Initial checkin of gRPC to third_party/

third_party/grpc/examples/cpp/helloworld/README.md

+# gRPC C++ Hello World Tutorial
+
+### Install gRPC
+Make sure you have installed gRPC on your system. Follow the instructions here:
+[https://github.com/grpc/grpc/blob/master/INSTALL](../../../INSTALL.md).
+
+### Get the tutorial source code
+
+The example code for this and our other examples lives in the `examples`
+directory. Clone this repository to your local machine by running the
+following command:
+
+
+```sh
+$ git clone https://github.com/grpc/grpc.git
+```
+
+Change your current directory to examples/cpp/helloworld
+
+```sh
+$ cd examples/cpp/helloworld/
+```
+
+### Defining a service
+
+The first step in creating our example is to define a *service*: an RPC
+service specifies the methods that can be called remotely with their parameters
+and return types. As you saw in the
+[overview](#protocolbuffers) above, gRPC does this using [protocol
+buffers](https://developers.google.com/protocol-buffers/docs/overview). We
+use the protocol buffers interface definition language (IDL) to define our
+service methods, and define the parameters and return
+types as protocol buffer message types. Both the client and the
+server use interface code generated from the service definition.
+
+Here's our example service definition, defined using protocol buffers IDL in
+[helloworld.proto](../../protos/helloworld.proto). The `Greeting`
+service has one method, `hello`, that lets the server receive a single
+`HelloRequest`
+message from the remote client containing the user's name, then send back
+a greeting in a single `HelloReply`. This is the simplest type of RPC you
+can specify in gRPC - we'll look at some other types later in this document.
+
+```protobuf
+syntax = "proto3";
+
+option java_package = "ex.grpc";
+
+package helloworld;
+
+// The greeting service definition.
+service Greeter {
+  // Sends a greeting
+  rpc SayHello (HelloRequest) returns (HelloReply) {}
+}
+
+// The request message containing the user's name.
+message HelloRequest {
+  string name = 1;
+}
+
+// The response message containing the greetings
+message HelloReply {
+  string message = 1;
+}
+
+```
+
+<a name="generating"></a>
+### Generating gRPC code
+
+Once we've defined our service, we use the protocol buffer compiler
+`protoc` to generate the special client and server code we need to create
+our application. The generated code contains both stub code for clients to
+use and an abstract interface for servers to implement, both with the method
+defined in our `Greeting` service.
+
+To generate the client and server side interfaces:
+
+```sh
+$ make helloworld.grpc.pb.cc helloworld.pb.cc
+```
+Which internally invokes the proto-compiler as:
+
+```sh
+$ protoc -I ../../protos/ --grpc_out=. --plugin=protoc-gen-grpc=grpc_cpp_plugin ../../protos/helloworld.proto
+$ protoc -I ../../protos/ --cpp_out=. ../../protos/helloworld.proto
+```
+
+### Writing a client
+
+- Create a channel. A channel is a logical connection to an endpoint. A gRPC
+  channel can be created with the target address, credentials to use and
+  arguments as follows
+
+    ```cpp
+    auto channel = CreateChannel("localhost:50051", InsecureChannelCredentials());
+    ```
+
+- Create a stub. A stub implements the rpc methods of a service and in the
+  generated code, a method is provided to created a stub with a channel:
+
+    ```cpp
+    auto stub = helloworld::Greeter::NewStub(channel);
+    ```
+
+- Make a unary rpc, with `ClientContext` and request/response proto messages.
+
+    ```cpp
+    ClientContext context;
+    HelloRequest request;
+    request.set_name("hello");
+    HelloReply reply;
+    Status status = stub->SayHello(&context, request, &reply);
+    ```
+
+- Check returned status and response.
+
+    ```cpp
+    if (status.ok()) {
+      // check reply.message()
+    } else {
+      // rpc failed.
+    }
+    ```
+
+For a working example, refer to [greeter_client.cc](greeter_client.cc).
+
+### Writing a server
+
+- Implement the service interface
+
+    ```cpp
+    class GreeterServiceImpl final : public Greeter::Service {
+      Status SayHello(ServerContext* context, const HelloRequest* request,
+          HelloReply* reply) override {
+        std::string prefix("Hello ");
+        reply->set_message(prefix + request->name());
+        return Status::OK;
+      }
+    };
+
+    ```
+
+- Build a server exporting the service
+
+    ```cpp
+    GreeterServiceImpl service;
+    ServerBuilder builder;
+    builder.AddListeningPort("0.0.0.0:50051", grpc::InsecureServerCredentials());
+    builder.RegisterService(&service);
+    std::unique_ptr<Server> server(builder.BuildAndStart());
+    ```
+
+For a working example, refer to [greeter_server.cc](greeter_server.cc).
+
+### Writing asynchronous client and server
+
+gRPC uses `CompletionQueue` API for asynchronous operations. The basic work flow
+is
+- bind a `CompletionQueue` to a rpc call
+- do something like a read or write, present with a unique `void*` tag
+- call `CompletionQueue::Next` to wait for operations to complete. If a tag
+  appears, it indicates that the corresponding operation is complete.
+
+#### Async client
+
+The channel and stub creation code is the same as the sync client.
+
+- Initiate the rpc and create a handle for the rpc. Bind the rpc to a
+  `CompletionQueue`.
+
+    ```cpp
+    CompletionQueue cq;
+    auto rpc = stub->AsyncSayHello(&context, request, &cq);
+    ```
+
+- Ask for reply and final status, with a unique tag
+
+    ```cpp
+    Status status;
+    rpc->Finish(&reply, &status, (void*)1);
+    ```
+
+- Wait for the completion queue to return the next tag. The reply and status are
+  ready once the tag passed into the corresponding `Finish()` call is returned.
+
+    ```cpp
+    void* got_tag;
+    bool ok = false;
+    cq.Next(&got_tag, &ok);
+    if (ok && got_tag == (void*)1) {
+      // check reply and status
+    }
+    ```
+
+For a working example, refer to [greeter_async_client.cc](greeter_async_client.cc).
+
+#### Async server
+
+The server implementation requests a rpc call with a tag and then wait for the
+completion queue to return the tag. The basic flow is
+
+- Build a server exporting the async service
+
+    ```cpp
+    helloworld::Greeter::AsyncService service;
+    ServerBuilder builder;
+    builder.AddListeningPort("0.0.0.0:50051", InsecureServerCredentials());
+    builder.RegisterAsyncService(&service);
+    auto cq = builder.AddCompletionQueue();
+    auto server = builder.BuildAndStart();
+    ```
+
+- Request one rpc
+
+    ```cpp
+    ServerContext context;
+    HelloRequest request;
+    ServerAsyncResponseWriter<HelloReply> responder;
+    service.RequestSayHello(&context, &request, &responder, &cq, &cq, (void*)1);
+    ```
+
+- Wait for the completion queue to return the tag. The context, request and
+  responder are ready once the tag is retrieved.
+
+    ```cpp
+    HelloReply reply;
+    Status status;
+    void* got_tag;
+    bool ok = false;
+    cq.Next(&got_tag, &ok);
+    if (ok && got_tag == (void*)1) {
+      // set reply and status
+      responder.Finish(reply, status, (void*)2);
+    }
+    ```
+
+- Wait for the completion queue to return the tag. The rpc is finished when the
+  tag is back.
+
+    ```cpp
+    void* got_tag;
+    bool ok = false;
+    cq.Next(&got_tag, &ok);
+    if (ok && got_tag == (void*)2) {
+      // clean up
+    }
+    ```
+
+To handle multiple rpcs, the async server creates an object `CallData` to
+maintain the state of each rpc and use the address of it as the unique tag. For
+simplicity the server only uses one completion queue for all events, and runs a
+main loop in `HandleRpcs` to query the queue.
+
+For a working example, refer to [greeter_async_server.cc](greeter_async_server.cc).
+
+
+
+