As discussed in previous sections, there are different data transmission protocols using which messages can be transmitted among sender, receiver, and message queues. It is difficult to cover all such protocols in the scope of this book. However, it is important to understand how these data transmission protocols work and why it is an important design decision for your message-oriented application integration architecture. In the light of this, we will cover one example of such a protocol: Advance Message Queuing Protocol also known as AQMP.
AQMP is an open protocol for asynchronous message queuing that developed and matured over several years. AMQP provides richer sets of messaging functionalities that can be used to support very advanced messaging scenarios. As depicted in the following figure, there are three main components in any AQMP-based messaging system:
As the name suggests, producers sends messages to brokers that in turn deliver them to consumers. Every broker has a component called exchange that is responsible for routing the messages from producers to appropriate message queues.
An AQMP messaging system consists of three main components:
- Publisher(s)
- Consumer(s)
- Broker/server(s)
Each component can be multiple in number and situated on independent hosts. Publishers and consumers communicate with each other through message queues bound to exchanges within the brokers. AQMP provides reliable, guaranteed, in-order message delivery. Message exchanges in an AQMP model can follow various methods. Let's look at each one of them:
- Direct exchange: This is a key-based routing mechanism. In this, a message is delivered to the queue whose name is equal to the routing key of the message.
- Fan-out exchange: A fan-out exchange routes messages to all of the queues that are bound to it and the routing key is ignored. If N queues are bound to a fan-out exchange, when a new message is published to that exchange, a copy of the message is delivered to all N queues. Fan-out exchanges are ideal for the broadcast routing of messages. In other words, the message is cloned and sent to all queues connected to this exchange.
- Topic exchange: In topic exchange, the message can be routed to some of the connected queues using wildcards. The topic exchange type is often used to implement various publish/subscribe pattern variations. Topic exchanges are commonly used for the multicast routing of messages.