StreamObserver Interface in gRPC

In gRPC, the StreamObserver interface manages responses in streaming remote procedure calls (RPCs). Streaming RPCs enable the continuous exchange of messages between the client and server, making them appropriate for scenarios in which data flows in one or both directions over an extended period. The StreamObserver interface provides methods to manage these streams of data asynchronously, offering precise control and flexibility to client and server applications alike.

Methods of the StreamObserver interface

StreamObserver interface offers functions to manage the flow of data between clients and servers during streaming RPCs. It is used for sending and receiving stream messages. The StreamObserver methods, onNext(), onError(), and onCompleted(), handle asynchronous data streams, errors, and signal the successful completion of an RPC.

gRPC provides a StreamObserver to the application for sending (request) messages whereas the application implements a StreamObserver for handling incoming (response) messages.

We will discuss each method below, focusing on the handling of response messages.

onNext method

The onNext method allows handling of incoming response data in real-time, asynchronously, and sequentially. It lets developers create responsive applications that can process data as it arrives, for example in like real-time communication, data streaming, and live updates. Both the client and server can implement the onNext method depending on the use case. Whenever either the server or the client sends a new message, this method is invoked. In the context of streaming RPCs, as the server or client continuously sends a stream of messages, the onNext method is responsible for processing these messages in a sequential and asynchronous manner. The method signature is shown below:

The ResponseType parameter represents the type of the response message. Response messages are defined using Protocol Buffers in the .proto file and are ...