How does core triggers cleanup of expired sessions in database?

Question

core/lib/Drupal/Core/Session/SessionHandler.php defines a method (gc()) to clear expired sessions from the database (sessions table)

when is that triggered ? is there a way to trigger it from the interface ?

i know i can just truncate sessions manually or with a cron or a custom drush command..., but i wonder how does core uses it and what triggers it by default

Thanks

Answer 1

Configuring Garbage Collection

When a session opens, PHP will call the gc handler randomly according to the probability set by session.gc_probability / session.gc_divisor. For example if these were set to 5/100 respectively, it would mean a probability of 5%. Similarly, 3/4 would mean a 3 in 4 chance of being called, i.e. 75%.

If the garbage collection handler is invoked, PHP will pass the value stored in the php.ini directive session.gc_maxlifetime. The meaning in this context is that any stored session that was saved more than gc_maxlifetime ago should be deleted. This allows one to expire records based on idle time.

However, some operating systems (e.g. Debian) do their own session handling and set the session.gc_probability variable to 0 to stop PHP doing garbage collection. That's why Symfony now overwrites this value to 1.

If you wish to use the original value set in your php.ini, add the following configuration:

# config/packages/framework.yaml
framework:
    session:
        gc_probability: null

_{Source: Symphony's official docs on Configuring Sessions and Save Handlers}

In Drupal this is seen in /sites/default/default.settings.php

/**
 * Some distributions of Linux (most notably Debian) ship their PHP
 * installations with garbage collection (gc) disabled. Since Drupal depends on
 * PHP's garbage collection for clearing sessions, ensure that garbage
 * collection occurs by using the most common settings.
 */
ini_set('session.gc_probability', 1);
ini_set('session.gc_divisor', 100);

Since the above "most common settings" is set to 1/100, Garbage Collection would run 1% of the time.

Now, as per the code comment, I presume this does not get put into settings.php when it gets auto-generated if your distribution has one already set one in one of its files as I don't see this in my settings.php

You can also set this in services.yml as seen in this answer.

---

In Conclusion

Note that while Drupal is built on top of Symphony, it does not rely on Symphony for this. I just quoted their nice explanation from their documentation.

We learned that PHP automatically calls this gc() method every session.gc_probability/session.gc_divisor % of the time and if you look at PHP docs on SessionHandler.

SessionHandler is a special class that can be used to expose the current internal PHP session save handler by inheritance. There are seven methods which wrap the seven internal session save handler callbacks (open, close, read, write, destroy, gc and create_sid).

_{Source: http://php.net/manual/en/class.sessionhandler.php}

core/lib/Drupal/Core/Session/SessionHandler.php

/**
 * Default session handler.
 */

class SessionHandler extends AbstractProxy implements \SessionHandlerInterface { ...

 /**
   * {@inheritdoc}
   */
  public function gc($lifetime) {
    // Be sure to adjust 'php_value session.gc_maxlifetime' to a large enough
    // value. For example, if you want user sessions to stay in your database
    // for three weeks before deleting them, you need to set gc_maxlifetime
    // to '1814400'. At that value, only after a user doesn't log in after
    // three weeks (1814400 seconds) will his/her session be removed.
    $this->connection->delete('sessions')
      ->condition('timestamp', REQUEST_TIME - $lifetime, '<')
      ->execute();
    return TRUE;
  }