I can't really defend Twitter's architecture, I am simply interested in the best way to implement something like that as well - but not as a centralized service.
Just a note: email does polling, too. Most email clients poll the server on a regular basis. But that is usually every 5 minutes or less? Twitter has just limited polling to 30 times per minute for external clients, if I remember correctly.
I haven't thought too deeply about it, but I wonder if a mixture between polling and pushing could work. Either it could be decided on the nature of the client if it should push or poll (ie how many subscribers does it have, how active is it), or clients could connect and say "please push for the next 20 minutes" or just send keep-alives. If client alive, push, otherwise let him poll? I doubt that polling could be eliminated completely. If an email server is offline for an extended period of time, the message never arrives. Mabye that is acceptable, though.
Just a note: email does polling, too. Most email clients poll the server on a regular basis. But that is usually every 5 minutes or less? Twitter has just limited polling to 30 times per minute for external clients, if I remember correctly.
I haven't thought too deeply about it, but I wonder if a mixture between polling and pushing could work. Either it could be decided on the nature of the client if it should push or poll (ie how many subscribers does it have, how active is it), or clients could connect and say "please push for the next 20 minutes" or just send keep-alives. If client alive, push, otherwise let him poll? I doubt that polling could be eliminated completely. If an email server is offline for an extended period of time, the message never arrives. Mabye that is acceptable, though.