This repository contains some feasible solutions to resolve the problem of using NestJS microservice with Kafka.

According to Confluent, it is recommended to group events about same entity in the same topic. Managing fine-grained topics is not beneficial in terms of performance and clarity.

However, it is not easy to do that in NestJS microservice(using @nestjs/microservices). The reason is that NestJS microservice uses @EventPattern to subscribe events, and @EventPattern maps a event pattern to a single topic.

There is relative issue on nestjs/nest Github. Kamil Mysliwiec, the creator of NestJS, suggests to have single method to subscribe single topic, which manage groups of event. Then broadcast another internal events using @nestjs/event-emitter package.

It is a possible solution, but doesn't seems elegant personally. Emitting internal event after consuming message from Kafka could make the system more complex and give difficulty to debug.

Here I came up with another solution. It is to use custom deserializer and server to convert topic to handler pattern and vice versa so that we can group multiple events in a single topic.

flowchart LR
  consumer-->|"1. subscribes specific events\n @EventPattern(HandlerPatternTopicTransformer.createHandlerPatternFrom())"|custom-server
  custom-server-->|"2. group each events to single topic\n HandlerPatternTopicTransformer.toTopic()"|kafka

flowchart LR
  producer-->|"serialize event in a format\n MessageProducer.enqueue()"|kafka

flowchart LR
    kafka-->|1. consume message|custom-deserializer
    custom-deserializer-->|"2. convert topic to handlerPattern and route to handler\n HandlerPatternTopicTransformer.createHandlerPatternFrom()"|consumer

(TBD)