event-publisher

As we are developing event driven microservices. We are publishing events to different destinations like SNS, SQS, Kafka, Kinesis or any other in house or cloud service.

This repository has a simple example of publishing event to SQS or SNS when there is any update in user details based on event type. In case there is no explicit event type to destination mapping event will be published to SNS.

For simplicity kept only 3 events getting published to SQS/SNS.

1. USER_CREATED to SNS
2. USER_UPDATED to SQS
3. USER_DELETED to SNS as it is default destination.

As usual spring boot application, we have a controller, service where handling request and response transformation or any business logic and publishing the event and repository for DB operations.

You can send a POST request to create a new user and publish USER_CREATED event. As you can see in below code snipped once we are saving the user to database, we are calling publish method of publisher service.

public Mono<String> create(CreateUserRequest createUserRequest) {
    return userRepository.save(createUserRequest.toUser())
            .doOnSuccess(user -> eventPublisherService.publish(USER_CREATED, user))
            .map(user -> user.getId());
}

Based on the event publisher service will get the EventPublisher object based on event type.

@Service
@RequiredArgsConstructor
@Slf4j
public class EventPublisherServiceImpl implements EventPublisherService {
    private final Map<String, EventPublisher> eventPublishers;
    private final ObjectMapper objectMapper;
}

This class is most interesting part of the complete application. As you can see in the above code Map<String, EventPublisher> eventPublishers is injected from constructor, but we have not created any such bean in our code. So the question is how is it going to work if we don't have any bean. Answer is simple Spring application context is giving it to us out of the box. We just need to give a name to our EventPublisher beans.

@Service("snsPublisher")
@Slf4j
public class SnsEventPublisher implements EventPublisher

@Service("sqsPublisher")
@Slf4j
public class SqsEventPublisher implements EventPublisher

Let's see how we are getting publisher from event type. I have hardcoded the eventDestinations, but these can be fetched from your DB or can be loaded during startup from configuration properties. I am using SNS as default publisher.

private EventPublisher publisher(String eventType) {
    Map<String, EventDestination> eventDestinations = Map.of(
            USER_CREATED, SNS,
            USER_UPDATED, EventDestination.SQS);

    var destination = eventDestinations.getOrDefault(eventType, SNS);
    return eventPublishers.get(destination.getBeanName());
}

In our EventDestination enum we have added the bean name of the publisher for that destination. So from bean name we are getting from our enum we are getting the actual bean from the eventPublishers map.

@AllArgsConstructor
@Getter
public enum EventDestination {
    SQS("sqsPublisher"), SNS("snsPublisher");
    private final String beanName;
}

This is kind of a factory which is giving you publisher and publishing the message to required publisher.