gRPC Server Reflection Tutorial

gRPC Server Reflection provides information about publicly-accessible gRPC
services on a server, and assists clients at runtime with constructing RPC
requests and responses without precompiled service information. It is used by
the gRPC command line tool (gRPC CLI), which can be used to introspect server
protos and send/receive test RPCs. Reflection is only supported for
proto-based services.

Enable Server Reflection

gRPC-Java Server Reflection is implemented by
io.grpc.protobuf.services.ProtoReflectionService in the grpc-services
package. To enable server reflection, you need to add the
ProtoReflectionService to your gRPC server.

For example, to enable server reflection in
examples/src/main/java/io/grpc/examples/helloworld/HelloWorldServer.java, we
need to make the following changes:

--- a/examples/build.gradle
+++ b/examples/build.gradle
@@ -27,6 +27,7 @@
 dependencies {
   compile "io.grpc:grpc-netty-shaded:${grpcVersion}"
   compile "io.grpc:grpc-protobuf:${grpcVersion}"
+  compile "io.grpc:grpc-services:${grpcVersion}"
   compile "io.grpc:grpc-stub:${grpcVersion}"
 
   testCompile "junit:junit:4.12"
--- a/examples/src/main/java/io/grpc/examples/helloworld/HelloWorldServer.java
+++ b/examples/src/main/java/io/grpc/examples/helloworld/HelloWorldServer.java
@@ -33,6 +33,7 @@ package io.grpc.examples.helloworld;
 
 import io.grpc.Server;
 import io.grpc.ServerBuilder;
+import io.grpc.protobuf.services.ProtoReflectionService;
 import io.grpc.stub.StreamObserver;
 import java.io.IOException;
 import java.util.logging.Logger;
@@ -50,6 +51,7 @@ public class HelloWorldServer {
     int port = 50051;
     server = ServerBuilder.forPort(port)
         .addService(new GreeterImpl())
+        .addService(ProtoReflectionService.newInstance())
         .build()
         .start();
     logger.info("Server started, listening on " + port);

In the following examples, we assume you have made these changes to
enable reflection in HelloWorldServer.java.

gRPC CLI

After enabling server reflection in a server application, you can use gRPC
CLI to get information about its available services. gRPC CLI is written
in C++. Instructions on how to build gRPC CLI can be found at
command_line_tool.md.

Use gRPC CLI to check services

First, build and start the hello-world-server:

$ cd examples/
$ ./gradlew installDist
$ build/install/examples/bin/hello-world-server

Open a new terminal and make sure you are in the directory where the grpc_cli
binary is located:

$ 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

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 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

sh $ ./grpc_cli type localhost:50051 helloworld.HelloRequest

output:
sh 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

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

output:
sh message: "Hello gRPC CLI"