How are expired codeigniter sessions cleaned up?

Question

Experts,

I'm trying to understand how sessions are cleaned up in Codeigniter in case the app is configured to store sessions in a database table.

In my case, I have three expired sessions in that session table and one active one for the same user. The manual states: "Note: The Session class has built-in garbage collection which clears out expired sessions so you do not need to write your own routine to do it."

Hmm, so when are my 'old' session in the db session table being cleared out or am I missing something?

thanks!

Answer 1

Here's the relevant source code of the Session class:

/**
 * Garbage collection
 *
 * This deletes expired session rows from database
 * if the probability percentage is met
 *
 * @access  public
 * @return  void
 */
function _sess_gc()
{
    if ($this->sess_use_database != TRUE)
    {
        return;
    }

    srand(time());
    if ((rand() % 100) < $this->gc_probability)
    {
        $expire = $this->now - $this->sess_expiration;

        $this->CI->db->where("last_activity < {$expire}");
        $this->CI->db->delete($this->sess_table_name);

        log_message('debug', 'Session garbage collection performed.');
    }
}

This function is called in one place, in the constructor of the Session class (almost the last line), so once per request under normal circumstances.

$this->gc_probability is hardcoded to 5 at the top of the class and it doesn't appear to be possible to change it. I'm not sure, but I believe this means that 5% of the time (randomly) the garbage collection will run, clearing out old entries from the sessions DB table.

Note that these old entries are of no significance or harm, the cleanup is only done so your database table does not get overloaded with old, useless records.