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

gRPC-go Server Reflection is implemented in package reflection. To enable server reflection, you need to import this package and register reflection service on your gRPC server.

For example, to enable server reflection in example/helloworld, we need to make the following changes:

--- a/examples/helloworld/greeter_server/main.go
+++ b/examples/helloworld/greeter_server/main.go
@@ -40,6 +40,7 @@ import (
        "google.golang.org/grpc"
        pb "google.golang.org/grpc/examples/helloworld/helloworld"
+       "google.golang.org/grpc/reflection"
 )

 const (
@@ -61,6 +62,8 @@ func main() {
        }
        s := grpc.NewServer()
        pb.RegisterGreeterService(s, &pb.GreeterService{SayHello: sayHello})
+       // Register reflection service on gRPC server.
+       reflection.Register(s)
        if err := s.Serve(lis); err != nil {
                log.Fatalf("failed to serve: %v", err)
        }

An example server with reflection registered can be found at examples/features/reflection/server.

gRPC CLI

After enabling Server Reflection in a server application, you can use gRPC CLI to check its services. gRPC CLI is only available in c++. Instructions on how to build and use gRPC CLI can be found at command_line_tool.md.

Use gRPC CLI to check services

First, start the helloworld server in grpc-go directory:

$ cd <grpc-go-directory>
$ go run examples/features/reflection/server/main.go

Open a new terminal and make sure you are in the directory where grpc_cli lives:

$ cd <grpc-cpp-directory>/bins/opt

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:

  grpc.examples.echo.Echo
  grpc.reflection.v1alpha.ServerReflection
  helloworld.Greeter

• 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[json_name = "name"];
  }

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"