A common dir means there is only one place to look when woken up to process input. The file name itself can contain unique information like what user sent it and what the sequence number is. A "kill -SIGUSR1 `<pid_file`" can wake up the service.

If you make it a UDP service, the server blocks on the UDP socket waiting for incoming messages, processing them one at a time. They can be queued in the socket buffer, which can be made large enough to deal with the probalbe backlog. The process reading the socket can just log the messages for further processing, so there is no liklihood of a big backlog. The service can frame the bytes in the log into different discussions. Once the log gets to a certain size, it can be rotated out for another. The log could even be a zip file for the day, with each message an archived file in appropriate interanl folder paths. You can glob through zip files as if they were directories with some tools like MS Windows Compressed folders and HXTT JDBC CSV/Text Drivers. UDP also allows for a multicast of messages back to participating users. The size is a bit restricted, not as much as twitter, 65k, but 1,440 is a more reliable size.