What does a message broker (Kafka, RabbitMQ) do?

A message broker is a component that passes messages between services in a distributed system, making their interaction asynchronous and reliable.

#The main idea

Services do not call each other directly, but exchange messages through a broker:

• one service publishes an event ("Payment completed"),
• another subscribes to such events and reacts ("Create a delivery record").

This removes rigid dependencies between services.

#What a message broker does

1. Accepts and stores messages acts as a buffer between publishers and consumers.
2. Delivers messages guarantees delivery even when the recipient is temporarily unavailable.
3. Routes streams sends the message to the right queue or topic.
4. Supports scaling distributes the load among several subscribers.
5. Ensures reliability and order stores acknowledgments, redelivers lost messages, and preserves the order of events.

#Message broker examples

• RabbitMQ: a classic queue broker (the AMQP protocol), great for business events and processing tasks.
• Apache Kafka: a high-performance streaming platform for data streams and event logs.

Summary:

A message broker is the "mail carrier" between microservices: it accepts, stores, and delivers messages, providing asynchrony, resilience, and loose coupling between parts of the system.