c->state_tracker);
@@ -381,9 +387,18 @@ grpc_subchannel* grpc_subchannel_create(grpc_connector* connector,
   const grpc_arg* arg =
       grpc_channel_args_find(c->args, GRPC_ARG_ENABLE_CHANNELZ);
   bool channelz_enabled = grpc_channel_arg_get_bool(arg, false);
+  arg = grpc_channel_args_find(c->args,
+                               GRPC_ARG_MAX_CHANNEL_TRACE_EVENTS_PER_NODE);
+  const grpc_integer_options options = {0, 0, INT_MAX};
+  size_t channel_tracer_max_nodes =
+      (size_t)grpc_channel_arg_get_integer(arg, options);
   if (channelz_enabled) {
-    c->channelz_subchannel =
-        grpc_core::MakeRefCounted<grpc_core::channelz::SubchannelNode>();
+    c->channelz_subchannel = grpc_core::MakeRefCounted<
+        grpc_core::channelz::ClientChannelSubchannelNode>(
+        channel_tracer_max_nodes, c);
+    c->channelz_subchannel->trace()->AddTraceEvent(
+        grpc_core::channelz::ChannelTrace::Severity::Info,
+        grpc_slice_from_static_string("Subchannel created"));
   }
 
   return grpc_subchannel_index_register(key, c);
@@ -757,6 +772,14 @@ void grpc_get_subchannel_address_arg(const grpc_channel_args* args,
   }
 }
 
+const char* grpc_subchannel_get_target(grpc_subchannel* subchannel) {
+  const grpc_arg* addr_arg =
+      grpc_channel_args_find(subchannel->args, GRPC_ARG_SUBCHANNEL_ADDRESS);
+  const char* addr_str = grpc_channel_arg_get_string(addr_arg);
+  GPR_ASSERT(addr_str != nullptr);  // Should have been set by LB policy.
+  return addr_str;
+}
+
 const char* grpc_get_subchannel_address_uri_arg(const grpc_channel_args* args) {
   const grpc_arg* addr_arg =
       grpc_channel_args_find(args, GRPC_ARG_SUBCHANNEL_ADDRESS);
diff --git a/src/core/ext/filters/client_channel/subchannel.h b/src/core/ext/filters/client_channel/subchannel.h
index 9e53f7d542..dd3a2d9621 100644
--- a/src/core/ext/filters/client_channel/subchannel.h
+++ b/src/core/ext/filters/client_channel/subchannel.h
@@ -177,6 +177,8 @@ grpc_subchannel* grpc_subchannel_create(grpc_connector* connector,
 void grpc_get_subchannel_address_arg(const grpc_channel_args* args,
                                      grpc_resolved_address* addr);
 
+const char* grpc_subchannel_get_target(grpc_subchannel* subchannel);
+
 /// Returns the URI string for the address to connect to.
 const char* grpc_get_subchannel_address_uri_arg(const grpc_channel_args* args);
 
diff --git a/src/core/lib/channel/channel_trace.cc b/src/core/lib/channel/channel_trace.cc
index b3443310ac..dd6fdeed54 100644
--- a/src/core/lib/channel/channel_trace.cc
+++ b/src/core/lib/channel/channel_trace.cc
@@ -178,24 +178,26 @@ grpc_json* ChannelTrace::RenderJson() const {
   if (!max_list_size_)
     return nullptr;  // tracing is disabled if max_events == 0
   grpc_json* json = grpc_json_create(GRPC_JSON_OBJECT);
-  char* num_events_logged_str;
-  gpr_asprintf(&num_events_logged_str, "%" PRId64, num_events_logged_);
   grpc_json* json_iterator = nullptr;
-  json_iterator =
-      grpc_json_create_child(json_iterator, json, "numEventsLogged",
-                             num_events_logged_str, GRPC_JSON_STRING, true);
+  if (num_events_logged_ > 0) {
+    json_iterator = grpc_json_add_number_string_child(
+        json, json_iterator, "numEventsLogged", num_events_logged_);
+  }
   json_iterator = grpc_json_create_child(
       json_iterator, json, "creationTimestamp",
       gpr_format_timespec(time_created_), GRPC_JSON_STRING, true);
-  grpc_json* events = grpc_json_create_child(json_iterator, json, "events",
-                                             nullptr, GRPC_JSON_ARRAY, false);
-  json_iterator = nullptr;
-  TraceEvent* it = head_trace_;
-  while (it != nullptr) {
-    json_iterator = grpc_json_create_child(json_iterator, events, nullptr,
-                                           nullptr, GRPC_JSON_OBJECT, false);
-    it->RenderTraceEvent(json_iterator);
-    it = it->next();
+  // only add in the event list if it is non-empty.
+  if (num_events_logged_ > 0) {
+    grpc_json* events = grpc_json_create_child(json_iterator, json, "events",
+                                               nullptr, GRPC_JSON_ARRAY, false);
+    json_iterator = nullptr;
+    TraceEvent* it = head_trace_;
+    while (it != nullptr) {
+      json_iterator = grpc_json_create_child(json_iterator, events, nullptr,
+                                             nullptr, GRPC_JSON_OBJECT, false);
+      it->RenderTraceEvent(json_iterator);
+      it = it->next();
+    }
   }
   return json;
 }
diff --git a/src/core/lib/channel/channelz.cc b/src/core/lib/channel/channelz.cc
index 9d6002ed8a..7fe086da3c 100644
--- a/src/core/lib/channel/channelz.cc
+++ b/src/core/lib/channel/channelz.cc
@@ -41,69 +41,33 @@
 namespace grpc_core {
 namespace channelz {
 
-ChannelNode::ChannelNode(grpc_channel* channel, size_t channel_tracer_max_nodes,
-                         bool is_top_level_channel)
-    : channel_(channel),
-      target_(nullptr),
-      channel_uuid_(-1),
-      is_top_level_channel_(is_top_level_channel) {
+CallCountingBase::CallCountingBase(size_t channel_tracer_max_nodes) {
   trace_.Init(channel_tracer_max_nodes);
-  target_ = UniquePtr<char>(grpc_channel_get_target(channel_));
-  channel_uuid_ = ChannelzRegistry::RegisterChannelNode(this);
   gpr_atm_no_barrier_store(&last_call_started_millis_,
                            (gpr_atm)ExecCtx::Get()->Now());
 }
 
-ChannelNode::~ChannelNode() {
-  trace_.Destroy();
-  ChannelzRegistry::UnregisterChannelNode(channel_uuid_);
-}
+CallCountingBase::~CallCountingBase() { trace_.Destroy(); }
 
-void ChannelNode::RecordCallStarted() {
+void CallCountingBase::RecordCallStarted() {
   gpr_atm_no_barrier_fetch_add(&calls_started_, (gpr_atm)1);
   gpr_atm_no_barrier_store(&last_call_started_millis_,
                            (gpr_atm)ExecCtx::Get()->Now());
 }
 
-void ChannelNode::PopulateConnectivityState(grpc_json* json) {}
-
-void ChannelNode::PopulateChildRefs(grpc_json* json) {}
-
-grpc_json* ChannelNode::RenderJson() {
-  // We need to track these three json objects to build our object
-  grpc_json* top_level_json = grpc_json_create(GRPC_JSON_OBJECT);
-  grpc_json* json = top_level_json;
-  grpc_json* json_iterator = nullptr;
-  // create and fill the ref child
-  json_iterator = grpc_json_create_child(json_iterator, json, "ref", nullptr,
-                                         GRPC_JSON_OBJECT, false);
-  json = json_iterator;
-  json_iterator = nullptr;
-  json_iterator = grpc_json_add_number_string_child(json, json_iterator,
-                                                    "channelId", channel_uuid_);
-  // reset json iterators to top level object
-  json = top_level_json;
-  json_iterator = nullptr;
-  // create and fill the data child.
-  grpc_json* data = grpc_json_create_child(json_iterator, json, "data", nullptr,
-                                           GRPC_JSON_OBJECT, false);
-  json = data;
-  json_iterator = nullptr;
-  PopulateConnectivityState(json);
-  GPR_ASSERT(target_.get() != nullptr);
-  json_iterator = grpc_json_create_child(
-      json_iterator, json, "target", target_.get(), GRPC_JSON_STRING, false);
+void CallCountingBase::PopulateTrace(grpc_json* json) {
   // fill in the channel trace if applicable
-  grpc_json* trace = trace_->RenderJson();
-  if (trace != nullptr) {
+  grpc_json* trace_json = trace_->RenderJson();
+  if (trace_json != nullptr) {
     // we manually link up and fill the child since it was created for us in
     // ChannelTrace::RenderJson
-    trace->key = "trace";  // this object is named trace in channelz.proto
-    json_iterator = grpc_json_link_child(json, trace, json_iterator);
+    trace_json->key = "trace";  // this object is named trace in channelz.proto
+    grpc_json_link_child(json, trace_json, nullptr);
   }
-  // reset the parent to be the data object.
-  json = data;
-  json_iterator = nullptr;
+}
+
+void CallCountingBase::PopulateCallData(grpc_json* json) {
+  grpc_json* json_iterator = nullptr;
   if (calls_started_ != 0) {
     json_iterator = grpc_json_add_number_string_child(
         json, json_iterator, "callsStarted", calls_started_);
@@ -121,19 +85,62 @@ grpc_json* ChannelNode::RenderJson() {
   json_iterator =
       grpc_json_create_child(json_iterator, json, "lastCallStartedTimestamp",
                              gpr_format_timespec(ts), GRPC_JSON_STRING, true);
-  json = top_level_json;
-  json_iterator = nullptr;
-  PopulateChildRefs(json);
-  return top_level_json;
 }
 
-char* ChannelNode::RenderJsonString() {
+char* CallCountingBase::RenderJsonString() {
   grpc_json* json = RenderJson();
   char* json_str = grpc_json_dump_to_string(json, 0);
   grpc_json_destroy(json);
   return json_str;
 }
 
+ChannelNode::ChannelNode(grpc_channel* channel, size_t channel_tracer_max_nodes,
+                         bool is_top_level_channel)
+    : CallCountingBase(channel_tracer_max_nodes),
+      channel_(channel),
+      is_top_level_channel_(is_top_level_channel) {
+  target_ = UniquePtr<char>(grpc_channel_get_target(channel_));
+  channel_uuid_ = ChannelzRegistry::RegisterChannelNode(this);
+}
+
+ChannelNode::~ChannelNode() {
+  ChannelzRegistry::UnregisterChannelNode(channel_uuid_);
+}
+
+void ChannelNode::PopulateTarget(grpc_json* json) {
+  GPR_ASSERT(target_.get() != nullptr);
+  grpc_json_create_child(nullptr, json, "target", target_.get(),
+                         GRPC_JSON_STRING, false);
+}
+
+grpc_json* ChannelNode::RenderJson() {
+  // We need to track these three json objects to build our object
+  grpc_json* top_level_json = grpc_json_create(GRPC_JSON_OBJECT);
+  grpc_json* json = top_level_json;
+  grpc_json* json_iterator = nullptr;
+  // create and fill the ref child
+  json_iterator = grpc_json_create_child(json_iterator, json, "ref", nullptr,
+                                         GRPC_JSON_OBJECT, false);
+  json = json_iterator;
+  json_iterator = nullptr;
+  json_iterator = grpc_json_add_number_string_child(json, json_iterator,
+                                                    "channelId", channel_uuid_);
+  // reset json iterators to top level object
+  json = top_level_json;
+  json_iterator = nullptr;
+  // create and fill the data child.
+  grpc_json* data = grpc_json_create_child(json_iterator, json, "data", nullptr,
+                                           GRPC_JSON_OBJECT, false);
+  json = data;
+  json_iterator = nullptr;
+  PopulateConnectivityState(json);
+  PopulateTarget(json);
+  PopulateTrace(json);
+  PopulateCallData(json);
+  PopulateChildRefs(json);
+  return top_level_json;
+}
+
 RefCountedPtr<ChannelNode> ChannelNode::MakeChannelNode(
     grpc_channel* channel, size_t channel_tracer_max_nodes,
     bool is_top_level_channel) {
@@ -141,7 +148,8 @@ RefCountedPtr<ChannelNode> ChannelNode::MakeChannelNode(
       channel, channel_tracer_max_nodes, is_top_level_channel);
 }
 
-SubchannelNode::SubchannelNode() {
+SubchannelNode::SubchannelNode(size_t channel_tracer_max_nodes)
+    : CallCountingBase(channel_tracer_max_nodes) {
   subchannel_uuid_ = ChannelzRegistry::RegisterSubchannelNode(this);
 }
 
@@ -149,5 +157,31 @@ SubchannelNode::~SubchannelNode() {
   ChannelzRegistry::UnregisterSubchannelNode(subchannel_uuid_);
 }
 
+grpc_json* SubchannelNode::RenderJson() {
+  grpc_json* top_level_json = grpc_json_create(GRPC_JSON_OBJECT);
+  grpc_json* json = top_level_json;
+  grpc_json* json_iterator = nullptr;
+  json_iterator = grpc_json_create_child(json_iterator, json, "ref", nullptr,
+                                         GRPC_JSON_OBJECT, false);
+  json = json_iterator;
+  json_iterator = nullptr;
+  json_iterator = grpc_json_add_number_string_child(
+      json, json_iterator, "subchannelId", subchannel_uuid_);
+  // reset json iterators to top level object
+  json = top_level_json;
+  json_iterator = nullptr;
+  // create and fill the data child.
+  grpc_json* data = grpc_json_create_child(json_iterator, json, "data", nullptr,
+                                           GRPC_JSON_OBJECT, false);
+  json = data;
+  json_iterator = nullptr;
+  PopulateConnectivityState(json);
+  PopulateTarget(json);
+  PopulateTrace(json);
+  PopulateCallData(json);
+  PopulateChildRefs(json);
+  return top_level_json;
+}
+
 }  // namespace channelz
 }  // namespace grpc_core
diff --git a/src/core/lib/channel/channelz.h b/src/core/lib/channel/channelz.h
index 07eb73d626..97e27c69fc 100644
--- a/src/core/lib/channel/channelz.h
+++ b/src/core/lib/channel/channelz.h
@@ -46,11 +46,44 @@ namespace testing {
 class ChannelNodePeer;
 }
 
-class ChannelNode : public RefCounted<ChannelNode> {
+// base class for all channelz entities
+class ChannelzBaseNode : public RefCounted<ChannelzBaseNode> {
  public:
-  static RefCountedPtr<ChannelNode> MakeChannelNode(
-      grpc_channel* channel, size_t channel_tracer_max_nodes,
-      bool is_top_level_channel);
+  ChannelzBaseNode() {}
+  virtual ~ChannelzBaseNode() {}
+ private:
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_DELETE
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_NEW
+};
+
+// Handles channelz bookkeeping for sockets
+// TODO(ncteisen): implement in subsequent PR.
+class SocketNode : public ChannelzBaseNode {
+ public:
+  SocketNode() : ChannelzBaseNode() {}
+  ~SocketNode() override {}
+ private:
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_DELETE
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_NEW
+};
+
+// This class is the parent for the channelz entities that deal with Channels
+// Subchannels, and Servers, since those have similar proto definitions.
+// This class has the ability to:
+//   - track calls_{started,succeeded,failed}
+//   - track last_call_started_timestamp
+//   - hold the channel trace.
+//   - perform common rendering.
+//
+// This class also defines some fat interfaces so that its children can
+// implement the functionality different. For example, querying the
+// connectivity state looks different for channels and subchannels, and does
+// not make sense for servers. So servers will not override, and channels and
+// subchannels will override with their own way to query connectivity state.
+class CallCountingBase : public ChannelzBaseNode {
+ public:
+  CallCountingBase(size_t channel_tracer_max_nodes);
+  ~CallCountingBase() override;
 
   void RecordCallStarted();
   void RecordCallFailed() {
@@ -59,66 +92,106 @@ class ChannelNode : public RefCounted<ChannelNode> {
   void RecordCallSucceeded() {
     gpr_atm_no_barrier_fetch_add(&calls_succeeded_, (gpr_atm(1)));
   }
+  ChannelTrace* trace() { return trace_.get(); }
+
+  // Fat interface for ConnectivityState. Default is to leave it out, however,
+  // things like Channel and Subchannel will override with their mechanism
+  // for querying connectivity state.
+  virtual void PopulateConnectivityState(grpc_json* json) {}
+
+  // Fat interface for Targets.
+  virtual void PopulateTarget(grpc_json* json) {}
+
+  // Fat interface for ChildRefs. Allows children to populate with whatever
+  // combination of child_refs, subchannel_refs, and socket_refs is correct.
+  virtual void PopulateChildRefs(grpc_json* json) {}
 
-  grpc_json* RenderJson();
+  // All children must implement their custom JSON rendering.
+  virtual grpc_json* RenderJson() GRPC_ABSTRACT;
+
+  // Common rendering of the channel trace.
+  void PopulateTrace(grpc_json* json);
+
+  // Common rendering of the call count data and last_call_started_timestamp.
+  void PopulateCallData(grpc_json* json);
+
+  // Common rendering of grpc_json from RenderJson() to allocated string.
   char* RenderJsonString();
 
-  // helper for getting and populating connectivity state. It is virtual
-  // because it allows the client_channel specific code to live in ext/
-  // instead of lib/
-  virtual void PopulateConnectivityState(grpc_json* json);
+ private:
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_DELETE
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_NEW
 
-  virtual void PopulateChildRefs(grpc_json* json);
+  gpr_atm calls_started_ = 0;
+  gpr_atm calls_succeeded_ = 0;
+  gpr_atm calls_failed_ = 0;
+  gpr_atm last_call_started_millis_ = 0;
+  ManualConstructor<ChannelTrace> trace_;
+};
 
-  ChannelTrace* trace() { return trace_.get(); }
+// Handles channelz bookkeeping for servers
+// TODO(ncteisen): implement in subsequent PR.
+class ServerNode : public CallCountingBase {
+ public:
+  ServerNode(size_t channel_tracer_max_nodes)
+      : CallCountingBase(channel_tracer_max_nodes) {}
+  ~ServerNode() override {}
+ private:
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_DELETE
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_NEW
+};
+
+// Overrides Channel specific functionality.
+class ChannelNode : public CallCountingBase {
+ public:
+  static RefCountedPtr<ChannelNode> MakeChannelNode(
+      grpc_channel* channel, size_t channel_tracer_max_nodes,
+      bool is_top_level_channel);
 
   void MarkChannelDestroyed() {
     GPR_ASSERT(channel_ != nullptr);
     channel_ = nullptr;
   }
 
+  grpc_json* RenderJson() override;
+
+  void PopulateTarget(grpc_json* json) override;
+
   bool ChannelIsDestroyed() { return channel_ == nullptr; }
 
   intptr_t channel_uuid() { return channel_uuid_; }
   bool is_top_level_channel() { return is_top_level_channel_; }
 
  protected:
-  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_DELETE
-  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_NEW
   ChannelNode(grpc_channel* channel, size_t channel_tracer_max_nodes,
               bool is_top_level_channel);
-  virtual ~ChannelNode();
+  ~ChannelNode() override;
 
  private:
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_DELETE
+  GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_NEW
+
   // testing peer friend.
   friend class testing::ChannelNodePeer;
 
   grpc_channel* channel_ = nullptr;
   UniquePtr<char> target_;
-  gpr_atm calls_started_ = 0;
-  gpr_atm calls_succeeded_ = 0;
-  gpr_atm calls_failed_ = 0;
-  gpr_atm last_call_started_millis_ = 0;
   intptr_t channel_uuid_;
   bool is_top_level_channel_ = true;
-  ManualConstructor<ChannelTrace> trace_;
 };
 
-// Placeholds channelz class for subchannels. All this can do now is track its
-// uuid (this information is needed by the parent channelz class).
-// TODO(ncteisen): build this out to support the GetSubchannel channelz request.
-class SubchannelNode : public RefCounted<SubchannelNode> {
+// Overrides Subchannel specific functionality.
+class SubchannelNode : public CallCountingBase {
  public:
-  SubchannelNode();
-  virtual ~SubchannelNode();
-
+  SubchannelNode(size_t channel_tracer_max_nodes);
+  ~SubchannelNode() override;
+  grpc_json* RenderJson() override;
   intptr_t subchannel_uuid() { return subchannel_uuid_; }
 
- protected:
+ private:
   GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_DELETE
   GPRC_ALLOW_CLASS_TO_USE_NON_PUBLIC_NEW
-
- private:
+  
   intptr_t subchannel_uuid_;
 };
 
diff --git a/src/core/lib/channel/channelz_registry.cc b/src/core/lib/channel/channelz_registry.cc
index 38496b3d78..aeeb6958e4 100644
--- a/src/core/lib/channel/channelz_registry.cc
+++ b/src/core/lib/channel/channelz_registry.cc
@@ -142,3 +142,19 @@ char* grpc_channelz_get_channel(intptr_t channel_id) {
   grpc_json_destroy(top_level_json);
   return json_str;
 }
+
+char* grpc_channelz_get_subchannel(intptr_t subchannel_id) {
+  grpc_core::channelz::SubchannelNode* subchannel_node =
+      grpc_core::channelz::ChannelzRegistry::GetSubchannelNode(subchannel_id);
+  if (subchannel_node == nullptr) {
+    return nullptr;
+  }
+  grpc_json* top_level_json = grpc_json_create(GRPC_JSON_OBJECT);
+  grpc_json* json = top_level_json;
+  grpc_json* subchannel_json = subchannel_node->RenderJson();
+  subchannel_json->key = "subchannel";
+  grpc_json_link_child(json, subchannel_json, nullptr);
+  char* json_str = grpc_json_dump_to_string(top_level_json, 0);
+  grpc_json_destroy(top_level_json);
+  return json_str;
+}
diff --git a/src/core/lib/surface/channel.cc b/src/core/lib/surface/channel.cc
index 7cbd61adef..01caadaaba 100644
--- a/src/core/lib/surface/channel.cc
+++ b/src/core/lib/surface/channel.cc
@@ -417,6 +417,9 @@ void grpc_channel_internal_unref(grpc_channel* c REF_ARG) {
 static void destroy_channel(void* arg, grpc_error* error) {
   grpc_channel* channel = static_cast<grpc_channel*>(arg);
   if (channel->channelz_channel != nullptr) {
+    channel->channelz_channel->trace()->AddTraceEvent(
+        grpc_core::channelz::ChannelTrace::Severity::Info,
+        grpc_slice_from_static_string("Channel destroyed"));
     channel->channelz_channel->MarkChannelDestroyed();
     channel->channelz_channel.reset();
   }
diff --git a/test/core/channel/channelz_test.cc b/test/core/channel/channelz_test.cc
index ad5f86d934..b8e65ebfb3 100644
--- a/test/core/channel/channelz_test.cc
+++ b/test/core/channel/channelz_test.cc
@@ -163,6 +163,14 @@ void ValidateChannel(ChannelNode* channel, validate_channel_data_args args) {
   gpr_free(core_api_json_str);
 }
 
+void ValidateSubchannel(SubchannelNode* subchannel,
+                        validate_channel_data_args args) {
+  char* json_str = subchannel->RenderJsonString();
+  grpc::testing::ValidateSubchannelProtoJsonTranslation(json_str);
+  ValidateCounters(json_str, args);
+  gpr_free(json_str);
+}
+
 grpc_millis GetLastCallStartedMillis(ChannelNode* channel) {
   ChannelNodePeer peer(channel);
   return peer.last_call_started_millis();
@@ -275,8 +283,29 @@ TEST(ChannelzGetTopChannelsTest, InternalChannelTest) {
   grpc_channel_destroy(internal_channel);
 }
 
+class ChannelzSubchannelTest : public ::testing::TestWithParam<size_t> {};
+
+TEST_P(ChannelzSubchannelTest, BasicTest) {
+  grpc_core::ExecCtx exec_ctx;
+  RefCountedPtr<SubchannelNode> channelz_subchannel =
+      MakeRefCounted<SubchannelNode>(GetParam());
+  channelz_subchannel->RecordCallStarted();
+  channelz_subchannel->RecordCallFailed();
+  channelz_subchannel->RecordCallSucceeded();
+  ValidateSubchannel(channelz_subchannel.get(), {1, 1, 1});
+  channelz_subchannel->RecordCallStarted();
+  channelz_subchannel->RecordCallFailed();
+  channelz_subchannel->RecordCallSucceeded();
+  channelz_subchannel->RecordCallStarted();
+  channelz_subchannel->RecordCallFailed();
+  channelz_subchannel->RecordCallSucceeded();
+  ValidateSubchannel(channelz_subchannel.get(), {3, 3, 3});
+}
+
 INSTANTIATE_TEST_CASE_P(ChannelzChannelTestSweep, ChannelzChannelTest,
                         ::testing::Values(0, 1, 2, 6, 10, 15));
+INSTANTIATE_TEST_CASE_P(ChannelzSubchannelTestSweep, ChannelzSubchannelTest,
+                        ::testing::Values(0, 1, 10, 15));
 
 }  // namespace testing
 }  // namespace channelz
diff --git a/test/cpp/util/channel_trace_proto_helper.cc b/test/cpp/util/channel_trace_proto_helper.cc
index b4704bfe6a..e416a0375f 100644
--- a/test/cpp/util/channel_trace_proto_helper.cc
+++ b/test/cpp/util/channel_trace_proto_helper.cc
@@ -82,5 +82,9 @@ void ValidateGetChannelResponseProtoJsonTranslation(char* json_c_str) {
       json_c_str);
 }
 
+void ValidateSubchannelProtoJsonTranslation(char* json_c_str) {
+  VaidateProtoJsonTranslation<grpc::channelz::v1::Subchannel>(json_c_str);
+}
+
 }  // namespace testing
 }  // namespace grpc
diff --git a/test/cpp/util/channel_trace_proto_helper.h b/test/cpp/util/channel_trace_proto_helper.h
index 18e3d54b6b..a1ebbcd110 100644
--- a/test/cpp/util/channel_trace_proto_helper.h
+++ b/test/cpp/util/channel_trace_proto_helper.h
@@ -26,6 +26,7 @@ void ValidateChannelTraceProtoJsonTranslation(char* json_c_str);
 void ValidateChannelProtoJsonTranslation(char* json_c_str);
 void ValidateGetTopChannelsResponseProtoJsonTranslation(char* json_c_str);
 void ValidateGetChannelResponseProtoJsonTranslation(char* json_c_str);
+void ValidateSubchannelProtoJsonTranslation(char* json_c_str);
 
 }  // namespace testing
 }  // namespace grpc