gRPC servers with Dagger

gRPC servers without Dagger

To implement a gRPC service, you create a class that extends a base service implementation generated from the Protocol Buffer definition.

service HelloService {
  rpc SayHello(HelloRequest) returns (HelloResponse);
}

The service definition above generates a class HelloServiceGrpc that contains a base implementation HelloServiceImplBase:

public class HelloServiceGrpc {
  public static class HelloServiceImplBase implements BindableService {
    public void sayHello(
        HelloRequest request, StreamObserver<HelloResponse> responseObserver);
  }
}

You then extend that class:

class Hello extends HelloServiceImplBase {
  @Override
  public void sayHello(
      HelloRequest request, StreamObserver<HelloResponse> responseObserver) {
    // ...
  }
}

Typically, you create the server by adding an instance of Hello to a [ServerBuilder]:

Hello helloService = new Hello(...);
serverBuilder.add(helloService);
Server server = serverBuilder.build();

But because you have to create your service instance before you create your server, if you’re using Dagger to create the service, Hello can’t inject anything that isn’t unscoped or at @Singleton scope.

Configuring a service using Dagger-gRPC

You can use Dagger-gRPC to instantiate your service implementation once for each call. To do so, follow these steps:

  1. Annotate your service implementation with @GrpcService, setting the grpcClass parameter to the top-level class that contains the base implementation it extends, and give it an @Inject constructor.

    @GrpcService(grpcClass = HelloServiceGrpc.class)
    class Hello extends HelloServiceImplBase {
      @Inject Hello(...) {...}
      ...
    }
    

    Make sure the Dagger-gRPC server annotation processor runs over your source code.

    That will generate an interface HelloServiceDefinition and a module HelloUnscopedGrpcServiceModule.

  2. Create a @Singleton component that implements HelloServiceDefinition, and install HelloUnscopedGrpcServiceModule into that component. In another module you install on that component, bind HelloServiceDefinition to your component type.

    Also make sure to bind a list of ServerInterceptors for the service, annotated with @ForGrpcService(serviceClass). The list can be empty.

    @Singleton
    @Component(modules = {HelloUnscopedGrpcServiceModule.class, MyModule.class})
    interface MyComponent extends HelloServiceDefinition {}
    
    @Module
    abstract class MyModule {
      @Binds
      abstract HelloServiceDefinition helloComponent(MyComponent myComponent);
    
      @Provides
      @ForGrpcService(HelloServiceGrpc.class)
      static List<? extends ServerInterceptor> helloInterceptors(
          AuthInterceptor authInterceptor,
          LoggingInterceptor loggingInterceptor) {
        return Arrays.asList(authInterceptor, loggingInterceptor);
      }
    }
    
  3. Install NettyServerModule (or [InProcessServerModule] for testing) into your @Singleton component. That component will provide the gRPC Server object.

    @Singleton
    @Component(modules = {
      NettyServerModule.class,
      HelloUnscopedGrpcServiceModule.class,
      MyModule.class,
    })
    interface MyComponent {
      Server server();
    }
    

Now you can start your server, and a new instance of Hello will be injected to handle each call.

int port = ...;
MyComponent component =
    DaggerMyComponent.builder()
        .nettyServerModule(NettyServerModule.bindingToPort(port))
        .build();
Server server = component.server();
server.start();

Binding in call scope <a name=call-scope></a>

If you want to bind objects in a scope whose lifetime is the same as one gRPC ServerCall, Dagger-gRPC supports that too.

  1. Instead of making your @Singleton component implement HelloServiceDefinition, create a subcomponent type that implements HelloServiceDefinition and is in @CallScope. Install HelloGrpcServiceModule into it. If you want bindings in that component to be able to depend on the gRPC call metadata, then also install GrpcCallMetadataModule.

    @CallScope
    @Subcomponent(modules = {
     HelloGrpcServiceModule.class,
     GrpcCallMetadataModule.class,
     ...})
    interface ServiceComponent extends HelloServiceDefinition {}
    
  2. Instead of HelloUnscopedGrpcServiceModule, install HelloGrpcProxyModule into your @Singleton component. In your other module, instead of providing HelloServiceDefinition, provide an instance of HelloServiceDefinition.Factory that calls the proper subcomponent factory and/or builder method.

    @Singleton
    @Component(modules = {HelloGrpcProxyModule.class, MyModule.class})
    interface MyComponent {
     ...
     ServiceComponent serviceComponent(GrpcCallMetadataModule metadataModule);
    }
    
    @Module
    class MyModule {
     @Provide
     static HelloServiceDefinition.Factory helloComponent(
         final MyComponent myComponent) {
       return new HelloServiceDefinition.Factory() {
         @Override
         public HelloServiceDefinition grpcService(
             GrpcCallMetadataModule metadataModule) {
           return myComponent.serviceComponent(metadataModule);
         }
       };
     }
    }
    
  3. Make sure that a module installed into your HelloServiceDefinition subcomponent or any ancestor component binds the list of interceptors for that service, just like above:

    @Module
    class MyModuleInstalledInSubcomponentOrAncestor {
     @Provides
     @ForGrpcService(HelloServiceGrpc.class)
     static List<? extends ServerInterceptor> helloInterceptors(
         AuthInterceptor authInterceptor,
         LoggingInterceptor loggingInterceptor) {
       return Arrays.asList(authInterceptor, loggingInterceptor);
     }
    }
    

Now any binding annotated with @CallScope will be used only once per call, with the bound object shared among all its dependent bindings.

If you’ve installed GrpcCallMetadataModule onto the subcomponent, then you’ll also be able to inject the [headers][Metadata] from the call into any binding in the subcomponent.

You create and start the server in the same way as above.