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Note

Publish-Subscribe vs. Event-Based

There are two important differences that differentiate the publish-subscribe architectural style from the event-based architectural style.

First, unlike the event-based architectural style, the components are either publishers or subscribers. Publishers send messages, while subscribers can subscribe or unsubscribe to receive these messages. A component cannot be both a publisher and a subscriber.

And second, the relationship between the publisher and the subscribers may vary in form and closeness.

Similarities to Event-Based

Like the event-based architectural style, the publish-subscribe architectural style relies on implicit invocation. Publishers and subscribers are not explicitly connected, but instead, there is a loose coupling between them.

Implementation

In a simple implementation of the publish-subscribe architectural style; to receive messages, a subscriber registers their interest to publishers through a call-back.

Publishers maintain a list of subscribers and communicate to them through procedure calls to the registered call-backs. In this way, subscribers are informed of messages implicitly.

This implementation is well suited to applications where each publisher has a relatively small number of subscribers. Every time a new message is ready to be published, the publisher makes a procedure call to each subscriber to deliver the new message. This can become a lot of work for the publisher as the number of subscribers grows.

Large Scale Implementations

Alternatively, the publish-subscribe architectural style can be adapted to deal with large scale applications, where a single publisher may have many subscribers, by designing a system with more loosely coupled components.

This is achieved by introducing a connector to distribute and possibly filter the messages to the subscribers. The connector could be an event bus or a network protocol.

The connector creates more distance between publishers and subscribers and relieves work for the publisher. These connectors may provide complex filtering of messages before they are delivered to the subscribers.

If an event bus is used as a connector, the system would be similar to the event-based architectural style, in that subscribers register with the event bus to receive messages from a particular publisher, and the publisher sends messages to the event bus for delivery to its subscribers.

Because the event bus mediates communication, publishers may not be aware of the subscribers. However, remember there is an important difference here in the publish-subscribe architectural style, publishers can only send messages, not receive them, and subscribers can only receive messages, not send them.

CONCLUSION The flexible loose coupling between publishers and subscribers makes publish-subscribe systems scalable and flexible.

While designing your system, it’s important to consider that using this architectural style may make it impossible to modify a published message. Since each subscriber has their own copy of the message, a correction message may be the only way to convey a change.

Some example systems that can manifest this style are mailing lists, RSS feeds, and mobile messaging subscriptions.