Configure your installation for memcached sessions (see config-dist.php). To make the effect manifest more quickly, adjust the lock timeout value to 10 seconds: $CFG->session_memcached_acquire_lock_timeout = 10; Simulate a long-running request by editing a repository plugin, in this case use repository/recent/lib.php, line 90, insert the statement "sleep(15);" to cause a delay in returning the result. Open your Moodle site, and navigate to a course where you have editing (teacher) capability, turn editing on for the course, and click "Add an activity or resource", select the File resource option, click Add. When the modedit interface (Add a new File) interface appears, click on the files display to cause the Filepicker dialog to appear. Access the developer tools extension for your browser (e.g. Firefox & Chrome, Ctrl + Shift + I), and select the tab to display network activity. From here you can click alternatively between 'Upload a file' and 'Recent files', allowing the request to finish between alternating clicks. When clicking on 'Recent files' you will see the two XHRs dispatched, one for setuserpref.php, and one for repository_ajax.php. When clicking on 'Upload files' expect to see only the call to setuserpref.php. Most often, you will see the setuserpref.php call completed almost immediately, and the subsequent call to repository_ajax.php will be pending until the simulated 15 sec. delay has elapsed. If the list of network calls gets too cluttered, you can clear the list. It may take several attempts toggling back and forth, but eventually you will see both requests pending, which means the repository_ajax.php call acquired the session key lock first. After 10 seconds (the session acquire lock timeout), the setuserpref.php will return, then 5 seconds afterward the repository_ajax call will return. Once this error condition has occurred you should notice the session cookie key value has changed--there was a new cookie value returned with the setuserpref.php operation. The mdl_sessions row corresponding to the original session cookie key value has been deleted. And, if using manual (Moodle internal) authentication, you should see that you now have to re-authenticate. An alternative testing method would be to toggle quickly, repeatedly, between 'Recent files', 'Upload files' and any of the other repositories. Those requests will be queued up, and while at least one of them is a 'Recent files' request where the simulated delay is in effect, the other requests will timeout waiting for a session key lock.