Project: gRPC (Orchestration & Management / Remote Procedure Call)
File: server_reflection_tutorial.md

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. C++ Server Reflection is an add-on library, `libgrpc++_reflection`. 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 dynamic linking and `--whole-archive` is needed for static linking. This demonstrates enabling c++ server reflection on Linux and MacOS. 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 , 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 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. `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 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) {} ``` 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; } ``` 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" ``` Server Reflection can be used by clients to get information about gRPC services at runtime. We've provided a descriptor database called which implements the 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(serveraddress, servercred); grpc::ProtoReflectionDescriptorDatabase reflection_db(channel); ``` Then use this instance to feed a . ```c++ google::protobuf::DescriptorPool descpool(&reflectiondb); ``` 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 and create messages dynamically. ```c++ const google::protobuf::Descriptor* request_desc = desc_pool->FindMessageTypeByName("helloworld.HelloRequest"); google::protobuf::DynamicMessageFactory dmf; google::protobuf::Message* request = dmf.GetPrototype(request_desc)->New(); ``` See .