XMPP is much better than most people realize, anyhow.
No, it is actually worse than most people realize. You only begin to realize the magnitude of the failure once you try to implement a client, and most people don't do that. XMPP is a trainwreck. It has never seen serious adoption because nobody wants to touch it - and for good reasons.
XMPP is what I cite when I try to explain the "XML mindset". It leads to bad things. It leads to ridiculous overengineering through layered complexity. It leads to a client/server ecosystem where each implementation speaks a different dialect because it's nearly impossible to get the protocol right.
There was a time when my roster would get screwed up in new, random, interesting ways whenever I launched a different client. Some clients would even manage to unsubscribe existing contacts for inexplicable reasons. And don't get me started on "Transports".
However, instant messaging is not rocket science. Neither is semi-decentralized instant messaging. XMPP makes it seem like a much harder problem than it really is, but only because XMPP is broken beyond repair.
Most people do not realize how much of the messiness comes from the problem and not the protocol.
Take a lesson from IRC, a group-chat protocol that, despite its age, works and scales amazingly well. A protocol that, despite an immense range of features, can easily be typed by a human on a telnet prompt, in real time.
It wouldn't take much fix the warts on IRC and extend it to cover everything that XMPP tries to do. This is what the XMPP author should have done in first place.
I have implemented two clients, a generalized libpurple transport, and spent significant time on a server in a professional environment for sale to real customers. IRC isn't a replacement for what it does, and yeah, the problems come from the problem domain, not the protocol. I'd know the difference. None of the "professional" protocols seem to be significantly simpler, because they can't be... the domain complexity forbids it. I spent much more time working on semantic issues than I ever did working on the raw XML, orders of magnitude difference. The XML was always the easy part, even if JSON would have been even easier. The trappings are irrelevant. I could transform the entire XMPP protocol stack into JSON in a week, with implementation in ejabberd and one of my clients. XML is a sideshow.
The problem is getting the standardized semantics. If XMPP taught me anything, it is that it doesn't matter how many specs you throw at a programmer, they're just going to bash on the program until it sort of works most of the time and release it. That's where your roster problems come from, it's where a not insignificant number of your transport problems come from too. (Though the transport protocol is one of the spottier bits of the protocol.) The core bits of XMPP are generally reasonably well specified and in my experience actually held up surprisingly well as I bent and spindled it a little bit. (Corporate customers don't "get" rosters, don't get that you can start with a blank roster and work your way up, so I added a module to build rosters based on grouping criteria specified by the user and driven by outside input. Well beyond the stock ejabberd shared rosters, but pretty custom to our environment. XMPP actually dealt with these semi-magical roster entries just fine, to my surprise.) Those specifications really matter and just sort of bashing some stuff out that's 90% correct most of the time isn't good enough when you're trying to communicate with so many different systems. XMPP actually managed to avoid a lot of problems that even the "professional" systems had, having learned from their experiences; AIM last I knew still had some encoding corner cases you wouldn't expect in a modern program, all of the protocols had major encoding growing pains, surprising versioning issues, all these little quirks inside them that you never noticed because you can paper over a lot when you control both the clients and the servers. Again, I know it's not perfect but in the space of "deployed IM protocols" it does not make a bad showing.
IRC clients are actually just as quirky, IRC just doesn't hang on to anywhere near as much state or you'd see it mangled, spindled, and mutilated too. (Also part of the reason it's not a replacement, real users want that state.) This is fine, too, I don't have a problem with IRC for what it is, but you can't just drop it in everywhere you see an XMPP server.
If you try to work IRC up to be a real, true XMPP replacement, you'll be complaining about how hard it sucks in no time. Too much suckage is in the problem space.
If you try to work IRC up to be a real, true XMPP replacement, you'll be complaining about how hard it sucks in no time.
I doubt that.
The parts that actually somewhat work in XMPP would be fairly straightforward to add to IRC (mostly related to persistence and identity). From there the question is where you'd want to take it, not what idiocies XMPP fell for. I.e. the task would be to do it right, not to imitate a broken protocol.
And then tell me with a straight face the complexity is "inherent to the problem". No. It's not.
IRC handles very similar problems to XMPP already (and then some that XMPP doesn't have) and the specification, in its entirety, is only 3643 lines long. Extending that for distributed, message-persisting operation would not bring it anywhere near the insanity of XMPP.
Naturally that's an academic exercise, nobody would actually re-shape IRC into an IM system that way. However, when cherry-picking concepts for a new protocol then IRC should be high on the list, and XMPP rather low.
It's a lot easier to write a functional IRC client by glancing at the RFC and banging on a piece of code until it mostly works. For example, http://lists.canonical.org/pipermail/kragen-hacks/2008-Febru... was written that way. Given that the only protocol message it has special handling for is PING, though, I'm pretty sure you could have written it based solely on the RFC.
However, I think it's a big mistake to claim IRC "scales amazingly well". The biggest IRC network today has tens of thousands of users (at the moment, freenode has 64000, undernet has 58000, and EFNet is down in the 32000 range) and the IRC networks are constantly suffering from breakdowns from overcapacity. Compare this to Skype, Facebook, or Gmail, with tens of millions of concurrent users.
I challenge you to write a functional irc client based solely on the rfc.
I've actually written IRC bots mostly from the top of my head.
The protocol and semantics are really simple.
Type this into a console near you:
nc irc.freenode.org 6667
USER foo bar batz boo
PRIVMSG #testchannel hello world
PRIVMSG test345 hello self
Yes, that's all it takes for a minimal, functional client.
(just remember to type PONG every once in a while)
I'm not sure what you mean by fractured. Like every protocol it has a few rough edges, but those are nowhere near the semantic nightmare that I witnessed when trying to dabble with XMPP (which admittedly was more than a year ago).
I remember attempting to write an IRC bot a while back and finding that RFC severely lacking when it came to connecting to whatever irc network I had chosen to test it against. The problem was that the handshake to join the server was different in the RFC than what the server was expecting. This sentiment was mirrored by others I consulted with over IRC that were devs on the epic3 irc client. So, while writing a IRC bot may be simple, I would imagine that writing a client isn't. The protocol itself was the same (in the "COMMAND args" sense), but the conventions differed.
Speaking as someone who spent a long time working on Undernet trying to make IRC work. The RFC is poorly specified (eg ~ and ^ are missed from the case equivalent list). IRC scales poorly (the protocol relies heavily on global state). IRC doesn't do UTF-8, or in fact, any sane character set (see the case mapping mentioned above). Client's ignore the parsing rules (eg the : marking multiword last arguments) and kludge around them. Each network went off and did their own thing, fragmenting the protocol space, and then declared themselves as being the One True IRC Protocol. IRC puts a lot of trust in all the server admins on the network, making it difficult to federate, and so on.
IRC was a great protocol in the 1980's, it's been dead for a while, just there are no good replacements.
Yes, IRC has warts, I didn't mean to declare it as the end of all things.
The point I was trying to make is that IRC would be a more sane starting point than XMPP. Even despite all the shortcomings you mentioned and some more that you didn't. And even despite it being a strictly centralized design that would require more server-side changes than XMPP to turn it into a distributed system.
I'll lean out of the window and even claim you could make a distributed IRCd backwards compatible to existing IRC clients, as far as the core business of presence/state, chat and group-chat are concerned.