Streaming 通信模式

具体用例可以参考：dubbo-samples-triple/pojo;

Stream (流)

Stream 是 Dubbo3 新提供的一种调用类型，在以下场景时建议使用流的方式:

接口需要发送大量数据，这些数据无法被放在一个 RPC 的请求或响应中，需要分批发送，但应用层如果按照传统的多次 RPC 方式无法解决顺序和性能的问题，如果需要保证有序，则只能串行发送
流式场景，数据需要按照发送顺序处理, 数据本身是没有确定边界的
推送类场景，多个消息在同一个调用的上下文中被发送和处理

Stream 分为以下三种:

SERVER_STREAM(服务端流)
CLIENT_STREAM(客户端流)
BIDIRECTIONAL_STREAM(双向流)

由于 java 语言的限制，BIDIRECTIONAL_STREAM 和 CLIENT_STREAM 的实现是一样的。

在 Dubbo3 中，流式接口以 SteamObserver 声明和使用，用户可以通过使用和实现这个接口来发送和处理流的数据、异常和结束。

对于 Dubbo2 用户来说，可能会对StreamObserver感到陌生，这是Dubbo3定义的一种流类型，Dubbo2 中并不存在 Stream 的类型，所以对于迁移场景没有任何影响。

流的语义保证

提供消息边界，可以方便地对消息单独处理
严格有序，发送端的顺序和接收端顺序一致
全双工，发送不需要等待
支持取消和超时

非 PB 序列化的流

public interface IWrapperGreeter {

    StreamObserver<String> sayHelloStream(StreamObserver<String> response);

    void sayHelloServerStream(String request, StreamObserver<String> response);
}

Stream 方法的方法入参和返回值是严格约定的，为防止写错而导致问题，Dubbo3 框架侧做了对参数的检查, 如果出错则会抛出异常。对于 双向流(BIDIRECTIONAL_STREAM), 需要注意参数中的 StreamObserver 是响应流，返回参数中的 StreamObserver 为请求流。

实现类

public class WrapGreeterImpl implements WrapGreeter {

    //...

    @Override
    public StreamObserver<String> sayHelloStream(StreamObserver<String> response) {
        return new StreamObserver<String>() {
            @Override
            public void onNext(String data) {
                System.out.println(data);
                response.onNext("hello,"+data);
            }

            @Override
            public void onError(Throwable throwable) {
                throwable.printStackTrace();
            }

            @Override
            public void onCompleted() {
                System.out.println("onCompleted");
                response.onCompleted();
            }
        };
    }

    @Override
    public void sayHelloServerStream(String request, StreamObserver<String> response) {
        for (int i = 0; i < 10; i++) {
            response.onNext("hello," + request);
        }
        response.onCompleted();
    }
}

调用方式

delegate.sayHelloServerStream("server stream", new StreamObserver<String>() {
    @Override
    public void onNext(String data) {
        System.out.println(data);
    }

    @Override
    public void onError(Throwable throwable) {
        throwable.printStackTrace();
    }

    @Override
    public void onCompleted() {
        System.out.println("onCompleted");
    }
});


StreamObserver<String> request = delegate.sayHelloStream(new StreamObserver<String>() {
    @Override
    public void onNext(String data) {
        System.out.println(data);
    }

    @Override
    public void onError(Throwable throwable) {
        throwable.printStackTrace();
    }

    @Override
    public void onCompleted() {
        System.out.println("onCompleted");
    }
});
for (int i = 0; i < n; i++) {
    request.onNext("stream request" + i);
}
request.onCompleted();

使用 Protobuf 序列化的流

对于 Protobuf 序列化方式，推荐编写 IDL 使用 compiler 插件进行编译生成。生成的代码大致如下:

public interface PbGreeter {

    static final String JAVA_SERVICE_NAME = "org.apache.dubbo.sample.tri.PbGreeter";
    static final String SERVICE_NAME = "org.apache.dubbo.sample.tri.PbGreeter";

    static final boolean inited = PbGreeterDubbo.init();
    
    //...

    void greetServerStream(org.apache.dubbo.sample.tri.GreeterRequest request, org.apache.dubbo.common.stream.StreamObserver<org.apache.dubbo.sample.tri.GreeterReply> responseObserver);

    org.apache.dubbo.common.stream.StreamObserver<org.apache.dubbo.sample.tri.GreeterRequest> greetStream(org.apache.dubbo.common.stream.StreamObserver<org.apache.dubbo.sample.tri.GreeterReply> responseObserver);
}

完整用例

编写 Java 接口

import org.apache.dubbo.common.stream.StreamObserver;
import org.apache.dubbo.hello.HelloReply;
import org.apache.dubbo.hello.HelloRequest;

public interface IGreeter {
    /**
     * <pre>
     *  Sends greeting by stream
     * </pre>
     */
    StreamObserver<HelloRequest> sayHello(StreamObserver<HelloReply> replyObserver);

}

编写实现类

public class IStreamGreeterImpl implements IStreamGreeter {

    @Override
    public StreamObserver<HelloRequest> sayHello(StreamObserver<HelloReply> replyObserver) {

        return new StreamObserver<HelloRequest>() {
            private List<HelloReply> replyList = new ArrayList<>();

            @Override
            public void onNext(HelloRequest helloRequest) {
                System.out.println("onNext receive request name:" + helloRequest.getName());
                replyList.add(HelloReply.newBuilder()
                    .setMessage("receive name:" + helloRequest.getName())
                    .build());
            }

            @Override
            public void onError(Throwable cause) {
                System.out.println("onError");
                replyObserver.onError(cause);
            }

            @Override
            public void onCompleted() {
                System.out.println("onComplete receive request size:" + replyList.size());
                for (HelloReply reply : replyList) {
                    replyObserver.onNext(reply);
                }
                replyObserver.onCompleted();
            }
        };
    }
}

创建 Provider

public class StreamProvider {
    public static void main(String[] args) throws InterruptedException {
        ServiceConfig<IStreamGreeter> service = new ServiceConfig<>();
        service.setInterface(IStreamGreeter.class);
        service.setRef(new IStreamGreeterImpl());
        service.setProtocol(new ProtocolConfig(CommonConstants.TRIPLE, 50051));
        service.setApplication(new ApplicationConfig("stream-provider"));
        service.setRegistry(new RegistryConfig("zookeeper://127.0.0.1:2181"));
        service.export();
        System.out.println("dubbo service started");
        new CountDownLatch(1).await();
    }
}

创建 Consumer

public class StreamConsumer {
    public static void main(String[] args) throws InterruptedException, IOException {
        ReferenceConfig<IStreamGreeter> ref = new ReferenceConfig<>();
        ref.setInterface(IStreamGreeter.class);
        ref.setCheck(false);
        ref.setProtocol(CommonConstants.TRIPLE);
        ref.setLazy(true);
        ref.setTimeout(100000);
        ref.setApplication(new ApplicationConfig("stream-consumer"));
        ref.setRegistry(new RegistryConfig("zookeeper://mse-6e9fda00-p.zk.mse.aliyuncs.com:2181"));
        final IStreamGreeter iStreamGreeter = ref.get();

        System.out.println("dubbo ref started");
        try {

            StreamObserver<HelloRequest> streamObserver = iStreamGreeter.sayHello(new StreamObserver<HelloReply>() {
                @Override
                public void onNext(HelloReply reply) {
                    System.out.println("onNext");
                    System.out.println(reply.getMessage());
                }

                @Override
                public void onError(Throwable throwable) {
                    System.out.println("onError:" + throwable.getMessage());
                }

                @Override
                public void onCompleted() {
                    System.out.println("onCompleted");
                }
            });

            streamObserver.onNext(HelloRequest.newBuilder()
                .setName("tony")
                .build());

            streamObserver.onNext(HelloRequest.newBuilder()
                .setName("nick")
                .build());

            streamObserver.onCompleted();
        } catch (Throwable t) {
            t.printStackTrace();
        }
        System.in.read();
    }
}

运行 Provider 和 Consumer ,可以看到请求正常返回了
onNext
receive name:tony
onNext
receive name:nick
onCompleted

常见问题

protobuf 类找不到

由于 Triple 协议底层需要依赖 protobuf 协议进行传输，即使定义的服务接口不使用 protobuf 也需要在环境中引入 protobuf 的依赖。

        <dependency>
            <groupId>com.google.protobuf</groupId>
            <artifactId>protobuf-java</artifactId>
            <version>3.19.4</version>
        </dependency>

最后修改 May 23, 2023: Update streaming.md (#2630) (ce2ba2e374)