Revert "Merge pull request #16600 from grpc/revert-16574-health_checking_service"

This reverts commit de11358660b6474ffea480d8d756c23e58a7cf07, reversing
changes made to 107d10ea73f77dc9bb498c9b91e1fcd0188dfb45.

6 files changed, 700 insertions, 81 deletions

diff --git a/src/cpp/server/health/default_health_check_service.cc b/src/cpp/server/health/default_health_check_service.cc
index bfda67d086..fc3db1bba7 100644
--- a/src/cpp/server/health/default_health_check_service.cc
+++ b/src/cpp/server/health/default_health_check_service.cc
@@ -30,29 +30,162 @@
 #include "src/cpp/server/health/health.pb.h"
 
 namespace grpc {
+
+//
+// DefaultHealthCheckService
+//
+
+DefaultHealthCheckService::DefaultHealthCheckService() {
+  services_map_[""].SetServingStatus(SERVING);
+}
+
+void DefaultHealthCheckService::SetServingStatus(
+    const grpc::string& service_name, bool serving) {
+  std::unique_lock<std::mutex> lock(mu_);
+  services_map_[service_name].SetServingStatus(serving ? SERVING : NOT_SERVING);
+}
+
+void DefaultHealthCheckService::SetServingStatus(bool serving) {
+  const ServingStatus status = serving ? SERVING : NOT_SERVING;
+  std::unique_lock<std::mutex> lock(mu_);
+  for (auto& p : services_map_) {
+    ServiceData& service_data = p.second;
+    service_data.SetServingStatus(status);
+  }
+}
+
+DefaultHealthCheckService::ServingStatus
+DefaultHealthCheckService::GetServingStatus(
+    const grpc::string& service_name) const {
+  std::lock_guard<std::mutex> lock(mu_);
+  auto it = services_map_.find(service_name);
+  if (it == services_map_.end()) {
+    return NOT_FOUND;
+  }
+  const ServiceData& service_data = it->second;
+  return service_data.GetServingStatus();
+}
+
+void DefaultHealthCheckService::RegisterCallHandler(
+    const grpc::string& service_name,
+    std::shared_ptr<HealthCheckServiceImpl::CallHandler> handler) {
+  std::unique_lock<std::mutex> lock(mu_);
+  ServiceData& service_data = services_map_[service_name];
+  service_data.AddCallHandler(handler /* copies ref */);
+  handler->SendHealth(std::move(handler), service_data.GetServingStatus());
+}
+
+void DefaultHealthCheckService::UnregisterCallHandler(
+    const grpc::string& service_name,
+    std::shared_ptr<HealthCheckServiceImpl::CallHandler> handler) {
+  std::unique_lock<std::mutex> lock(mu_);
+  auto it = services_map_.find(service_name);
+  if (it == services_map_.end()) return;
+  ServiceData& service_data = it->second;
+  service_data.RemoveCallHandler(std::move(handler));
+  if (service_data.Unused()) {
+    services_map_.erase(it);
+  }
+}
+
+DefaultHealthCheckService::HealthCheckServiceImpl*
+DefaultHealthCheckService::GetHealthCheckService(
+    std::unique_ptr<ServerCompletionQueue> cq) {
+  GPR_ASSERT(impl_ == nullptr);
+  impl_.reset(new HealthCheckServiceImpl(this, std::move(cq)));
+  return impl_.get();
+}
+
+//
+// DefaultHealthCheckService::ServiceData
+//
+
+void DefaultHealthCheckService::ServiceData::SetServingStatus(
+    ServingStatus status) {
+  status_ = status;
+  for (auto& call_handler : call_handlers_) {
+    call_handler->SendHealth(call_handler /* copies ref */, status);
+  }
+}
+
+void DefaultHealthCheckService::ServiceData::AddCallHandler(
+    std::shared_ptr<HealthCheckServiceImpl::CallHandler> handler) {
+  call_handlers_.insert(std::move(handler));
+}
+
+void DefaultHealthCheckService::ServiceData::RemoveCallHandler(
+    std::shared_ptr<HealthCheckServiceImpl::CallHandler> handler) {
+  call_handlers_.erase(std::move(handler));
+}
+
+//
+// DefaultHealthCheckService::HealthCheckServiceImpl
+//
+
 namespace {
 const char kHealthCheckMethodName[] = "/grpc.health.v1.Health/Check";
+const char kHealthWatchMethodName[] = "/grpc.health.v1.Health/Watch";
 }  // namespace
 
 DefaultHealthCheckService::HealthCheckServiceImpl::HealthCheckServiceImpl(
-    DefaultHealthCheckService* service)
-    : service_(service), method_(nullptr) {
-  internal::MethodHandler* handler =
-      new internal::RpcMethodHandler<HealthCheckServiceImpl, ByteBuffer,
-                                     ByteBuffer>(
-          std::mem_fn(&HealthCheckServiceImpl::Check), this);
-  method_ = new internal::RpcServiceMethod(
-      kHealthCheckMethodName, internal::RpcMethod::NORMAL_RPC, handler);
-  AddMethod(method_);
-}
-
-Status DefaultHealthCheckService::HealthCheckServiceImpl::Check(
-    ServerContext* context, const ByteBuffer* request, ByteBuffer* response) {
-  // Decode request.
-  std::vector<Slice> slices;
-  if (!request->Dump(&slices).ok()) {
-    return Status(StatusCode::INVALID_ARGUMENT, "");
+    DefaultHealthCheckService* database,
+    std::unique_ptr<ServerCompletionQueue> cq)
+    : database_(database), cq_(std::move(cq)) {
+  // Add Check() method.
+  check_method_ = new internal::RpcServiceMethod(
+      kHealthCheckMethodName, internal::RpcMethod::NORMAL_RPC, nullptr);
+  AddMethod(check_method_);
+  // Add Watch() method.
+  watch_method_ = new internal::RpcServiceMethod(
+      kHealthWatchMethodName, internal::RpcMethod::SERVER_STREAMING, nullptr);
+  AddMethod(watch_method_);
+  // Create serving thread.
+  thread_ = std::unique_ptr<::grpc_core::Thread>(
+      new ::grpc_core::Thread("grpc_health_check_service", Serve, this));
+}
+
+DefaultHealthCheckService::HealthCheckServiceImpl::~HealthCheckServiceImpl() {
+  // We will reach here after the server starts shutting down.
+  shutdown_ = true;
+  {
+    std::unique_lock<std::mutex> lock(cq_shutdown_mu_);
+    cq_->Shutdown();
+  }
+  thread_->Join();
+}
+
+void DefaultHealthCheckService::HealthCheckServiceImpl::StartServingThread() {
+  thread_->Start();
+}
+
+void DefaultHealthCheckService::HealthCheckServiceImpl::Serve(void* arg) {
+  HealthCheckServiceImpl* service =
+      reinterpret_cast<HealthCheckServiceImpl*>(arg);
+  // TODO(juanlishen): This is a workaround to wait for the cq to be ready.
+  // Need to figure out why cq is not ready after service starts.
+  gpr_sleep_until(gpr_time_add(gpr_now(GPR_CLOCK_MONOTONIC),
+                               gpr_time_from_seconds(1, GPR_TIMESPAN)));
+  CheckCallHandler::CreateAndStart(service->cq_.get(), service->database_,
+                                   service);
+  WatchCallHandler::CreateAndStart(service->cq_.get(), service->database_,
+                                   service);
+  void* tag;
+  bool ok;
+  while (true) {
+    if (!service->cq_->Next(&tag, &ok)) {
+      // The completion queue is shutting down.
+      GPR_ASSERT(service->shutdown_);
+      break;
+    }
+    auto* next_step = static_cast<CallableTag*>(tag);
+    next_step->Run(ok);
   }
+}
+
+bool DefaultHealthCheckService::HealthCheckServiceImpl::DecodeRequest(
+    const ByteBuffer& request, grpc::string* service_name) {
+  std::vector<Slice> slices;
+  if (!request.Dump(&slices).ok()) return false;
   uint8_t* request_bytes = nullptr;
   bool request_bytes_owned = false;
   size_t request_size = 0;
@@ -64,14 +197,13 @@ Status DefaultHealthCheckService::HealthCheckServiceImpl::Check(
     request_size = slices[0].size();
   } else {
     request_bytes_owned = true;
-    request_bytes = static_cast<uint8_t*>(gpr_malloc(request->Length()));
+    request_bytes = static_cast<uint8_t*>(gpr_malloc(request.Length()));
     uint8_t* copy_to = request_bytes;
     for (size_t i = 0; i < slices.size(); i++) {
       memcpy(copy_to, slices[i].begin(), slices[i].size());
       copy_to += slices[i].size();
     }
   }
-
   if (request_bytes != nullptr) {
     pb_istream_t istream = pb_istream_from_buffer(request_bytes, request_size);
     bool decode_status = pb_decode(
@@ -79,26 +211,22 @@ Status DefaultHealthCheckService::HealthCheckServiceImpl::Check(
     if (request_bytes_owned) {
       gpr_free(request_bytes);
     }
-    if (!decode_status) {
-      return Status(StatusCode::INVALID_ARGUMENT, "");
-    }
-  }
-
-  // Check status from the associated default health checking service.
-  DefaultHealthCheckService::ServingStatus serving_status =
-      service_->GetServingStatus(
-          request_struct.has_service ? request_struct.service : "");
-  if (serving_status == DefaultHealthCheckService::NOT_FOUND) {
-    return Status(StatusCode::NOT_FOUND, "");
+    if (!decode_status) return false;
   }
+  *service_name = request_struct.has_service ? request_struct.service : "";
+  return true;
+}
 
-  // Encode response
+bool DefaultHealthCheckService::HealthCheckServiceImpl::EncodeResponse(
+    ServingStatus status, ByteBuffer* response) {
   grpc_health_v1_HealthCheckResponse response_struct;
   response_struct.has_status = true;
   response_struct.status =
-      serving_status == DefaultHealthCheckService::SERVING
-          ? grpc_health_v1_HealthCheckResponse_ServingStatus_SERVING
-          : grpc_health_v1_HealthCheckResponse_ServingStatus_NOT_SERVING;
+      status == NOT_FOUND
+          ? grpc_health_v1_HealthCheckResponse_ServingStatus_SERVICE_UNKNOWN
+          : status == SERVING
+                ? grpc_health_v1_HealthCheckResponse_ServingStatus_SERVING
+                : grpc_health_v1_HealthCheckResponse_ServingStatus_NOT_SERVING;
   pb_ostream_t ostream;
   memset(&ostream, 0, sizeof(ostream));
   pb_encode(&ostream, grpc_health_v1_HealthCheckResponse_fields,
@@ -108,48 +236,282 @@ Status DefaultHealthCheckService::HealthCheckServiceImpl::Check(
                                    GRPC_SLICE_LENGTH(response_slice));
   bool encode_status = pb_encode(
       &ostream, grpc_health_v1_HealthCheckResponse_fields, &response_struct);
-  if (!encode_status) {
-    return Status(StatusCode::INTERNAL, "Failed to encode response.");
-  }
+  if (!encode_status) return false;
   Slice encoded_response(response_slice, Slice::STEAL_REF);
   ByteBuffer response_buffer(&encoded_response, 1);
   response->Swap(&response_buffer);
-  return Status::OK;
+  return true;
 }
 
-DefaultHealthCheckService::DefaultHealthCheckService() {
-  services_map_.emplace("", true);
+//
+// DefaultHealthCheckService::HealthCheckServiceImpl::CheckCallHandler
+//
+
+void DefaultHealthCheckService::HealthCheckServiceImpl::CheckCallHandler::
+    CreateAndStart(ServerCompletionQueue* cq,
+                   DefaultHealthCheckService* database,
+                   HealthCheckServiceImpl* service) {
+  std::shared_ptr<CallHandler> self =
+      std::make_shared<CheckCallHandler>(cq, database, service);
+  CheckCallHandler* handler = static_cast<CheckCallHandler*>(self.get());
+  {
+    std::unique_lock<std::mutex> lock(service->cq_shutdown_mu_);
+    if (service->shutdown_) return;
+    // Request a Check() call.
+    handler->next_ =
+        CallableTag(std::bind(&CheckCallHandler::OnCallReceived, handler,
+                              std::placeholders::_1, std::placeholders::_2),
+                    std::move(self));
+    service->RequestAsyncUnary(0, &handler->ctx_, &handler->request_,
+                               &handler->writer_, cq, cq, &handler->next_);
+  }
 }
 
-void DefaultHealthCheckService::SetServingStatus(
-    const grpc::string& service_name, bool serving) {
-  std::lock_guard<std::mutex> lock(mu_);
-  services_map_[service_name] = serving;
+DefaultHealthCheckService::HealthCheckServiceImpl::CheckCallHandler::
+    CheckCallHandler(ServerCompletionQueue* cq,
+                     DefaultHealthCheckService* database,
+                     HealthCheckServiceImpl* service)
+    : cq_(cq), database_(database), service_(service), writer_(&ctx_) {}
+
+void DefaultHealthCheckService::HealthCheckServiceImpl::CheckCallHandler::
+    OnCallReceived(std::shared_ptr<CallHandler> self, bool ok) {
+  if (!ok) {
+    // The value of ok being false means that the server is shutting down.
+    return;
+  }
+  // Spawn a new handler instance to serve the next new client. Every handler
+  // instance will deallocate itself when it's done.
+  CreateAndStart(cq_, database_, service_);
+  // Process request.
+  gpr_log(GPR_DEBUG, "[HCS %p] Health check started for handler %p", service_,
+          this);
+  grpc::string service_name;
+  grpc::Status status = Status::OK;
+  ByteBuffer response;
+  if (!service_->DecodeRequest(request_, &service_name)) {
+    status = Status(StatusCode::INVALID_ARGUMENT, "");
+  } else {
+    ServingStatus serving_status = database_->GetServingStatus(service_name);
+    if (serving_status == NOT_FOUND) {
+      status = Status(StatusCode::NOT_FOUND, "service name unknown");
+    } else if (!service_->EncodeResponse(serving_status, &response)) {
+      status = Status(StatusCode::INTERNAL, "");
+    }
+  }
+  // Send response.
+  {
+    std::unique_lock<std::mutex> lock(service_->cq_shutdown_mu_);
+    if (!service_->shutdown_) {
+      next_ =
+          CallableTag(std::bind(&CheckCallHandler::OnFinishDone, this,
+                                std::placeholders::_1, std::placeholders::_2),
+                      std::move(self));
+      if (status.ok()) {
+        writer_.Finish(response, status, &next_);
+      } else {
+        writer_.FinishWithError(status, &next_);
+      }
+    }
+  }
 }
 
-void DefaultHealthCheckService::SetServingStatus(bool serving) {
-  std::lock_guard<std::mutex> lock(mu_);
-  for (auto iter = services_map_.begin(); iter != services_map_.end(); ++iter) {
-    iter->second = serving;
+void DefaultHealthCheckService::HealthCheckServiceImpl::CheckCallHandler::
+    OnFinishDone(std::shared_ptr<CallHandler> self, bool ok) {
+  if (ok) {
+    gpr_log(GPR_DEBUG, "[HCS %p] Health check call finished for handler %p",
+            service_, this);
   }
 }
 
-DefaultHealthCheckService::ServingStatus
-DefaultHealthCheckService::GetServingStatus(
-    const grpc::string& service_name) const {
-  std::lock_guard<std::mutex> lock(mu_);
-  const auto& iter = services_map_.find(service_name);
-  if (iter == services_map_.end()) {
-    return NOT_FOUND;
+//
+// DefaultHealthCheckService::HealthCheckServiceImpl::WatchCallHandler
+//
+
+void DefaultHealthCheckService::HealthCheckServiceImpl::WatchCallHandler::
+    CreateAndStart(ServerCompletionQueue* cq,
+                   DefaultHealthCheckService* database,
+                   HealthCheckServiceImpl* service) {
+  std::shared_ptr<CallHandler> self =
+      std::make_shared<WatchCallHandler>(cq, database, service);
+  WatchCallHandler* handler = static_cast<WatchCallHandler*>(self.get());
+  {
+    std::unique_lock<std::mutex> lock(service->cq_shutdown_mu_);
+    if (service->shutdown_) return;
+    // Request AsyncNotifyWhenDone().
+    handler->on_done_notified_ =
+        CallableTag(std::bind(&WatchCallHandler::OnDoneNotified, handler,
+                              std::placeholders::_1, std::placeholders::_2),
+                    self /* copies ref */);
+    handler->ctx_.AsyncNotifyWhenDone(&handler->on_done_notified_);
+    // Request a Watch() call.
+    handler->next_ =
+        CallableTag(std::bind(&WatchCallHandler::OnCallReceived, handler,
+                              std::placeholders::_1, std::placeholders::_2),
+                    std::move(self));
+    service->RequestAsyncServerStreaming(1, &handler->ctx_, &handler->request_,
+                                         &handler->stream_, cq, cq,
+                                         &handler->next_);
   }
-  return iter->second ? SERVING : NOT_SERVING;
 }
 
-DefaultHealthCheckService::HealthCheckServiceImpl*
-DefaultHealthCheckService::GetHealthCheckService() {
-  GPR_ASSERT(impl_ == nullptr);
-  impl_.reset(new HealthCheckServiceImpl(this));
-  return impl_.get();
+DefaultHealthCheckService::HealthCheckServiceImpl::WatchCallHandler::
+    WatchCallHandler(ServerCompletionQueue* cq,
+                     DefaultHealthCheckService* database,
+                     HealthCheckServiceImpl* service)
+    : cq_(cq),
+      database_(database),
+      service_(service),
+      stream_(&ctx_),
+      call_state_(WAITING_FOR_CALL) {}
+
+void DefaultHealthCheckService::HealthCheckServiceImpl::WatchCallHandler::
+    OnCallReceived(std::shared_ptr<CallHandler> self, bool ok) {
+  if (ok) {
+    call_state_ = CALL_RECEIVED;
+  } else {
+    // AsyncNotifyWhenDone() needs to be called before the call starts, but the
+    // tag will not pop out if the call never starts (
+    // https://github.com/grpc/grpc/issues/10136). So we need to manually
+    // release the ownership of the handler in this case.
+    GPR_ASSERT(on_done_notified_.ReleaseHandler() != nullptr);
+  }
+  if (!ok || shutdown_) {
+    // The value of ok being false means that the server is shutting down.
+    Shutdown(std::move(self), "OnCallReceived");
+    return;
+  }
+  // Spawn a new handler instance to serve the next new client. Every handler
+  // instance will deallocate itself when it's done.
+  CreateAndStart(cq_, database_, service_);
+  // Parse request.
+  if (!service_->DecodeRequest(request_, &service_name_)) {
+    on_finish_done_ =
+        CallableTag(std::bind(&WatchCallHandler::OnFinishDone, this,
+                              std::placeholders::_1, std::placeholders::_2),
+                    std::move(self));
+    stream_.Finish(Status(StatusCode::INVALID_ARGUMENT, ""), &on_finish_done_);
+    call_state_ = FINISH_CALLED;
+    return;
+  }
+  // Register the call for updates to the service.
+  gpr_log(GPR_DEBUG,
+          "[HCS %p] Health check watch started for service \"%s\" "
+          "(handler: %p)",
+          service_, service_name_.c_str(), this);
+  database_->RegisterCallHandler(service_name_, std::move(self));
+}
+
+void DefaultHealthCheckService::HealthCheckServiceImpl::WatchCallHandler::
+    SendHealth(std::shared_ptr<CallHandler> self, ServingStatus status) {
+  std::unique_lock<std::mutex> lock(mu_);
+  // If there's already a send in flight, cache the new status, and
+  // we'll start a new send for it when the one in flight completes.
+  if (send_in_flight_) {
+    pending_status_ = status;
+    return;
+  }
+  // Start a send.
+  SendHealthLocked(std::move(self), status);
+}
+
+void DefaultHealthCheckService::HealthCheckServiceImpl::WatchCallHandler::
+    SendHealthLocked(std::shared_ptr<CallHandler> self, ServingStatus status) {
+  std::unique_lock<std::mutex> cq_lock(service_->cq_shutdown_mu_);
+  if (service_->shutdown_) {
+    cq_lock.release()->unlock();
+    Shutdown(std::move(self), "SendHealthLocked");
+    return;
+  }
+  send_in_flight_ = true;
+  call_state_ = SEND_MESSAGE_PENDING;
+  // Construct response.
+  ByteBuffer response;
+  if (!service_->EncodeResponse(status, &response)) {
+    on_finish_done_ =
+        CallableTag(std::bind(&WatchCallHandler::OnFinishDone, this,
+                              std::placeholders::_1, std::placeholders::_2),
+                    std::move(self));
+    stream_.Finish(Status(StatusCode::INTERNAL, ""), &on_finish_done_);
+    return;
+  }
+  next_ = CallableTag(std::bind(&WatchCallHandler::OnSendHealthDone, this,
+                                std::placeholders::_1, std::placeholders::_2),
+                      std::move(self));
+  stream_.Write(response, &next_);
+}
+
+void DefaultHealthCheckService::HealthCheckServiceImpl::WatchCallHandler::
+    OnSendHealthDone(std::shared_ptr<CallHandler> self, bool ok) {
+  if (!ok || shutdown_) {
+    Shutdown(std::move(self), "OnSendHealthDone");
+    return;
+  }
+  call_state_ = CALL_RECEIVED;
+  {
+    std::unique_lock<std::mutex> lock(mu_);
+    send_in_flight_ = false;
+    // If we got a new status since we started the last send, start a
+    // new send for it.
+    if (pending_status_ != NOT_FOUND) {
+      auto status = pending_status_;
+      pending_status_ = NOT_FOUND;
+      SendHealthLocked(std::move(self), status);
+    }
+  }
+}
+
+void DefaultHealthCheckService::HealthCheckServiceImpl::WatchCallHandler::
+    OnDoneNotified(std::shared_ptr<CallHandler> self, bool ok) {
+  GPR_ASSERT(ok);
+  done_notified_ = true;
+  if (ctx_.IsCancelled()) {
+    is_cancelled_ = true;
+  }
+  gpr_log(GPR_DEBUG,
+          "[HCS %p] Healt check call is notified done (handler: %p, "
+          "is_cancelled: %d).",
+          service_, this, static_cast<int>(is_cancelled_));
+  Shutdown(std::move(self), "OnDoneNotified");
+}
+
+// TODO(roth): This method currently assumes that there will be only one
+// thread polling the cq and invoking the corresponding callbacks.  If
+// that changes, we will need to add synchronization here.
+void DefaultHealthCheckService::HealthCheckServiceImpl::WatchCallHandler::
+    Shutdown(std::shared_ptr<CallHandler> self, const char* reason) {
+  if (!shutdown_) {
+    gpr_log(GPR_DEBUG,
+            "[HCS %p] Shutting down the handler (service_name: \"%s\", "
+            "handler: %p, reason: %s).",
+            service_, service_name_.c_str(), this, reason);
+    shutdown_ = true;
+  }
+  // OnCallReceived() may be called after OnDoneNotified(), so we need to
+  // try to Finish() every time we are in Shutdown().
+  if (call_state_ >= CALL_RECEIVED && call_state_ < FINISH_CALLED) {
+    std::unique_lock<std::mutex> lock(service_->cq_shutdown_mu_);
+    if (!service_->shutdown_) {
+      on_finish_done_ =
+          CallableTag(std::bind(&WatchCallHandler::OnFinishDone, this,
+                                std::placeholders::_1, std::placeholders::_2),
+                      std::move(self));
+      // TODO(juanlishen): Maybe add a message proto for the client to
+      // explicitly cancel the stream so that we can return OK status in such
+      // cases.
+      stream_.Finish(Status::CANCELLED, &on_finish_done_);
+      call_state_ = FINISH_CALLED;
+    }
+  }
+}
+
+void DefaultHealthCheckService::HealthCheckServiceImpl::WatchCallHandler::
+    OnFinishDone(std::shared_ptr<CallHandler> self, bool ok) {
+  if (ok) {
+    gpr_log(GPR_DEBUG,
+            "[HCS %p] Health check call finished (service_name: \"%s\", "
+            "handler: %p).",
+            service_, service_name_.c_str(), this);
+  }
 }
 
 }  // namespace grpc
diff --git a/src/cpp/server/health/default_health_check_service.h b/src/cpp/server/health/default_health_check_service.h
index a1ce5aa64e..edad594936 100644
--- a/src/cpp/server/health/default_health_check_service.h
+++ b/src/cpp/server/health/default_health_check_service.h
@@ -19,42 +19,268 @@
 #ifndef GRPC_INTERNAL_CPP_SERVER_DEFAULT_HEALTH_CHECK_SERVICE_H
 #define GRPC_INTERNAL_CPP_SERVER_DEFAULT_HEALTH_CHECK_SERVICE_H
 
+#include <atomic>
 #include <mutex>
+#include <set>
 
+#include <grpc/support/log.h>
+#include <grpcpp/grpcpp.h>
 #include <grpcpp/health_check_service_interface.h>
+#include <grpcpp/impl/codegen/async_generic_service.h>
+#include <grpcpp/impl/codegen/async_unary_call.h>
 #include <grpcpp/impl/codegen/service_type.h>
 #include <grpcpp/support/byte_buffer.h>
 
+#include "src/core/lib/gprpp/thd.h"
+
 namespace grpc {
 
 // Default implementation of HealthCheckServiceInterface. Server will create and
 // own it.
 class DefaultHealthCheckService final : public HealthCheckServiceInterface {
  public:
+  enum ServingStatus { NOT_FOUND, SERVING, NOT_SERVING };
+
   // The service impl to register with the server.
   class HealthCheckServiceImpl : public Service {
    public:
-    explicit HealthCheckServiceImpl(DefaultHealthCheckService* service);
+    // Base class for call handlers.
+    class CallHandler {
+     public:
+      virtual ~CallHandler() = default;
+      virtual void SendHealth(std::shared_ptr<CallHandler> self,
+                              ServingStatus status) = 0;
+    };
 
-    Status Check(ServerContext* context, const ByteBuffer* request,
-                 ByteBuffer* response);
+    HealthCheckServiceImpl(DefaultHealthCheckService* database,
+                           std::unique_ptr<ServerCompletionQueue> cq);
+
+    ~HealthCheckServiceImpl();
+
+    void StartServingThread();
 
    private:
-    const DefaultHealthCheckService* const service_;
-    internal::RpcServiceMethod* method_;
+    // A tag that can be called with a bool argument. It's tailored for
+    // CallHandler's use. Before being used, it should be constructed with a
+    // method of CallHandler and a shared pointer to the handler. The
+    // shared pointer will be moved to the invoked function and the function
+    // can only be invoked once. That makes ref counting of the handler easier,
+    // because the shared pointer is not bound to the function and can be gone
+    // once the invoked function returns (if not used any more).
+    class CallableTag {
+     public:
+      using HandlerFunction =
+          std::function<void(std::shared_ptr<CallHandler>, bool)>;
+
+      CallableTag() {}
+
+      CallableTag(HandlerFunction func, std::shared_ptr<CallHandler> handler)
+          : handler_function_(std::move(func)), handler_(std::move(handler)) {
+        GPR_ASSERT(handler_function_ != nullptr);
+        GPR_ASSERT(handler_ != nullptr);
+      }
+
+      // Runs the tag. This should be called only once. The handler is no
+      // longer owned by this tag after this method is invoked.
+      void Run(bool ok) {
+        GPR_ASSERT(handler_function_ != nullptr);
+        GPR_ASSERT(handler_ != nullptr);
+        handler_function_(std::move(handler_), ok);
+      }
+
+      // Releases and returns the shared pointer to the handler.
+      std::shared_ptr<CallHandler> ReleaseHandler() {
+        return std::move(handler_);
+      }
+
+     private:
+      HandlerFunction handler_function_ = nullptr;
+      std::shared_ptr<CallHandler> handler_;
+    };
+
+    // Call handler for Check method.
+    // Each handler takes care of one call. It contains per-call data and it
+    // will access the members of the parent class (i.e.,
+    // DefaultHealthCheckService) for per-service health data.
+    class CheckCallHandler : public CallHandler {
+     public:
+      // Instantiates a CheckCallHandler and requests the next health check
+      // call. The handler object will manage its own lifetime, so no action is
+      // needed from the caller any more regarding that object.
+      static void CreateAndStart(ServerCompletionQueue* cq,
+                                 DefaultHealthCheckService* database,
+                                 HealthCheckServiceImpl* service);
+
+      // This ctor is public because we want to use std::make_shared<> in
+      // CreateAndStart(). This ctor shouldn't be used elsewhere.
+      CheckCallHandler(ServerCompletionQueue* cq,
+                       DefaultHealthCheckService* database,
+                       HealthCheckServiceImpl* service);
+
+      // Not used for Check.
+      void SendHealth(std::shared_ptr<CallHandler> self,
+                      ServingStatus status) override {}
+
+     private:
+      // Called when we receive a call.
+      // Spawns a new handler so that we can keep servicing future calls.
+      void OnCallReceived(std::shared_ptr<CallHandler> self, bool ok);
+
+      // Called when Finish() is done.
+      void OnFinishDone(std::shared_ptr<CallHandler> self, bool ok);
+
+      // The members passed down from HealthCheckServiceImpl.
+      ServerCompletionQueue* cq_;
+      DefaultHealthCheckService* database_;
+      HealthCheckServiceImpl* service_;
+
+      ByteBuffer request_;
+      GenericServerAsyncResponseWriter writer_;
+      ServerContext ctx_;
+
+      CallableTag next_;
+    };
+
+    // Call handler for Watch method.
+    // Each handler takes care of one call. It contains per-call data and it
+    // will access the members of the parent class (i.e.,
+    // DefaultHealthCheckService) for per-service health data.
+    class WatchCallHandler : public CallHandler {
+     public:
+      // Instantiates a WatchCallHandler and requests the next health check
+      // call. The handler object will manage its own lifetime, so no action is
+      // needed from the caller any more regarding that object.
+      static void CreateAndStart(ServerCompletionQueue* cq,
+                                 DefaultHealthCheckService* database,
+                                 HealthCheckServiceImpl* service);
+
+      // This ctor is public because we want to use std::make_shared<> in
+      // CreateAndStart(). This ctor shouldn't be used elsewhere.
+      WatchCallHandler(ServerCompletionQueue* cq,
+                       DefaultHealthCheckService* database,
+                       HealthCheckServiceImpl* service);
+
+      void SendHealth(std::shared_ptr<CallHandler> self,
+                      ServingStatus status) override;
+
+     private:
+      // Called when we receive a call.
+      // Spawns a new handler so that we can keep servicing future calls.
+      void OnCallReceived(std::shared_ptr<CallHandler> self, bool ok);
+
+      // Requires holding mu_.
+      void SendHealthLocked(std::shared_ptr<CallHandler> self,
+                            ServingStatus status);
+
+      // When sending a health result finishes.
+      void OnSendHealthDone(std::shared_ptr<CallHandler> self, bool ok);
+
+      // Called when Finish() is done.
+      void OnFinishDone(std::shared_ptr<CallHandler> self, bool ok);
+
+      // Called when AsyncNotifyWhenDone() notifies us.
+      void OnDoneNotified(std::shared_ptr<CallHandler> self, bool ok);
+
+      void Shutdown(std::shared_ptr<CallHandler> self, const char* reason);
+
+      // The members passed down from HealthCheckServiceImpl.
+      ServerCompletionQueue* cq_;
+      DefaultHealthCheckService* database_;
+      HealthCheckServiceImpl* service_;
+
+      ByteBuffer request_;
+      grpc::string service_name_;
+      GenericServerAsyncWriter stream_;
+      ServerContext ctx_;
+
+      std::mutex mu_;
+      bool send_in_flight_ = false;               // Guarded by mu_.
+      ServingStatus pending_status_ = NOT_FOUND;  // Guarded by mu_.
+
+      // The state of the RPC progress.
+      enum CallState {
+        WAITING_FOR_CALL,
+        CALL_RECEIVED,
+        SEND_MESSAGE_PENDING,
+        FINISH_CALLED
+      } call_state_;
+
+      bool shutdown_ = false;
+      bool done_notified_ = false;
+      bool is_cancelled_ = false;
+      CallableTag next_;
+      CallableTag on_done_notified_;
+      CallableTag on_finish_done_;
+    };
+
+    // Handles the incoming requests and drives the completion queue in a loop.
+    static void Serve(void* arg);
+
+    // Returns true on success.
+    static bool DecodeRequest(const ByteBuffer& request,
+                              grpc::string* service_name);
+    static bool EncodeResponse(ServingStatus status, ByteBuffer* response);
+
+    // Needed to appease Windows compilers, which don't seem to allow
+    // nested classes to access protected members in the parent's
+    // superclass.
+    using Service::RequestAsyncServerStreaming;
+    using Service::RequestAsyncUnary;
+
+    DefaultHealthCheckService* database_;
+    std::unique_ptr<ServerCompletionQueue> cq_;
+    internal::RpcServiceMethod* check_method_;
+    internal::RpcServiceMethod* watch_method_;
+
+    // To synchronize the operations related to shutdown state of cq_, so that
+    // we don't enqueue new tags into cq_ after it is already shut down.
+    std::mutex cq_shutdown_mu_;
+    std::atomic_bool shutdown_{false};
+    std::unique_ptr<::grpc_core::Thread> thread_;
   };
 
   DefaultHealthCheckService();
+
   void SetServingStatus(const grpc::string& service_name,
                         bool serving) override;
   void SetServingStatus(bool serving) override;
-  enum ServingStatus { NOT_FOUND, SERVING, NOT_SERVING };
+
   ServingStatus GetServingStatus(const grpc::string& service_name) const;
-  HealthCheckServiceImpl* GetHealthCheckService();
+
+  HealthCheckServiceImpl* GetHealthCheckService(
+      std::unique_ptr<ServerCompletionQueue> cq);
 
  private:
+  // Stores the current serving status of a service and any call
+  // handlers registered for updates when the service's status changes.
+  class ServiceData {
+   public:
+    void SetServingStatus(ServingStatus status);
+    ServingStatus GetServingStatus() const { return status_; }
+    void AddCallHandler(
+        std::shared_ptr<HealthCheckServiceImpl::CallHandler> handler);
+    void RemoveCallHandler(
+        std::shared_ptr<HealthCheckServiceImpl::CallHandler> handler);
+    bool Unused() const {
+      return call_handlers_.empty() && status_ == NOT_FOUND;
+    }
+
+   private:
+    ServingStatus status_ = NOT_FOUND;
+    std::set<std::shared_ptr<HealthCheckServiceImpl::CallHandler>>
+        call_handlers_;
+  };
+
+  void RegisterCallHandler(
+      const grpc::string& service_name,
+      std::shared_ptr<HealthCheckServiceImpl::CallHandler> handler);
+
+  void UnregisterCallHandler(
+      const grpc::string& service_name,
+      std::shared_ptr<HealthCheckServiceImpl::CallHandler> handler);
+
   mutable std::mutex mu_;
-  std::map<grpc::string, bool> services_map_;
+  std::map<grpc::string, ServiceData> services_map_;  // Guarded by mu_.
   std::unique_ptr<HealthCheckServiceImpl> impl_;
 };
 
diff --git a/src/cpp/server/health/health.pb.c b/src/cpp/server/health/health.pb.c
index 09bd98a3d9..5c214c7160 100644
--- a/src/cpp/server/health/health.pb.c
+++ b/src/cpp/server/health/health.pb.c
@@ -2,7 +2,6 @@
 /* Generated by nanopb-0.3.7-dev */
 
 #include "src/cpp/server/health/health.pb.h"
-
 /* @@protoc_insertion_point(includes) */
 #if PB_PROTO_HEADER_VERSION != 30
 #error Regenerate this file with the current version of nanopb generator.
diff --git a/src/cpp/server/health/health.pb.h b/src/cpp/server/health/health.pb.h
index 29e1f3bacb..9d54ccd618 100644
--- a/src/cpp/server/health/health.pb.h
+++ b/src/cpp/server/health/health.pb.h
@@ -17,11 +17,12 @@ extern "C" {
 typedef enum _grpc_health_v1_HealthCheckResponse_ServingStatus {
     grpc_health_v1_HealthCheckResponse_ServingStatus_UNKNOWN = 0,
     grpc_health_v1_HealthCheckResponse_ServingStatus_SERVING = 1,
-    grpc_health_v1_HealthCheckResponse_ServingStatus_NOT_SERVING = 2
+    grpc_health_v1_HealthCheckResponse_ServingStatus_NOT_SERVING = 2,
+    grpc_health_v1_HealthCheckResponse_ServingStatus_SERVICE_UNKNOWN = 3
 } grpc_health_v1_HealthCheckResponse_ServingStatus;
 #define _grpc_health_v1_HealthCheckResponse_ServingStatus_MIN grpc_health_v1_HealthCheckResponse_ServingStatus_UNKNOWN
-#define _grpc_health_v1_HealthCheckResponse_ServingStatus_MAX grpc_health_v1_HealthCheckResponse_ServingStatus_NOT_SERVING
-#define _grpc_health_v1_HealthCheckResponse_ServingStatus_ARRAYSIZE ((grpc_health_v1_HealthCheckResponse_ServingStatus)(grpc_health_v1_HealthCheckResponse_ServingStatus_NOT_SERVING+1))
+#define _grpc_health_v1_HealthCheckResponse_ServingStatus_MAX grpc_health_v1_HealthCheckResponse_ServingStatus_SERVICE_UNKNOWN
+#define _grpc_health_v1_HealthCheckResponse_ServingStatus_ARRAYSIZE ((grpc_health_v1_HealthCheckResponse_ServingStatus)(grpc_health_v1_HealthCheckResponse_ServingStatus_SERVICE_UNKNOWN+1))
 
 /* Struct definitions */
 typedef struct _grpc_health_v1_HealthCheckRequest {
diff --git a/src/cpp/server/server_cc.cc b/src/cpp/server/server_cc.cc
index 36f7eb81f9..bf612d2d24 100644
--- a/src/cpp/server/server_cc.cc
+++ b/src/cpp/server/server_cc.cc
@@ -559,16 +559,20 @@ void Server::Start(ServerCompletionQueue** cqs, size_t num_cqs) {
 
   // Only create default health check service when user did not provide an
   // explicit one.
+  ServerCompletionQueue* health_check_cq = nullptr;
+  DefaultHealthCheckService::HealthCheckServiceImpl*
+      default_health_check_service_impl = nullptr;
   if (health_check_service_ == nullptr && !health_check_service_disabled_ &&
       DefaultHealthCheckServiceEnabled()) {
-    if (sync_server_cqs_ == nullptr || sync_server_cqs_->empty()) {
-      gpr_log(GPR_INFO,
-              "Default health check service disabled at async-only server.");
-    } else {
-      auto* default_hc_service = new DefaultHealthCheckService;
-      health_check_service_.reset(default_hc_service);
-      RegisterService(nullptr, default_hc_service->GetHealthCheckService());
-    }
+    auto* default_hc_service = new DefaultHealthCheckService;
+    health_check_service_.reset(default_hc_service);
+    health_check_cq = new ServerCompletionQueue(GRPC_CQ_DEFAULT_POLLING);
+    grpc_server_register_completion_queue(server_, health_check_cq->cq(),
+                                          nullptr);
+    default_health_check_service_impl =
+        default_hc_service->GetHealthCheckService(
+            std::unique_ptr<ServerCompletionQueue>(health_check_cq));
+    RegisterService(nullptr, default_health_check_service_impl);
   }
 
   grpc_server_start(server_);
@@ -583,6 +587,9 @@ void Server::Start(ServerCompletionQueue** cqs, size_t num_cqs) {
         new UnimplementedAsyncRequest(this, cqs[i]);
       }
     }
+    if (health_check_cq != nullptr) {
+      new UnimplementedAsyncRequest(this, health_check_cq);
+    }
   }
 
   // If this server has any support for synchronous methods (has any sync
@@ -595,6 +602,10 @@ void Server::Start(ServerCompletionQueue** cqs, size_t num_cqs) {
   for (auto it = sync_req_mgrs_.begin(); it != sync_req_mgrs_.end(); it++) {
     (*it)->Start();
   }
+
+  if (default_health_check_service_impl != nullptr) {
+    default_health_check_service_impl->StartServingThread();
+  }
 }
 
 void Server::ShutdownInternal(gpr_timespec deadline) {
diff --git a/src/proto/grpc/health/v1/health.proto b/src/proto/grpc/health/v1/health.proto
index 4b4677b8a4..38843ff1e7 100644
--- a/src/proto/grpc/health/v1/health.proto
+++ b/src/proto/grpc/health/v1/health.proto
@@ -34,10 +34,30 @@ message HealthCheckResponse {
     UNKNOWN = 0;
     SERVING = 1;
     NOT_SERVING = 2;
+    SERVICE_UNKNOWN = 3;  // Used only by the Watch method.
   }
   ServingStatus status = 1;
 }
 
 service Health {
+  // If the requested service is unknown, the call will fail with status
+  // NOT_FOUND.
   rpc Check(HealthCheckRequest) returns (HealthCheckResponse);
+
+  // Performs a watch for the serving status of the requested service.
+  // The server will immediately send back a message indicating the current
+  // serving status.  It will then subsequently send a new message whenever
+  // the service's serving status changes.
+  //
+  // If the requested service is unknown when the call is received, the
+  // server will send a message setting the serving status to
+  // SERVICE_UNKNOWN but will *not* terminate the call.  If at some
+  // future point, the serving status of the service becomes known, the
+  // server will send a new message with the service's serving status.
+  //
+  // If the call terminates with status UNIMPLEMENTED, then clients
+  // should assume this method is not supported and should not retry the
+  // call.  If the call terminates with any other status (including OK),
+  // clients should retry the call with appropriate exponential backoff.
+  rpc Watch(HealthCheckRequest) returns (stream HealthCheckResponse);
 }