Long polling issue with high traffic website

Question

Say I have script, that does long polling on server to check if user has any new mesages. Server side would be something like this

while counter < 5
    if something_changed
        push_changes_to_client
        break
    else
        counter++
        sleep 5

Which checks database 5 times and every time if there is no change, it waits 5s untill next check, which results in maximum execution time of about 25s.

What happens when client moves from one page to another really fast? I suppose the server script keep on running even after client move to different page, where it sends another request for changes.

Does this mean, that when lot of people are moving quickly around the site (less than the 25s max execution on each page), then the server has to keep running all the scripts, that are trying to respond to page that doesn't exist any more? Wouldn't this cause the server to use all of it's thread pool pretty fast?

Answer 1

In a thread-per-connection model with synchronous sleep calls, this indeed may tie up a large number of threads. However, if the "sleep" simply schedules a callback and returns, the thread pool logjam can be avoided.