Developing Feeds with RSS and Atom

Publish and Subscribe

The traditional guidelines for the use of syndication feeds present a small problem. There is always going to be a balance between the need to be up to date and the need to refrain from abusing the feed publisher’s server by requesting the feed every few seconds. While the norm is to request the feed a maximum of once an hour, many feeds deserve to be followed much more closely. Conversely, many feeds update only once a day, or less often. Requesting those once an hour is a waste of time and resources and potentially expensive for the publisher. On top of the compressed feeds and conditional GET already discussed, one other idea has long existed within the feed world: Publish and Subscribe.

Let’s think of the earlier situation. We have a feed, and this feed has its users. The users take the feed and do what they will—I’ve presented some examples of potential uses in the previous chapters—but each user 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.

Currently, there is no Publish and Subscribe mechanism for Atom, but there are two ways to implement such a system within RSS: one with the relevant elements ...