February 23, 2024

Earlier articles introduced what Protobuf is and the way it may be mixed with gRPC to implement easy synchronous API. Nevertheless, it didn’t current the true energy of gRPC, which is streaming, totally using the capabilities of HTTP/2.0.

Contract definition

We should outline the strategy with enter and output parameters just like the earlier service. To comply with the separation of considerations, let’s create a devoted service for GPS monitoring functions. Our current proto ought to be prolonged with the next snippet.

message SubscribeRequest 
  string vin = 1;

service GpsTracker 
  rpc Subscribe(SubscribeRequest) returns (stream Geolocation);

Probably the most essential half right here of enabling streaming is specifying it in enter or output sort. To try this, a key phrase stream is used. It signifies that the server will maintain the connection open, and we are able to count on Geolocation messages to be despatched by it.


public void subscribe(SubscribeRequest request, StreamObserver<Geolocation> responseObserver) 

The easy implementation of the strategy doesn’t differ from the implementation of a unary name. The one distinction is in how onNext the strategy behaves; in common synchronous implementation, the strategy can’t be invoked greater than as soon as. Nevertheless, for technique working on stream, onNext could also be invoked as many instances as you need.

As it’s possible you’ll discover on the hooked up screenshot, the geolocation place was returned however the connection continues to be established and the consumer awaits extra knowledge to be despatched within the stream. If the server needs to tell the consumer that there is no such thing as a extra knowledge, it ought to invoke: the onCompleted technique; nevertheless, sending single messages shouldn’t be why we wish to use stream.

Use circumstances for streaming capabilities are primarily transferring important responses as streams of knowledge chunks or real-time occasions. I’ll attempt to exhibit the second use case with this service. Implementation will likely be primarily based on the reactor (https://projectreactor.io/ ) as it really works effectively for the introduced use case.

Let’s put together a easy implementation of the service. To make it work, internet flux dependency will likely be required.

implementation 'org.springframework.boot:spring-boot-starter-webflux'

We should put together a service for publishing geolocation occasions for a selected car.


import com.grapeup.grpc.instance.mannequin.GeolocationEvent;
import org.springframework.stereotype.Service;
import reactor.core.writer.Flux;
import reactor.core.writer.Sinks;

public class InMemoryGeolocationService implements GeolocationService 

    personal remaining Sinks.Many<GeolocationEvent> sink = Sinks.many().multicast().directAllOrNothing();

    public void publish(GeolocationEvent occasion) 

    public Flux<GeolocationEvent> getRealTimeEvents(String vin) 
        return sink.asFlux().filter(occasion -> occasion.vin().equals(vin));

Let’s modify the GRPC service ready within the earlier article to insert the strategy and use our new service to publish occasions.

public void insert(Geolocation request, StreamObserver<Empty> responseObserver) 
    GeolocationEvent geolocationEvent = convertToGeolocationEvent(request);


Lastly, let’s transfer to our GPS tracker implementation; we are able to exchange the earlier dummy implementation with the next one:

public void subscribe(SubscribeRequest request, StreamObserver<Geolocation> responseObserver) 
        .subscribe(occasion -> responseObserver.onNext(toProto(occasion)),

Right here we reap the benefits of utilizing Reactor, as we not solely can subscribe for incoming occasions but in addition deal with errors and completion of stream in the identical means.

To map our inside mannequin to response, the next helper technique is used:

personal static Geolocation toProto(GeolocationEvent occasion) 
    return Geolocation.newBuilder()


As it’s possible you’ll be observed, we despatched the next requests with GPS place and obtained them in real-time from our open stream connection. Streaming knowledge utilizing gRPC or one other instrument like Kafka is extensively utilized in many IoT techniques, together with Automotive.

Bidirectional stream

What if our consumer wish to obtain knowledge for a number of autos however with out preliminary information about all autos they’re thinking about? Creating new connections for every car isn’t one of the best strategy. However fear no extra! Whereas utilizing gRPC, the consumer might reuse the identical connection because it helps bidirectional streaming, which signifies that each consumer and server might ship messages utilizing open channels.

rpc SubscribeMany(stream SubscribeRequest) returns (stream Geolocation);

Sadly, IntelliJ doesn’t enable us to check this performance with their built-in consumer, so we’ve got to develop one ourselves.

localhost:9090/com. grapeup.geolocation.GpsTracker/SubscribeMany

com.intellij.grpc.requests.RejectedRPCException: Unsupported technique is named

Our dummy consumer might look one thing like that, primarily based on generated lessons from the protobuf contract:

var channel = ManagedChannelBuilder.forTarget("localhost:9090")
var observer = GpsTrackerGrpc.newStub(channel)
    .subscribeMany(new StreamObserver<>() 
        public void onNext(Geolocation worth) 

        public void onError(Throwable t) 
            System.err.println("Error " + t.getMessage());

        public void onCompleted() 
whereas (true)  // to maintain consumer subscribing for demo functions :)

Should you ship the updates for the next random VINs: JF2SJAAC1EH511148, 1YVGF22C3Y5152251, you must have the ability to see the output within the console. Test it out!

Tip of the iceberg

Offered examples are simply gRPC fundamentals; there may be rather more to it, like disconnecting from the channel from each ends and reconnecting to the server in case of community failure. The next articles have been supposed to share with YOU that gRPC structure has a lot to supply, and there are many prospects for a way it may be utilized in techniques. Particularly in techniques requiring low latency or the power to supply consumer code with strict contract validation.