Merge pull request #8071 from y-zeng/reflection_example

Add server reflection tutorial

doc/server_reflection_tutorial.md

+# gRPC Server Reflection Tutorial
+
+gRPC Server Reflection provides information about publicly-accessible gRPC
+services on a server, and assists clients at runtime to construct RPC
+requests and responses without precompiled service information. It is used by
+gRPC CLI, which can be used to introspect server protos and send/receive test
+RPCs.
+
+## Enable Server Reflection
+
+### Enable server reflection in C++ servers
+
+C++ Server Reflection is an add-on library, `libgrpc++_reflction`. To enable C++
+server reflection, you can link this library to your server binary.
+
+Some platforms (e.g. Ubuntu 11.10 onwards) only link in libraries that directly
+contain symbols used by the application. On these platforms, LD flag
+`--no-as-needed` is needed for for dynamic linking and `--whole-archive` is
+needed for for static linking.
+
+This [Makefile](../examples/cpp/helloworld/Makefile#L37#L45) demonstrates
+enabling c++ server reflection on Linux and MacOS.
+
+## Test services using Server Reflection
+
+After enabling Server Reflection in a server application, you can use gRPC CLI
+to test its services.
+
+Instructions on how to use gRPC CLI can be found at
+[command_line_tool.md](command_line_tool.md), or using `grpc_cli help` command.
+
+Here we use `examples/cpp/helloworld` as an example to show the use of gRPC
+Server Reflection and gRPC CLI. First, we need to build gRPC CLI and setup an
+example server with Server Reflection enabled.
+
+- Setup an example server
+
+  Server Reflection has already been enabled in the
+  [Makefile](../examples/cpp/helloworld/Makefile) of the helloworld example. We
+  can simply make it and run the greeter_server.
+
+  ```sh
+  $ make -C examples/cpp/helloworld
+  $ examples/cpp/helloworld/greeter_server &
+  ```
+
+- Build gRPC CLI
+
+  ```sh
+  make grpc_cli
+  cd bins/opt
+  ```
+
+  gRPC CLI binary `grpc_cli` can be found at `bins/opt/` folder. This tool is
+  still new and does not have a `make install` target yet.
+
+### List services
+
+`grpc_cli ls` command lists services and methods exposed at a given port
+
+- List all the services exposed at a given port
+
+  ```sh
+  $ grpc_cli ls localhost:50051
+  ```
+
+  output:
+  ```sh
+  helloworld.Greeter
+  grpc.reflection.v1alpha.ServerReflection
+  ```
+
+- List one service with details
+
+  `grpc_cli ls` command inspects a service given its full name (in the format of
+  \<package\>.\<service\>). It can print information with a long listing format
+  when `-l` flag is set. This flag can be used to get more details about a
+  service.
+
+  ```sh
+  $ grpc_cli ls localhost:50051 helloworld.Greeter -l
+  ```
+
+  output:
+  ```sh
+  filename: helloworld.proto
+  package: helloworld;
+  service Greeter {
+    rpc SayHello(helloworld.HelloRequest) returns (helloworld.HelloReply) {}
+  }
+
+  ```
+
+### List methods
+
+- List one method with details
+
+  `grpc_cli ls` command also inspects a method given its full name (in the
+  format of \<package\>.\<service\>.\<method\>).
+
+  ```sh
+  $ grpc_cli ls localhost:50051 helloworld.Greeter.SayHello -l
+  ```
+
+  output:
+  ```sh
+    rpc SayHello(helloworld.HelloRequest) returns (helloworld.HelloReply) {}
+  ```
+
+### Inspect message types
+
+We can use`grpc_cli type` command to inspect request/response types given the
+full name of the type (in the format of \<package\>.\<type\>).
+
+- Get information about the request type
+
+  ```sh
+  $ grpc_cli type localhost:50051 helloworld.HelloRequest
+  ```
+
+  output:
+  ```sh
+  message HelloRequest {
+    optional string name = 1;
+  }
+  ```
+
+### Call a remote method
+
+We can send RPCs to a server and get responses using `grpc_cli call` command.
+
+- Call a unary method
+
+  ```sh
+  $ grpc_cli call localhost:50051 SayHello "name: 'gRPC CLI'"
+  ```
+
+  output:
+  ```sh
+  message: "Hello gRPC CLI"
+  ```
+
+## Use Server Reflection in a C++ client
+
+Server Reflection can be used by clients to get information about gRPC services
+at runtime. We've provided a descriptor database called
+[grpc::ProtoReflectionDescriptorDatabase](../test/cpp/util/proto_reflection_descriptor_database.h)
+which implements the
+[google::protobuf::DescriptorDatabase](https://developers.google.com/protocol-buffers/docs/reference/cpp/google.protobuf.descriptor_database#DescriptorDatabase)
+interface. It manages the communication between clients and reflection services
+and the storage of received information. Clients can use it as using a local
+descriptor database.
+
+- To use Server Reflection with grpc::ProtoReflectionDescriptorDatabase, first
+  initialize an instance with a grpc::Channel.
+
+  ```c++
+  std::shared_ptr<grpc::Channel> channel =
+      grpc::CreateChannel(server_address, server_cred);
+  grpc::ProtoReflectionDescriptorDatabase reflection_db(channel);
+  ```
+
+- Then use this instance to feed a
+  [google::protobuf::DescriptorPool](https://developers.google.com/protocol-buffers/docs/reference/cpp/google.protobuf.descriptor#DescriptorPool).
+
+  ```c++
+  google::protobuf::DescriptorPool desc_pool(&reflection_db);
+  ```
+
+- Example usage of this descriptor pool
+
+  * Get Service/method descriptors.
+
+    ```c++
+    const google::protobuf::ServiceDescriptor* service_desc =
+        desc_pool->FindServiceByName("helloworld.Greeter");
+    const google::protobuf::MethodDescriptor* method_desc =
+        desc_pool->FindMethodByName("helloworld.Greeter.SayHello");
+    ```
+
+  * Get message type descriptors.
+
+    ```c++
+    const google::protobuf::Descriptor* request_desc =
+        desc_pool->FindMessageTypeByName("helloworld.HelloRequest");
+    ```
+
+  * Feed [google::protobuf::DynamicMessageFactory](https://developers.google.com/protocol-buffers/docs/reference/cpp/google.protobuf.dynamic_message#DynamicMessageFactory).
+
+    ```c++
+    google::protobuf::DynamicMessageFactory(&desc_pool);
+    ```