[microservices] Extend EventPattern to listen for event types within a specified topic with Kafka [type:feat]

[taj-p]

Feature Request

The current strategy of topic creation for kafka microservices does not follow best practice. For each EventPattern decorator in some Kafka microservice controller, a new topic is created in Kafka. This is contrary to best practices (see: confluent article). Rather, I think it is argued that related event types should be grouped within a single topic.

This is important primarily for ordering of events. Kafka maintains the ordering of messages only within a topic-partition, not between topics. This means that listeners of some topics could see them in a nonsensical order. Moreover, this can cause issues for Kafka users who want to use EventSourcing or save their Kafka topics for replayability / recovery.

My suggestion is to allow a user to set the topic within an EventPattern and then the pattern matches an event type within that topic.

[kamilmysliwiec]

Rather, I think it is argued that related event types should be grouped within a single topic.

We can't presume what event types should be grouped within a single topic so we must leave this decision to developers. For example, in case you want to group several events together (under one topic), you can register a single EventPattern and then, within a handler emit another internal event (appropriate for the specific, incoming event) using the @nestjs/event-emitter package https://docs.nestjs.com/techniques/events

[taj-p]

Thanks for getting back to me 😄 . You're right - I didn't realize this technique was possible. Thank you for documenting the EventPattern tactic to handle multiple events from a single topic.