Building Event-Driven Microservices

Chapter 5. Event-Driven Processing Basics

Most event-driven microservices follow, at a minimum, the same three steps:

1. Consume an event from an input event stream.

2. Process that event.

3. Produce any necessary output events.

There are also event-driven microservices that derive their input event from a synchronous request-response interaction, which is covered more in Chapter 13. This chapter covers only microservices that source their events from event streams.

In stream-sourced event-driven microservices, the microservice instance will create a producer client and a consumer client and register itself with any necessary consumer groups, if applicable. The microservice starts a loop to poll the consumer client for new events, processing them as they come in and emitting any required output events. This workflow is shown in the following pseudocode. (Your implementation will of course vary according to your language, stream-processing framework, event-broker selection, and other technical factors.)

Consumer consumerClient = new consumerClient(consumerGroupName, ...);
Producer producerClient = new producerClient(...);

while(true) {
    InputEvent event = consumerClient.pollOneEvent(inputEventStream);
    OutputEvent output = processEvent(event);
    producerClient.produceEventToStream(outputEventStream, output);

    //At-least-once processing.
    consumerClient.commitOffsets();
}

The processEvent function is of particular interest. This is where the real event-processing work gets done, primarily ...