[jdev] How is a client crash communicated to the XMPP server?

[Jonathan Dickinson]

Good question.

You will find that the Coversant XMPP communicator does not send a <presence type="unavailable"/> stanza when it logs out. From what I can tell is that most servers will do that on behalf of the client: it caused me a few minutes of debugging :).

If you want to tell when the client disconnects unexpectedly from another client (e.g. if you have a contact that represents your server availability or something) you could do something like:

<presence type="available">
  ...
  <!-- NB: No 'graceful' element -->
</presence>

<presence type="unavailable">
  ...
  <graceful xmlns="http://www.tempuri.org/xmpp.org/graceful">
</presence>

If you receive a stanza without the graceful element you can be sure that the client quit unexpectedly (server sent 'unavailable' on behalf of the client).

This raises another important question: most servers AND clients are not XMPP compliant. It is very strange and annoying. Should I take a 'tough luck, you should behave properly' approach to non-compliant clients and servers?

> -----Original Message-----
> From: jdev-bounces at jabber.org [mailto:jdev-bounces at jabber.org] On Behalf Of
> JabberForum
> Sent: 02 June 2008 10:25 AM
> To: jdev at jabber.org
> Subject: [jdev] How is a client crash communicated to the XMPP server?
>
>
> Hi,
> I would like to discuss about what happens when an XMPP client crashes
> and how this information is communicated to the server..
>
> I have a server S and 2 clients C1 and C2 connected to the server. C1
> has C2 in its roster list and C2 has C1 in its roster list.
>
> If C1 gracefully logs out, then an iq stanza of type unavailable is
> sent to the server and the server broadcasts that stanza to all JIDs
> which have a subscription of from or both.
>
> However, I have been trying to search for what happens when the
> client crashes (or exits ungracefully). I havent arrived at the answer
> yet. Here is what RFC 3291 says :
>
> "5.1.5.  Unavailable Presence
>
> Before ending its session with a server, a client SHOULD gracefully
> become
> unavailable by sending a final presence stanza that possesses no 'to'
> attribute and that possesses a 'type' attribute whose value is
> "unavailable" (optionally, the final presence stanza MAY contain one or
>
> more <status/> elements specifying the reason why the user is no longer
>
> available). *However, the user's server MUST NOT depend on receiving
> final
> presence from an available resource, since the resource may become
> unavailable unexpectedly or may be timed out by the server. If one of
> the
> user's resources becomes unavailable for any reason (either gracefully
> or
> ungracefully), the user's server MUST broadcast unavailable presence to
>
> all contacts *(1) that are in the user's roster with a subscription
> type of
> "from" or "both", (2) to whom the user has not blocked outbound
> presence,
> and (3) from whom the server has not received a presence error during
> the
> user's session; the user's server MUST also send that unavailable
> presence
> stanza to any of the user's other available resources, as well as to
> any
> entities to which the user has sent directed presence during the user's
>
> session for that resource (if the user has not yet sent directed
> unavailable presence to that entity). Any presence stanza with no
> 'type'
> attribute and no 'to' attribute that is sent after sending directed
> unavailable presence or broadcasted unavailable presence MUST be
> broadcasted by the server to all subscribers. "
>
> Do we interpret that as : "the server will 'somehow' know that the
> client has crashed and then publish the unavailable stanzas to everyone
> subscribed to that JID ?
>
> Please clarify.. :-)
>
> -santhosh
>
>
> --
> santhosh.kulandaiyan
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