40k might be pushing it, but 10-15k should be no problem on current hardware.
More importantly IRC servers have a battle hardened scaling solution baked right in, based on the spanning tree protocol.
Anyone looking to write a scalable chat software would be well advised to study the evolution of IRC. Anyone looking to hack a scalable chat solution would be well advised to use a mature IRCd for the backend.
Edit: I hung out on DALnet back when twisted was still linked and when it got taken down the crush of the users, as the article mentions, rendered the network virtuall inaccessible. Goes to show what happens when you rely too much on one link.
More importantly IRC servers have a battle hardened scaling solution baked right in, based on the spanning tree protocol.
Anyone looking to write a scalable chat software would be well advised to study the evolution of IRC. Anyone looking to hack a scalable chat solution would be well advised to use a mature IRCd for the backend.