I worked on a large XMPP system for a while and realized it's actually not a very good protocol, for one reason: presence. "Presence" is the feature that shows a little indicator next to each one of your contacts indicating whether they're online or away.
The problem with presence is that it generates a LOT of traffic. It's N^2 with the average number of contacts each user has - every time a user changes presence, you have to send a message to every one of their contacts to update them about the presence. This happens whether they're paying attention to your client or not. So on a large system, the big majority of the traffic is just presence messages flying back and forth saying who is at their keyboard and who isn't. This is really wasteful in general. Worse, it's not a super important feature for a lot of people, so it's definitely not worth the effort. And ever since mobile computing became ubiquitous, it's not even clear what this is supposed to represent.
So, I say let XMPP die. It was a fine idea for its time. But the protocol has no place in the modern world.
The problem with presence is that it generates a LOT of traffic. It's N^2 with the average number of contacts each user has - every time a user changes presence, you have to send a message to every one of their contacts to update them about the presence. This happens whether they're paying attention to your client or not. So on a large system, the big majority of the traffic is just presence messages flying back and forth saying who is at their keyboard and who isn't. This is really wasteful in general. Worse, it's not a super important feature for a lot of people, so it's definitely not worth the effort. And ever since mobile computing became ubiquitous, it's not even clear what this is supposed to represent.
So, I say let XMPP die. It was a fine idea for its time. But the protocol has no place in the modern world.