Why are my XHR calls waiting for each other to return a response

Question

I have an iframe within a page that is continually polling the server for a session variable that is being actively updated by a \"main\" XHR.

So basically:

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Answer 1

Yes, default session manager (using files) locks the session file when you do session_start and releases it when you do session_write_close (or the scripts ends). Meanwhile other scripts trying to access session, wait to the release. a detailed article here or at the manual session-write-close