[Example] Added gRPC C++ generic API example (#35411)

Closes #35411

PiperOrigin-RevId: 599930218

examples/cpp/generic_api/BUILD

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+# Copyright 2023 the 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.
+
+licenses(["notice"])
+
+cc_binary(
+    name = "greeter_client",
+    srcs = ["greeter_client.cc"],
+    defines = ["BAZEL_BUILD"],
+    deps = [
+        "//:grpc++",
+        "//examples/protos:helloworld_cc_grpc",
+        "@com_google_absl//absl/flags:flag",
+        "@com_google_absl//absl/flags:parse",
+    ],
+)
+
+cc_binary(
+    name = "greeter_server",
+    srcs = ["greeter_server.cc"],
+    defines = ["BAZEL_BUILD"],
+    deps = [
+        "//:grpc++",
+        "//examples/protos:helloworld_cc_grpc",
+        "@com_google_absl//absl/flags:flag",
+        "@com_google_absl//absl/flags:parse",
+        "@com_google_absl//absl/strings:str_format",
+    ],
+)

examples/cpp/generic_api/CMakeLists.txt

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+# Copyright 2023 the 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.
+#
+# cmake build file for C++ helloworld example.
+# Assumes protobuf and gRPC have been installed using cmake.
+# See cmake_externalproject/CMakeLists.txt for all-in-one cmake build
+# that automatically builds all the dependencies before building helloworld.
+
+cmake_minimum_required(VERSION 3.8)
+
+project(GenericAPI C CXX)
+
+include(../cmake/common.cmake)
+
+# Proto files
+get_filename_component(hw_proto "../../protos/helloworld.proto" ABSOLUTE)
+get_filename_component(hw_proto_path "${hw_proto}" PATH)
+
+# Generated sources
+set(hw_proto_srcs "${CMAKE_CURRENT_BINARY_DIR}/helloworld.pb.cc")
+set(hw_proto_hdrs "${CMAKE_CURRENT_BINARY_DIR}/helloworld.pb.h")
+set(hw_grpc_srcs "${CMAKE_CURRENT_BINARY_DIR}/helloworld.grpc.pb.cc")
+set(hw_grpc_hdrs "${CMAKE_CURRENT_BINARY_DIR}/helloworld.grpc.pb.h")
+add_custom_command(
+      OUTPUT "${hw_proto_srcs}" "${hw_proto_hdrs}" "${hw_grpc_srcs}" "${hw_grpc_hdrs}"
+      COMMAND ${_PROTOBUF_PROTOC}
+      ARGS --grpc_out "${CMAKE_CURRENT_BINARY_DIR}"
+        --cpp_out "${CMAKE_CURRENT_BINARY_DIR}"
+        -I "${hw_proto_path}"
+        --plugin=protoc-gen-grpc="${_GRPC_CPP_PLUGIN_EXECUTABLE}"
+        "${hw_proto}"
+      DEPENDS "${hw_proto}")
+
+# Include generated *.pb.h files
+include_directories("${CMAKE_CURRENT_BINARY_DIR}")
+
+# hw_grpc_proto
+add_library(hw_grpc_proto
+  ${hw_grpc_srcs}
+  ${hw_grpc_hdrs}
+  ${hw_proto_srcs}
+  ${hw_proto_hdrs})
+target_link_libraries(hw_grpc_proto
+  ${_REFLECTION}
+  ${_GRPC_GRPCPP}
+  ${_PROTOBUF_LIBPROTOBUF})
+
+# Targets greeter_(client|server)
+foreach(_target
+  greeter_client greeter_server)
+  add_executable(${_target} "${_target}.cc")
+  target_link_libraries(${_target}
+    hw_grpc_proto
+    absl::flags
+    absl::flags_parse
+    ${_REFLECTION}
+    ${_GRPC_GRPCPP}
+    ${_PROTOBUF_LIBPROTOBUF})
+endforeach()

examples/cpp/generic_api/README.md

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+# Generic API Example
+
+## Overview
+
+While generated stub code is often the simpler and best choice for sending and handling API calls,
+generic APIs offer unique advantages in specific scenarios, such as proxy implementation.
+Their ability to manage multiple message types with a single function makes them particularly handy
+in these cases. This example demonstrates how to use generic APIs to achieve this flexibility.
+
+This example implements `greeter_callback_client` and `greeter_callback_server` using the generic APIs.
+Therefore, looking at the difference would be helpful to understand how to use generic APIs.
+
+### Try it!
+
+Once you have working gRPC, you can build this example using either bazel or cmake.
+
+Run the server, which will listen on port 50051:
+
+```sh
+$ ./greeter_server
+```
+
+Run the client (in a different terminal):
+
+```sh
+$ ./greeter_client
+```
+
+If things go smoothly, you will see the client output:
+
+```
+### Send: SayHello(name=World)
+Ok. ReplyMessage=Hello World
+### Send: SayHello(name=gRPC)
+Ok. ReplyMessage=Hello gRPC
+```

examples/cpp/generic_api/greeter_client.cc

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+// Copyright 2023 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 <condition_variable>
+#include <iostream>
+#include <memory>
+#include <mutex>
+#include <string>
+
+#include "absl/flags/flag.h"
+#include "absl/flags/parse.h"
+#include "absl/strings/str_format.h"
+
+#include <grpcpp/generic/generic_stub.h>
+#include <grpcpp/grpcpp.h>
+
+#ifdef BAZEL_BUILD
+#include "examples/protos/helloworld.grpc.pb.h"
+#else
+#include "helloworld.grpc.pb.h"
+#endif
+
+ABSL_FLAG(std::string, target, "localhost:50051", "Server address");
+
+using grpc::Channel;
+using grpc::ClientContext;
+using grpc::Status;
+using helloworld::Greeter;
+using helloworld::HelloReply;
+using helloworld::HelloRequest;
+
+using ProtoGenericStub =
+    ::grpc::TemplatedGenericStub<::google::protobuf::Message,
+                                 ::google::protobuf::Message>;
+
+class GreeterClient {
+ public:
+  GreeterClient(std::shared_ptr<Channel> channel)
+      : stub_(new ProtoGenericStub(channel)) {}
+
+  // Assembles the client's payload, sends it and prints the response back
+  // from the server.
+  void SayHello(const std::string& user) {
+    // Data we are sending to the server.
+    HelloRequest request;
+    request.set_name(user);
+    // Container for the data we expect from the server.
+    HelloReply reply;
+    // Context for the client. It could be used to convey extra information to
+    // the server and/or tweak certain RPC behaviors.
+    ClientContext context;
+    // The actual RPC.
+    std::mutex mu;
+    std::condition_variable cv;
+    bool done = false;
+    Status status;
+    std::cout << absl::StrFormat("### Send: SayHello(name=%s)", user)
+              << std::endl;
+    // Send a unary call using a generic stub. Unlike generated subs,
+    // this requires to specify the name of call.
+    stub_->UnaryCall(&context, "/helloworld.Greeter/SayHello",
+                     grpc::StubOptions(), &request, &reply, [&](Status s) {
+                       status = std::move(s);
+                       std::lock_guard<std::mutex> lock(mu);
+                       done = true;
+                       cv.notify_one();
+                     });
+    std::unique_lock<std::mutex> lock(mu);
+    while (!done) {
+      cv.wait(lock);
+    }
+    // Handles the reply
+    if (status.ok()) {
+      std::cout << absl::StrFormat("Ok. ReplyMessage=%s", reply.message())
+                << std::endl;
+    } else {
+      std::cout << absl::StrFormat("Failed. Code=%d Message=%s",
+                                   status.error_code(), status.error_message())
+                << std::endl;
+    }
+  }
+
+ private:
+  // Instead of `Greeter::Stub`, it uses `ProtoGenericStub` to send any calls.
+  std::unique_ptr<ProtoGenericStub> stub_;
+};
+
+int main(int argc, char** argv) {
+  absl::ParseCommandLine(argc, argv);
+  // Instantiate the client. It requires a channel, out of which the actual RPCs
+  // are created. This channel models a connection to an endpoint specified by
+  // the argument "--target=" which is the only expected argument.
+  std::string target_str = absl::GetFlag(FLAGS_target);
+  // We indicate that the channel isn't authenticated (use of
+  // InsecureChannelCredentials()).
+  GreeterClient greeter(
+      grpc::CreateChannel(target_str, grpc::InsecureChannelCredentials()));
+  greeter.SayHello("World");
+  greeter.SayHello("gRPC");
+  return 0;
+}

examples/cpp/generic_api/greeter_server.cc

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+// Copyright 2023 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 <iostream>
+#include <memory>
+#include <string>
+#include <unordered_set>
+
+#include "absl/flags/flag.h"
+#include "absl/flags/parse.h"
+#include "absl/strings/str_format.h"
+#include "absl/synchronization/mutex.h"
+
+#include <grpcpp/grpcpp.h>
+#include <grpcpp/health_check_service_interface.h>
+
+#ifdef BAZEL_BUILD
+#include "examples/protos/helloworld.grpc.pb.h"
+#else
+#include "helloworld.grpc.pb.h"
+#endif
+
+ABSL_FLAG(uint16_t, port, 50051, "Server port for the service");
+
+using grpc::ByteBuffer;
+using grpc::CallbackGenericService;
+using grpc::CallbackServerContext;
+using grpc::GenericCallbackServerContext;
+using grpc::ProtoBufferReader;
+using grpc::ProtoBufferWriter;
+using grpc::Server;
+using grpc::ServerBuilder;
+using grpc::ServerGenericBidiReactor;
+using grpc::Status;
+using grpc::StatusCode;
+using helloworld::HelloReply;
+using helloworld::HelloRequest;
+
+// Logic and data behind the server's behavior.
+class GreeterServiceImpl final : public CallbackGenericService {
+  ServerGenericBidiReactor* CreateReactor(
+      GenericCallbackServerContext* context) override {
+    if (context->method() == "/helloworld.Greeter/SayHello") {
+      // Let the SayHello reactor handle this now on.
+      return new SayHelloReactor();
+    } else {
+      // Forward this to the implementation of the base calss returning
+      // UNIMPLEMENTED.
+      return CallbackGenericService::CreateReactor(context);
+    }
+  }
+
+  class SayHelloReactor : public ServerGenericBidiReactor {
+   public:
+    SayHelloReactor() { StartRead(&request_); }
+
+   private:
+    Status OnSayHello(const HelloRequest& request, HelloReply* reply) {
+      if (request.name() == "") {
+        return Status(StatusCode::INVALID_ARGUMENT, "name is not specified");
+      }
+      reply->set_message(absl::StrFormat("Hello %s", request.name()));
+      return Status::OK;
+    }
+
+    void OnDone() override { delete this; }
+    void OnReadDone(bool ok) override {
+      if (!ok) {
+        return;
+      }
+      Status result;
+      // Deserialize a request message
+      HelloRequest request;
+      result = grpc::GenericDeserialize<ProtoBufferReader, HelloRequest>(
+          &request_, &request);
+      if (!result.ok()) {
+        Finish(result);
+        return;
+      }
+      // Call the SayHello handler
+      HelloReply reply;
+      result = OnSayHello(request, &reply);
+      if (!result.ok()) {
+        Finish(result);
+        return;
+      }
+      // Serialize a reply message
+      bool own_buffer;
+      result = grpc::GenericSerialize<ProtoBufferWriter, HelloReply>(
+          reply, &response_, &own_buffer);
+      if (!result.ok()) {
+        Finish(result);
+        return;
+      }
+      StartWrite(&response_);
+    }
+    void OnWriteDone(bool ok) override {
+      Finish(ok ? Status::OK
+                : Status(StatusCode::UNKNOWN, "Unexpected failure"));
+    }
+    ByteBuffer request_;
+    ByteBuffer response_;
+  };
+
+ private:
+  absl::Mutex mu_;
+};
+
+void RunServer(uint16_t port) {
+  std::string server_address = absl::StrFormat("0.0.0.0:%d", port);
+  GreeterServiceImpl service;
+  grpc::EnableDefaultHealthCheckService(true);
+  ServerBuilder builder;
+  // Listen on the given address without any authentication mechanism.
+  builder.AddListeningPort(server_address, grpc::InsecureServerCredentials());
+  // Register "service" as the instance through which we'll communicate with
+  // clients. In this case it corresponds to an *synchronous* service.
+  builder.RegisterCallbackGenericService(&service);
+  // Finally assemble the server.
+  std::unique_ptr<Server> server(builder.BuildAndStart());
+  std::cout << "Server listening on " << server_address << std::endl;
+
+  // Wait for the server to shutdown. Note that some other thread must be
+  // responsible for shutting down the server for this call to ever return.
+  server->Wait();
+}
+
+int main(int argc, char** argv) {
+  absl::ParseCommandLine(argc, argv);
+  RunServer(absl::GetFlag(FLAGS_port));
+  return 0;
+}