Merge pull request #18493 from vjpai/callback_comments

Comments for all callback API methods

include/grpcpp/generic/generic_stub.h

@@ -73,10 +73,15 @@ class GenericStub final {
    public:
     explicit experimental_type(GenericStub* stub) : stub_(stub) {}
 
+    /// Setup and start a unary call to a named method \a method using
+    /// \a context and specifying the \a request and \a response buffers.
     void UnaryCall(ClientContext* context, const grpc::string& method,
                    const ByteBuffer* request, ByteBuffer* response,
                    std::function<void(Status)> on_completion);
 
+    /// Setup a call to a named method \a method using \a context and tied to
+    /// \a reactor . Like any other bidi streaming RPC, it will not be activated
+    /// until StartCall is invoked on its reactor.
     void PrepareBidiStreamingCall(
         ClientContext* context, const grpc::string& method,
         experimental::ClientBidiReactor<ByteBuffer, ByteBuffer>* reactor);

include/grpcpp/impl/codegen/async_generic_service.h

@@ -85,13 +85,23 @@ class AsyncGenericService final {
 
 namespace experimental {
 
+/// \a ServerGenericBidiReactor is the reactor class for bidi streaming RPCs
+/// invoked on a CallbackGenericService. The API difference relative to
+/// ServerBidiReactor is that the argument to OnStarted is a
+/// GenericServerContext rather than a ServerContext. All other reaction and
+/// operation initiation APIs are the same as ServerBidiReactor.
 class ServerGenericBidiReactor
     : public ServerBidiReactor<ByteBuffer, ByteBuffer> {
  public:
+  /// Similar to ServerBidiReactor::OnStarted except for argument type.
+  ///
+  /// \param[in] context The context object associated with this RPC.
+  virtual void OnStarted(GenericServerContext* context) {}
+
+ private:
   void OnStarted(ServerContext* ctx) final {
     OnStarted(static_cast<GenericServerContext*>(ctx));
   }
-  virtual void OnStarted(GenericServerContext* ctx) {}
 };
 
 }  // namespace experimental
@@ -108,10 +118,18 @@ class UnimplementedGenericBidiReactor
 }  // namespace internal
 
 namespace experimental {
+
+/// \a CallbackGenericService is the base class for generic services implemented
+/// using the callback API and registered through the ServerBuilder using
+/// RegisterCallbackGenericService.
 class CallbackGenericService {
  public:
   CallbackGenericService() {}
   virtual ~CallbackGenericService() {}
+
+  /// The "method handler" for the generic API. This function should be
+  /// overridden to return a ServerGenericBidiReactor that implements the
+  /// application-level interface for this RPC.
   virtual ServerGenericBidiReactor* CreateReactor() {
     return new internal::UnimplementedGenericBidiReactor;
   }

include/grpcpp/impl/codegen/client_callback.h

@@ -157,28 +157,69 @@ class ClientCallbackWriter {
   }
 };
 
-// The user must implement this reactor interface with reactions to each event
-// type that gets called by the library. An empty reaction is provided by
-// default
+// The following classes are the reactor interfaces that are to be implemented
+// by the user. They are passed in to the library as an argument to a call on a
+// stub (either a codegen-ed call or a generic call). The streaming RPC is
+// activated by calling StartCall, possibly after initiating StartRead,
+// StartWrite, or AddHold operations on the streaming object. Note that none of
+// the classes are pure; all reactions have a default empty reaction so that the
+// user class only needs to override those classes that it cares about.
+
+/// \a ClientBidiReactor is the interface for a bidirectional streaming RPC.
 template <class Request, class Response>
 class ClientBidiReactor {
  public:
   virtual ~ClientBidiReactor() {}
-  virtual void OnDone(const Status& s) {}
-  virtual void OnReadInitialMetadataDone(bool ok) {}
-  virtual void OnReadDone(bool ok) {}
-  virtual void OnWriteDone(bool ok) {}
-  virtual void OnWritesDoneDone(bool ok) {}
 
+  /// Activate the RPC and initiate any reads or writes that have been Start'ed
+  /// before this call. All streaming RPCs issued by the client MUST have
+  /// StartCall invoked on them (even if they are canceled) as this call is the
+  /// activation of their lifecycle.
   void StartCall() { stream_->StartCall(); }
+
+  /// Initiate a read operation (or post it for later initiation if StartCall
+  /// has not yet been invoked).
+  ///
+  /// \param[out] resp Where to eventually store the read message. Valid when
+  ///                  the library calls OnReadDone
   void StartRead(Response* resp) { stream_->Read(resp); }
+
+  /// Initiate a write operation (or post it for later initiation if StartCall
+  /// has not yet been invoked).
+  ///
+  /// \param[in] req The message to be written. The library takes temporary
+  ///                ownership until OnWriteDone, at which point the application
+  ///                regains ownership of msg.
   void StartWrite(const Request* req) { StartWrite(req, WriteOptions()); }
+
+  /// Initiate/post a write operation with specified options.
+  ///
+  /// \param[in] req The message to be written. The library takes temporary
+  ///                ownership until OnWriteDone, at which point the application
+  ///                regains ownership of msg.
+  /// \param[in] options The WriteOptions to use for writing this message
   void StartWrite(const Request* req, WriteOptions options) {
     stream_->Write(req, std::move(options));
   }
+
+  /// Initiate/post a write operation with specified options and an indication
+  /// that this is the last write (like StartWrite and StartWritesDone, merged).
+  /// Note that calling this means that no more calls to StartWrite,
+  /// StartWriteLast, or StartWritesDone are allowed.
+  ///
+  /// \param[in] req The message to be written. The library takes temporary
+  ///                ownership until OnWriteDone, at which point the application
+  ///                regains ownership of msg.
+  /// \param[in] options The WriteOptions to use for writing this message
   void StartWriteLast(const Request* req, WriteOptions options) {
     StartWrite(req, std::move(options.set_last_message()));
   }
+
+  /// Indicate that the RPC will have no more write operations. This can only be
+  /// issued once for a given RPC. This is not required or allowed if
+  /// StartWriteLast is used since that already has the same implication.
+  /// Note that calling this means that no more calls to StartWrite,
+  /// StartWriteLast, or StartWritesDone are allowed.
   void StartWritesDone() { stream_->WritesDone(); }
 
   /// Holds are needed if (and only if) this stream has operations that take
@@ -196,14 +237,51 @@ class ClientBidiReactor {
   /// AddHold or AddMultipleHolds before StartCall.  If there is going to be,
   /// for example, a read-flow and a write-flow taking place outside the
   /// reactions, then call AddMultipleHolds(2) before StartCall. When the
-  /// application knows that it won't issue any more Read operations (such as
+  /// application knows that it won't issue any more read operations (such as
   /// when a read comes back as not ok), it should issue a RemoveHold(). It
   /// should also call RemoveHold() again after it does StartWriteLast or
-  /// StartWritesDone that indicates that there will be no more Write ops.
+  /// StartWritesDone that indicates that there will be no more write ops.
+  /// The number of RemoveHold calls must match the total number of AddHold
+  /// calls plus the number of holds added by AddMultipleHolds.
   void AddHold() { AddMultipleHolds(1); }
   void AddMultipleHolds(int holds) { stream_->AddHold(holds); }
   void RemoveHold() { stream_->RemoveHold(); }
 
+  /// Notifies the application that all operations associated with this RPC
+  /// have completed and provides the RPC status outcome.
+  ///
+  /// \param[in] s The status outcome of this RPC
+  virtual void OnDone(const Status& s) {}
+
+  /// Notifies the application that a read of initial metadata from the
+  /// server is done. If the application chooses not to implement this method,
+  /// it can assume that the initial metadata has been read before the first
+  /// call of OnReadDone or OnDone.
+  ///
+  /// \param[in] ok Was the initial metadata read successfully? If false, no
+  ///               further read-side operation will succeed.
+  virtual void OnReadInitialMetadataDone(bool ok) {}
+
+  /// Notifies the application that a StartRead operation completed.
+  ///
+  /// \param[in] ok Was it successful? If false, no further read-side operation
+  ///               will succeed.
+  virtual void OnReadDone(bool ok) {}
+
+  /// Notifies the application that a StartWrite operation completed.
+  ///
+  /// \param[in] ok Was it successful? If false, no further write-side operation
+  ///               will succeed.
+  virtual void OnWriteDone(bool ok) {}
+
+  /// Notifies the application that a StartWritesDone operation completed. Note
+  /// that this is only used on explicit StartWritesDone operations and not for
+  /// those that are implicitly invoked as part of a StartWriteLast.
+  ///
+  /// \param[in] ok Was it successful? If false, the application will later see
+  ///               the failure reflected as a bad status in OnDone.
+  virtual void OnWritesDoneDone(bool ok) {}
+
  private:
   friend class ClientCallbackReaderWriter<Request, Response>;
   void BindStream(ClientCallbackReaderWriter<Request, Response>* stream) {
@@ -212,13 +290,12 @@ class ClientBidiReactor {
   ClientCallbackReaderWriter<Request, Response>* stream_;
 };
 
+/// \a ClientReadReactor is the interface for a server-streaming RPC.
+/// All public methods behave as in ClientBidiReactor.
 template <class Response>
 class ClientReadReactor {
  public:
   virtual ~ClientReadReactor() {}
-  virtual void OnDone(const Status& s) {}
-  virtual void OnReadInitialMetadataDone(bool ok) {}
-  virtual void OnReadDone(bool ok) {}
 
   void StartCall() { reader_->StartCall(); }
   void StartRead(Response* resp) { reader_->Read(resp); }
@@ -227,20 +304,22 @@ class ClientReadReactor {
   void AddMultipleHolds(int holds) { reader_->AddHold(holds); }
   void RemoveHold() { reader_->RemoveHold(); }
 
+  virtual void OnDone(const Status& s) {}
+  virtual void OnReadInitialMetadataDone(bool ok) {}
+  virtual void OnReadDone(bool ok) {}
+
  private:
   friend class ClientCallbackReader<Response>;
   void BindReader(ClientCallbackReader<Response>* reader) { reader_ = reader; }
   ClientCallbackReader<Response>* reader_;
 };
 
+/// \a ClientWriteReactor is the interface for a client-streaming RPC.
+/// All public methods behave as in ClientBidiReactor.
 template <class Request>
 class ClientWriteReactor {
  public:
   virtual ~ClientWriteReactor() {}
-  virtual void OnDone(const Status& s) {}
-  virtual void OnReadInitialMetadataDone(bool ok) {}
-  virtual void OnWriteDone(bool ok) {}
-  virtual void OnWritesDoneDone(bool ok) {}
 
   void StartCall() { writer_->StartCall(); }
   void StartWrite(const Request* req) { StartWrite(req, WriteOptions()); }
@@ -256,6 +335,11 @@ class ClientWriteReactor {
   void AddMultipleHolds(int holds) { writer_->AddHold(holds); }
   void RemoveHold() { writer_->RemoveHold(); }
 
+  virtual void OnDone(const Status& s) {}
+  virtual void OnReadInitialMetadataDone(bool ok) {}
+  virtual void OnWriteDone(bool ok) {}
+  virtual void OnWritesDoneDone(bool ok) {}
+
  private:
   friend class ClientCallbackWriter<Request>;
   void BindWriter(ClientCallbackWriter<Request>* writer) { writer_ = writer; }

include/grpcpp/impl/codegen/server_callback.h

@@ -40,7 +40,12 @@ namespace internal {
 class ServerReactor {
  public:
   virtual ~ServerReactor() = default;
+
+  /// Notifies the application that all operations associated with this RPC
+  /// have completed.
   virtual void OnDone() {}
+
+  /// Notifies the application that this RPC has been cancelled.
   virtual void OnCancel() {}
 };
 
@@ -167,33 +172,110 @@ class ServerCallbackReaderWriter {
   }
 };
 
-// The following classes are reactors that are to be implemented
-// by the user, returned as the result of the method handler for
-// a callback method, and activated by the call to OnStarted
+// The following classes are the reactor interfaces that are to be implemented
+// by the user, returned as the result of the method handler for a callback
+// method, and activated by the call to OnStarted. Note that none of the classes
+// are pure; all reactions have a default empty reaction so that the user class
+// only needs to override those classes that it cares about.
+
+/// \a ServerBidiReactor is the interface for a bidirectional streaming RPC.
 template <class Request, class Response>
 class ServerBidiReactor : public internal::ServerReactor {
  public:
   ~ServerBidiReactor() = default;
-  virtual void OnStarted(ServerContext*) {}
-  virtual void OnSendInitialMetadataDone(bool ok) {}
-  virtual void OnReadDone(bool ok) {}
-  virtual void OnWriteDone(bool ok) {}
 
+  /// Send any initial metadata stored in the RPC context. If not invoked,
+  /// any initial metadata will be passed along with the first Write or the
+  /// Finish (if there are no writes).
   void StartSendInitialMetadata() { stream_->SendInitialMetadata(); }
-  void StartRead(Request* msg) { stream_->Read(msg); }
-  void StartWrite(const Response* msg) { StartWrite(msg, WriteOptions()); }
-  void StartWrite(const Response* msg, WriteOptions options) {
-    stream_->Write(msg, std::move(options));
-  }
-  void StartWriteAndFinish(const Response* msg, WriteOptions options,
+
+  /// Initiate a read operation.
+  ///
+  /// \param[out] req Where to eventually store the read message. Valid when
+  ///                 the library calls OnReadDone
+  void StartRead(Request* req) { stream_->Read(req); }
+
+  /// Initiate a write operation.
+  ///
+  /// \param[in] resp The message to be written. The library takes temporary
+  ///                 ownership until OnWriteDone, at which point the
+  ///                 application regains ownership of resp.
+  void StartWrite(const Response* resp) { StartWrite(resp, WriteOptions()); }
+
+  /// Initiate a write operation with specified options.
+  ///
+  /// \param[in] resp The message to be written. The library takes temporary
+  ///                 ownership until OnWriteDone, at which point the
+  ///                 application regains ownership of resp.
+  /// \param[in] options The WriteOptions to use for writing this message
+  void StartWrite(const Response* resp, WriteOptions options) {
+    stream_->Write(resp, std::move(options));
+  }
+
+  /// Initiate a write operation with specified options and final RPC Status,
+  /// which also causes any trailing metadata for this RPC to be sent out.
+  /// StartWriteAndFinish is like merging StartWriteLast and Finish into a
+  /// single step. A key difference, though, is that this operation doesn't have
+  /// an OnWriteDone reaction - it is considered complete only when OnDone is
+  /// available. An RPC can either have StartWriteAndFinish or Finish, but not
+  /// both.
+  ///
+  /// \param[in] resp The message to be written. The library takes temporary
+  ///                 ownership until Onone, at which point the application
+  ///                 regains ownership of resp.
+  /// \param[in] options The WriteOptions to use for writing this message
+  /// \param[in] s The status outcome of this RPC
+  void StartWriteAndFinish(const Response* resp, WriteOptions options,
                            Status s) {
-    stream_->WriteAndFinish(msg, std::move(options), std::move(s));
-  }
-  void StartWriteLast(const Response* msg, WriteOptions options) {
-    StartWrite(msg, std::move(options.set_last_message()));
+    stream_->WriteAndFinish(resp, std::move(options), std::move(s));
   }
+
+  /// Inform system of a planned write operation with specified options, but
+  /// allow the library to schedule the actual write coalesced with the writing
+  /// of trailing metadata (which takes place on a Finish call).
+  ///
+  /// \param[in] resp The message to be written. The library takes temporary
+  ///                 ownership until OnWriteDone, at which point the
+  ///                 application regains ownership of resp.
+  /// \param[in] options The WriteOptions to use for writing this message
+  void StartWriteLast(const Response* resp, WriteOptions options) {
+    StartWrite(resp, std::move(options.set_last_message()));
+  }
+
+  /// Indicate that the stream is to be finished and the trailing metadata and
+  /// RPC status are to be sent. Every RPC MUST be finished using either Finish
+  /// or StartWriteAndFinish (but not both), even if the RPC is already
+  /// cancelled.
+  ///
+  /// \param[in] s The status outcome of this RPC
   void Finish(Status s) { stream_->Finish(std::move(s)); }
 
+  /// Notify the application that a streaming RPC has started
+  ///
+  /// \param[in] context The context object now associated with this RPC
+  virtual void OnStarted(ServerContext* context) {}
+
+  /// Notifies the application that an explicit StartSendInitialMetadata
+  /// operation completed. Not used when the sending of initial metadata
+  /// piggybacks onto the first write.
+  ///
+  /// \param[in] ok Was it successful? If false, no further write-side operation
+  ///               will succeed.
+  virtual void OnSendInitialMetadataDone(bool ok) {}
+
+  /// Notifies the application that a StartRead operation completed.
+  ///
+  /// \param[in] ok Was it successful? If false, no further read-side operation
+  ///               will succeed.
+  virtual void OnReadDone(bool ok) {}
+
+  /// Notifies the application that a StartWrite (or StartWriteLast) operation
+  /// completed.
+  ///
+  /// \param[in] ok Was it successful? If false, no further write-side operation
+  ///               will succeed.
+  virtual void OnWriteDone(bool ok) {}
+
  private:
   friend class ServerCallbackReaderWriter<Request, Response>;
   void BindStream(ServerCallbackReaderWriter<Request, Response>* stream) {
@@ -203,18 +285,29 @@ class ServerBidiReactor : public internal::ServerReactor {
   ServerCallbackReaderWriter<Request, Response>* stream_;
 };
 
+/// \a ServerReadReactor is the interface for a client-streaming RPC.
 template <class Request, class Response>
 class ServerReadReactor : public internal::ServerReactor {
  public:
   ~ServerReadReactor() = default;
-  virtual void OnStarted(ServerContext*, Response* resp) {}
-  virtual void OnSendInitialMetadataDone(bool ok) {}
-  virtual void OnReadDone(bool ok) {}
 
+  /// The following operation initiations are exactly like ServerBidiReactor.
   void StartSendInitialMetadata() { reader_->SendInitialMetadata(); }
-  void StartRead(Request* msg) { reader_->Read(msg); }
+  void StartRead(Request* req) { reader_->Read(req); }
   void Finish(Status s) { reader_->Finish(std::move(s)); }
 
+  /// Similar to ServerBidiReactor::OnStarted, except that this also provides
+  /// the response object that the stream fills in before calling Finish.
+  /// (It must be filled in if status is OK, but it may be filled in otherwise.)
+  ///
+  /// \param[in] context The context object now associated with this RPC
+  /// \param[in] resp The response object to be used by this RPC
+  virtual void OnStarted(ServerContext* context, Response* resp) {}
+
+  /// The following notifications are exactly like ServerBidiReactor.
+  virtual void OnSendInitialMetadataDone(bool ok) {}
+  virtual void OnReadDone(bool ok) {}
+
  private:
   friend class ServerCallbackReader<Request>;
   void BindReader(ServerCallbackReader<Request>* reader) { reader_ = reader; }
@@ -222,28 +315,38 @@ class ServerReadReactor : public internal::ServerReactor {
   ServerCallbackReader<Request>* reader_;
 };
 
+/// \a ServerReadReactor is the interface for a server-streaming RPC.
 template <class Request, class Response>
 class ServerWriteReactor : public internal::ServerReactor {
  public:
   ~ServerWriteReactor() = default;
-  virtual void OnStarted(ServerContext*, const Request* req) {}
-  virtual void OnSendInitialMetadataDone(bool ok) {}
-  virtual void OnWriteDone(bool ok) {}
 
+  /// The following operation initiations are exactly like ServerBidiReactor.
   void StartSendInitialMetadata() { writer_->SendInitialMetadata(); }
-  void StartWrite(const Response* msg) { StartWrite(msg, WriteOptions()); }
-  void StartWrite(const Response* msg, WriteOptions options) {
-    writer_->Write(msg, std::move(options));
+  void StartWrite(const Response* resp) { StartWrite(resp, WriteOptions()); }
+  void StartWrite(const Response* resp, WriteOptions options) {
+    writer_->Write(resp, std::move(options));
   }
-  void StartWriteAndFinish(const Response* msg, WriteOptions options,
+  void StartWriteAndFinish(const Response* resp, WriteOptions options,
                            Status s) {
-    writer_->WriteAndFinish(msg, std::move(options), std::move(s));
+    writer_->WriteAndFinish(resp, std::move(options), std::move(s));
   }
-  void StartWriteLast(const Response* msg, WriteOptions options) {
-    StartWrite(msg, std::move(options.set_last_message()));
+  void StartWriteLast(const Response* resp, WriteOptions options) {
+    StartWrite(resp, std::move(options.set_last_message()));
   }
   void Finish(Status s) { writer_->Finish(std::move(s)); }
 
+  /// Similar to ServerBidiReactor::OnStarted, except that this also provides
+  /// the request object sent by the client.
+  ///
+  /// \param[in] context The context object now associated with this RPC
+  /// \param[in] req The request object sent by the client
+  virtual void OnStarted(ServerContext* context, const Request* req) {}
+
+  /// The following notifications are exactly like ServerBidiReactor.
+  virtual void OnSendInitialMetadataDone(bool ok) {}
+  virtual void OnWriteDone(bool ok) {}
+
  private:
   friend class ServerCallbackWriter<Response>;
   void BindWriter(ServerCallbackWriter<Response>* writer) { writer_ = writer; }

include/grpcpp/server_builder.h

@@ -231,6 +231,10 @@ class ServerBuilder {
       builder_->interceptor_creators_ = std::move(interceptor_creators);
     }
 
+    /// Register a generic service that uses the callback API.
+    /// Matches requests with any :authority
+    /// This is mostly useful for writing generic gRPC Proxies where the exact
+    /// serialization format is unknown
     ServerBuilder& RegisterCallbackGenericService(
         experimental::CallbackGenericService* service);