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Chapter 12. Publish and Subscribe > Introducing Publish and Subscribe

12.1. Introducing Publish and Subscribe

The problems of efficient delivery of information have existed for far longer than RSS has, as have techniques for addressing them. Indeed, both the systems described in this chapter follow the classic Gang of Four "Observer" design pattern. It is perhaps from the other title of this pattern that the name of the technique arises: Publish and Subscribe.

Let's think of the situation from above. We have a feed, and this feed has its users. The users take the feed and do what they will — we've already seen some examples of potential uses in the previous chapters — but each of the users depends on his copy of the feed (whether in memory, or converted to another format and saved) being up-to-date. In order for this to happen, the users subscribe to a system that watches the feed continuously. This system then publishes notifications to all of the users when the feed changes. The users are then responsible for updating their copies of the feed, ordinarily by requesting it from the server.


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