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  1. Moodle
  2. MDL-59866

Edge case affecting redis sessions

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    • MOODLE_31_STABLE, MOODLE_32_STABLE, MOODLE_33_STABLE, MOODLE_34_STABLE
    • MOODLE_33_STABLE, MOODLE_34_STABLE
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      Prerequisite is a working redis installation available on localhost.

      Add

      define('TEST_SESSION_REDIS_HOST', '127.0.0.1');
      		 
      $CFG->session_handler_class = '\core\session\redis';
      		 
      $CFG->session_redis_host = '127.0.0.1';
      		 
      $CFG->session_redis_acquire_lock_timeout = 120;
      		 
      $CFG->session_redis_lock_expire = 7200;

      To config.php

      Set up phpunit and run:
      vendor/bin/phpunit lib/tests/session_redis_test.php

      To verify that there are no regressions.

      Then download the forked_redis_sessions.php script from this issue and put it in dataroot and then run it as apache user:

      sudo -u www-data php path/to/moodle/forked_redis_sessions.php

      Show
      Prerequisite is a working redis installation available on localhost. Add define( 'TEST_SESSION_REDIS_HOST' , '127.0.0.1' ); $CFG->session_handler_class = '\core\session\redis' ; $CFG->session_redis_host = '127.0.0.1' ; $CFG->session_redis_acquire_lock_timeout = 120 ; $CFG->session_redis_lock_expire = 7200 ; To config.php Set up phpunit and run: vendor/bin/phpunit lib/tests/session_redis_test.php To verify that there are no regressions. Then download the forked_redis_sessions.php script from this issue and put it in dataroot and then run it as apache user: sudo -u www-data php path/to/moodle/forked_redis_sessions.php

      We recently had an issue with a client where students were getting booted out of the LMS and getting a "Exception - Redis went away" error especially when doing quizzes affecting about less than 0.0001% of requests. We narrowed it down to a potential edge case bug in the phpredis module detailed here.

      A potential workaround is just to have the session retry the connection and that seems to work within the first retry (confirmed with unit tests).

        1. forked_redis_sessions.php
          3 kB

            marcusboon Marcus Boon
            marcusboon Marcus Boon
            Damyon Wiese Damyon Wiese
            Andrew Lyons Andrew Lyons
            Ryan Wyllie Ryan Wyllie
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              Created:
              Updated:
              Resolved:

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