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 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, 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 of the helloworld example. We can simply make it and run the greeter_server.

  $ make -C examples/cpp/helloworld
  $ examples/cpp/helloworld/greeter_server &

• Build gRPC CLI

  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

  $ grpc_cli ls localhost:50051

  output:

  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.

  $ grpc_cli ls localhost:50051 helloworld.Greeter -l

  output:

  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>).

  $ grpc_cli ls localhost:50051 helloworld.Greeter.SayHello -l

  output:

    rpc SayHello(helloworld.HelloRequest) returns (helloworld.HelloReply) {}

Inspect message types

We can usegrpc_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

  $ grpc_cli type localhost:50051 helloworld.HelloRequest

  output:

  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

  $ grpc_cli call localhost:50051 SayHello "name: 'gRPC CLI'"

  output:

  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 which implements the google::protobuf::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.

  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.

  google::protobuf::DescriptorPool desc_pool(&reflection_db);

• Example usage of this descriptor pool

  • Get Service/method descriptors.

    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.

    const google::protobuf::Descriptor* request_desc =
        desc_pool->FindMessageTypeByName("helloworld.HelloRequest");

  • Feed google::protobuf::DynamicMessageFactory.

    google::protobuf::DynamicMessageFactory(&desc_pool);