Programming Jabber

The Presence Element

The <presence/> element is used to convey a Jabber entity’s availability. An entity can be available, which means that it’s connected and any messages sent to it will be delivered immediately, or it can be unavailable, which means that it’s not connected, and any messages sent to it will be stored and delivered the next time a connection is made.

For the large part, it is the entity itself, not the Jabber server to which it connects, that controls the availability information. The Jabber server will communicate an entity’s unavailability if that entity disconnects from the server but will do that only if the entity has communicated its availability beforehand.

Availability information isn’t a free-for-all. Presence in Jabber is usually exchanged within a subscription mechanism. See Section 5.4.2.3 for an explanation.

Presence Management

It’s worth noting that the entities referred to here are client entities, that is, clients (and therefore the users using those clients) connected to the Jabber server over an XML stream qualified by the jabber:client namespace (see Section 5.3). Presence is a feature that is used throughout Jabber; the Jabber Session Manager (JSM) manages presence on behalf of clients. External components that connect to the Jabber server backbone are separate from the JSM and therefore don’t have any concept of “managed” presence. That’s not to say they can’t partake in the sending and receiving of presence elements. They just have to ...