Programming Jabber

The IQ Element

The third and final element in the Jabber building block set is the <iq/> element (“iq” stands for “info/query”), which represents a mechanism for sending and receiving information. What the <iq/> element has over the <message/> element for this purpose is structure and inherent meaning. It is useful to liken the info/query mechanism to the request/response model of HTTP using GET and POST.

The <iq/> element allows a structured conversation between two Jabber entities. The conversation exists to exchange data, to retrieve or set it, and to notify the other party as to the success (or not) of that retrieve or set action. There are four states that an <iq/> element can be in, each reflecting one of the activities in this conversation:

get

Get information.

set

Set information.

result

Show the result when the get or set was successful.

error

Specify an error if the get or set was not successful.

These states are reflected in the type attribute of <iq/> elements. The relationship between two entities in such a structured conversation that convey these states is shown in Figure 5-4.

Figure 5-4. Entities in an <iq/>-based conversation

Note

As you can see, the combination of the <iq/> element specification and the type attribute is written like this:

IQ-type

For example, “IQ-get” refers to an <iq/> element with type='get', and so on.

Earlier in this chapter, we saw ...