[JDEV] Extending Jabber

[Jeremie Miller <jeremie@netins.net>]

I think this is going to be a confusing thing, even I get confused by it
at times... There are quite a few different ways to extend Jabber, each
with their own purpose/speciality, but many times the functionality
desired by the extender/developer can be achieved more than one way(which
is good, but confusing).

I think this has already been the source of some difficulties for a few.  
IF I get a chance I'll try to write up an official guide and post it on
the site somewhere, but for now I'll give it my go in ASCII-land:

Ways of extending Jabber:

	Servers
	Transports
	Modules
	Clients

Servers: You can write an entire server that will live on a port on some
IP somewhere on the net, and if it speaks Jabber, then all other Jabber
servers will talk to it no prob.

Transports: A layer just under the server, so that you can write an app to
just "translate" between incoming Jabber data and some other system, and
send replies.

Modules: This lives specifically underneath the "Jabber" transport, it
handles the details of users, authentication, roster list management,
offline messages, logging, etc... Apache style.

Clients: I think the name says it all... just a piece of running software
that represents a channel to the user (channel being the screen in most
cases).


There should be very few reasons you'd want to extend by just writing a
new server, but it's not all that hard so I'm sure it will happen.
Actually, in the cases where there will always only be one transport
behind a server, it might be good to just roll the two together... as
mentioned, a special purpose "ISP" server that only provides the basics
for Jabber clients might be one way of extending at the "Server" level.

Transports.  This is the way to go whenever you want to build a two way
channel between different systems or userbases... ICQ, AIM, IRC, etc...
Another way of putting it: whenever you need your own addressable space
userid@server where you have many difrerent "userid", you need a
transport.  I would try to avoid using Transports for any other purpose
than to build a TWO WAY system between real-live users, or anything that
would be uniquely identified as a single user/individual on a roster.  It
probably is overkill to use it for simpler purposes, but the reasons
exist, but it should be a last resort :)

Importantly, modules exist.  Not much focus has happened here yet and
there is still plenty of work that needs to be done here.  A Module is
probably the *best* way to provide more functionality or special
functionality for users.  Modules usually make the final decision about
many things on the server, yet they are quite easy to write and maintain.
The API is still in flux and mostly needs to grow as more development
effort happens in this area.  Adding functionality for users should happen
from withing a module... nuff said :)

And finally, the last yet commonly forgotten way of extending the "Jabber
platform" is by building software-only clients :)  Yes, that's right...
just software that logs into the Jabber server and pretends it's a client.
These work out very nicely since Jabber allows multiple simultaneous
logins for every user and this "software agent" can lower it's priority so
that it can't recieve messages unless they are specifically addressed to
it.  A good example might be for pagers... Say there is a "pager server"
that is always running and accepts input via various means and can send
pages to a set of pre-determined users.  It could also log into the Jabber
server for each user and set it's nickname to "Pager".  Then, that users
friends/co-workers could just click on that users "Pager" session and send
them a page.  Of course security would be in place on the "pager server"
to only accept pages from certian users/friends.


ALSO, to note, there are OTHER ways of extending the platform but from a
different point of view... every status packet and message packet contain
an <ext></ext> tag that can contain anything the client so desires, and is
always just passed on untouched by the server(s).  It could be used in
combination between special clients, or between special servers,
transports, or even between a special module and a client.  I'm sure these
will be fairly popular and I encourage their use.

Of course, because of the way XML works, the Jabber protocol could be
significantly built upon and extra tags and attributes added throughout,
and as long as the "extended" client was talking to an "extended" server,
all would be happy.


Anyway, these are just some things to keep in mind since there are so many
ways of extending Jabber *grin*... actually, is there anything that isn't
extendable about Jabber? :)  

<action>raising hands above the crowd</action>
<say voice="booming">you may now return to your <divine>terminals</divine>
and resume coding</say>

And all was good,

Jer