How would you organize the interaction of multiple services?

Messaging through brokers (Kafka, RabbitMQ), REST API, gRPC.

The choice depends on the requirements:

• Message broker: Asynchronous interaction, high scalability, reliability (guaranteed delivery), decoupled architecture. Suitable for event-driven architecture, background tasks, distributed transactions.

  // Example of sending a message to Kafka
  kafkaTemplate.send("topic-name", "message-payload");
  

• REST API: Synchronous interaction, ease of implementation, HTTP protocol. Suitable for data retrieval, idempotent operations.

  // Example of calling REST API using RestTemplate
  restTemplate.getForObject("http://service-url/resource", MyResponse.class);
  

• gRPC: Synchronous/streaming interaction, high performance due to Protobuf, bidirectional communication. Suitable for high-load systems, transmitting large volumes of data.

  // Example of calling a gRPC service
  MyServiceGrpc.MyServiceBlockingStub stub = MyServiceGrpc.newBlockingStub(channel);
  MyResponse response = stub.myMethod(MyRequest.newBuilder().build());
  

Combined approaches are often used, for example, REST for data requests and brokers for event processing. For service discovery and load balancing, Service Registry (Eureka, Consul) and API Gateway (Spring Cloud Gateway, Zuul) are used.