Most people who know about Jabber/XMPP think of it as an instant messaging platform. Of course, that is the primary use for Jabber at present but that may not always be the case.

The Jabber/XMPP network forms an XML based overlay network. Each message or packet of information carried by this overlay network is an XML stanza. You can think of Jabber servers as being XML routers and the clients as end nodes. In fact, the instant messaging portions of the XMPP standards are defined in a separate RFC from the core XML streaming technology (RFC 3920, RFC 3921).

One example of a non-IM use of Jabber is defined in JEP-0072: SOAP over XMPP. This document specifies how SOAP, which is normally used with HTTP to form web services, can be carried on top of Jabber/XMPP.

Another interesting non-IM use of Jabber comes from JEP-0060: Publish-Subscribe (aka pubsub). Pubsub is basically an event notification system that runs on top of Jabber/XMPP. In pubsub, a user publishes some XML data to a Jabber server which supports JEP-0060. Other users are then able to “subscribe” to this node. Whenever the node changes, a notification will be sent to all subscribed users.

There are lots of interesting things that could be done with pubsub. Off hand, here are a few of examples:

• You want to checkout a book from the local Library. Unfortunately, someone else already has the book. In order to find out when the book has been returned, you subscribe to the node that represents that book on the library’s pubsub server. Once the book is returned you will know instantly.
• You plan on purchasing a large, expensive TV in the near future. Rather than manually looking at the websites for several major retailers every few days, you subscribe to the pubsub node at each retailer for the particular TV model you are interested in. If any of the retailers have a sale, you find out instantly.
• If like many people you use a RSS reader to keep up with new posts on your favourite blogs, you know that RSS readers periodically poll all feeds on your list. Often there are no new posts and this polling is a waste of resources. Instead, a pubsub enabled blog could notify interested readers of a new post. Not only do you find out about the new post sooner, network resources are saved.

In all of the above examples, subscribing to the particular pubsub node could be as simple as clicking on a link (JEP-0147: XMPP URI Scheme Query Components).

Also interesting is JEP-0163: Personal Eventing Protocol which defines a subset of the full pubsub (JEP-0060) specification which can be used for simpler instant messaging related tasks such as providing current geographic location information (JEP-0080: User Geolocation) or providing contacts with information about the music you are currently listening too (JEP-0118: User Tune).

It will be interesting to see how pubsub will be integrated into other network applications such as RSS readers and Jabber IM clients. It seems likely that pubsub notifications will be handled either by a Jabber client separate from the one that is used for IM or at least the Jabber IM client will have to distinguish these events from normal IM traffic.

For a nice overview of pubsub (with pretty pictures) see Jive Software: All About Pubsub.