Message brokers translate messages from one application to another. Clients of message brokers use APIs to send messages that are serialized to a specific format and subscribe to queues to be notified about all the new messages. There is the AMQP (short for Advanced Message Queuing Protocol) protocol, which provides a common API that's compatible with different products.

The main concept of message brokers is the queue. It is an abstraction that represents an entity used to collect messages until they will be consumed by clients.

Why is the concept of message brokers cool? Because it's the simplest way to achieve loose coupling for services and maintain the possibility of smooth updates. You can use a common message format and write microservices to read specific types of messages. It helps you to reroute all paths of messages. For examples, you can add a specific handler for the specific message type, or set a balancing rule to load more powerful services.

There are a lot of message brokers that can be used, depending on your needs. Some popular products are described in the following sections.