Chapter 17. Messaging

17.1. Introduction

17.1.1. HornetQ

HornetQ is a multi-protocol, asynchronous messaging system developed by Red Hat. HornetQ provides high availability (HA) with automatic client failover to guarantee message reliability in the event of a server failure. HornetQ also supports flexible clustering solutions with load-balanced messages.

17.1.2. About Java Messaging Service (JMS)

Messaging systems allow you to loosely couple heterogeneous systems together with added reliability. Java Messaging Service (JMS) providers use a system of transactions, to commit or roll back changes atomically. Unlike systems based on a Remote Procedure Call (RPC) pattern, messaging systems primarily use an asynchronous message passing pattern with no tight relationship between requests and responses. Most messaging systems also support a request-response mode but this is not a primary feature of messaging systems.
Messaging systems decouple the senders of messages from the consumers of messages. The senders and consumers of messages are completely independent and know nothing of each other. This allows you to create flexible, loosely coupled systems. Often, large enterprises use a messaging system to implement a message bus which loosely couples heterogeneous systems together. Message buses often form the core of an Enterprise Service Bus (ESB). Using a message bus to decouple disparate systems can allow the system to grow and adapt more easily. It also allows more flexibility to add new systems or retire old ones since they don't have brittle dependencies on each other.

17.1.3. Supported Messaging Styles

HornetQ supports the following messaging styles:
Message Queue pattern
The Message Queue pattern involves sending a message to a queue. Once in the queue, the message is usually made persistent to guarantee delivery. Once the message has moved through the queue, the messaging system delivers it to a message consumer. The message consumer acknowledges the delivery of the message once it is processed.
When used with point-to-point messaging, the Message Queue pattern allows multiple consumers for a queue, but each message can only be received by a single consumer.
Publish-Subscribe pattern
The Publish-Subscribe pattern allows multiple senders to send messages to a single entity on the server. This entity is often known as a "topic". Each topic can be attended by multiple consumers, known as "subscriptions".
Each subscription receives a copy of every message sent to the topic. This differs from the Message Queue pattern, where each message is only consumed by a single consumer.
Subscriptions that are durable retain copies of each message sent to the topic until the subscriber consumes them. These copies are retained even in the event of a server restart. Non-durable subscriptions last only as long as the connection that created them.