XMPP is most definitely not a message bus, and message buses or message queues are two technologies that seem like the practical/workable implementation of mult

XMPP is most definitely not a message bus, and message buses or message queues are two technologies that seem like the practical/workable implementation of multicast for the purposes of pubsub. One thing that I think makes XMPP so heavyweight is that it's XML. And it took so long to get off the ground because the documentation was like "here, look, it's so easy to send messages over Jabber, it's just XML" but didn't really explain all the "presence" terminology nor provide libraries for a number of popular languages initially that hid all the complexity and let you actually join to a server. Even the perl XMPP library on CPAN talks about "XML stanzas" -- dude, all I want to do is join a server and send and receive messages, hook me up to an event loop! (admittedly, the perl module in question does this, there's just a lot of XMPP baggage to get around). XMPP also include{s,d} a lot of complexity for things that normally happen out of band for security purposes, like user creation on the servers. I mean, XMPP makes the IMAP protocol look simple.