How to store PHP sessions in APC Cache?

Question

Storing sessions in disk very slow and painful for me. I'm having very high traffic. I want to store session in Advanced PHP Cache, how can I do this?

Answer 1

I tried to lure better answers by offering 100 points as a bounty, but none of the answers were really satisfying.

I would aggregate the recommended solutions like this:

Using APC as a session storage

APC cannot really be used as a session store, because there is no mechanism available to APC that allows proper locking, But this locking is essential to ensure nobody alters the initially read session data before writing it back.

Bottom line: Avoid it, it won't work.

Alternatives

A number of session handlers might be available. Check the output of phpinfo() at the Session section for "Registered save handlers".

File storage on RAM disk

Works out-of-the-box, but needs a file system mounted as RAM disk for obvious reasons.

Shared memory (mm)

Is available when PHP is compiled with mm enabled. This is builtin on windows.

Memcache(d)

PHP comes with a dedicated session save handler for this. Requires installed memcache server and PHP client. Depending on which of the two memcache extensions is installed, the save handler is either called memcache or memcached.

Answer 2

In theory, you ought to be able to write a custom session handler which uses APC to do this transparently for you. However, I haven't actually been able to find anything really promising in a quick five-minute search; most people seem to be using APC for the bytecode cache and putting their sessions in memcached.

Answer 3

<?php

// to enable paste this line right before session_start():
//   new Session_APC;
class Session_APC
{
    protected $_prefix;
    protected $_ttl;
    protected $_lockTimeout = 10; // if empty, no session locking, otherwise seconds to lock timeout

    public function __construct($params=array())
    {
        $def = session_get_cookie_params();
        $this->_ttl = $def['lifetime'];
        if (isset($params['ttl'])) {
            $this->_ttl = $params['ttl'];
        }
        if (isset($params['lock_timeout'])) {
            $this->_lockTimeout = $params['lock_timeout'];
        }

        session_set_save_handler(
            array($this, 'open'), array($this, 'close'),
            array($this, 'read'), array($this, 'write'),
            array($this, 'destroy'), array($this, 'gc')
        );
    }

    public function open($savePath, $sessionName)
    {
        $this->_prefix = 'BSession/'.$sessionName;
        if (!apc_exists($this->_prefix.'/TS')) {
            // creating non-empty array @see http://us.php.net/manual/en/function.apc-store.php#107359
            apc_store($this->_prefix.'/TS', array(''));
            apc_store($this->_prefix.'/LOCK', array(''));
        }
        return true;
    }

    public function close()
    {
        return true;
    }

    public function read($id)
    {
        $key = $this->_prefix.'/'.$id;
        if (!apc_exists($key)) {
            return ''; // no session
        }

        // redundant check for ttl before read
        if ($this->_ttl) {
            $ts = apc_fetch($this->_prefix.'/TS');
            if (empty($ts[$id])) {
                return ''; // no session
            } elseif (!empty($ts[$id]) && $ts[$id] + $this->_ttl < time()) {
                unset($ts[$id]);
                apc_delete($key);
                apc_store($this->_prefix.'/TS', $ts);
                return ''; // session expired
            }
        }

        if (!$this->_lockTimeout) {
            $locks = apc_fetch($this->_prefix.'/LOCK');
            if (!empty($locks[$id])) {
                while (!empty($locks[$id]) && $locks[$id] + $this->_lockTimeout >= time()) {
                    usleep(10000); // sleep 10ms
                    $locks = apc_fetch($this->_prefix.'/LOCK');
                }
            }
            /*
            // by default will overwrite session after lock expired to allow smooth site function
            // alternative handling is to abort current process
            if (!empty($locks[$id])) {
                return false; // abort read of waiting for lock timed out
            }
            */
            $locks[$id] = time(); // set session lock
            apc_store($this->_prefix.'/LOCK', $locks);
        }

        return apc_fetch($key); // if no data returns empty string per doc
    }

    public function write($id, $data)
    {
        $ts = apc_fetch($this->_prefix.'/TS');
        $ts[$id] = time();
        apc_store($this->_prefix.'/TS', $ts);

        $locks = apc_fetch($this->_prefix.'/LOCK');
        unset($locks[$id]);
        apc_store($this->_prefix.'/LOCK', $locks);

        return apc_store($this->_prefix.'/'.$id, $data, $this->_ttl);
    }

    public function destroy($id)
    {
        $ts = apc_fetch($this->_prefix.'/TS');
        unset($ts[$id]);
        apc_store($this->_prefix.'/TS', $ts);

        $locks = apc_fetch($this->_prefix.'/LOCK');
        unset($locks[$id]);
        apc_store($this->_prefix.'/LOCK', $locks);

        return apc_delete($this->_prefix.'/'.$id);
    }

    public function gc($lifetime)
    {
        if ($this->_ttl) {
            $lifetime = min($lifetime, $this->_ttl);
        }
        $ts = apc_fetch($this->_prefix.'/TS');
        foreach ($ts as $id=>$time) {
            if ($time + $lifetime < time()) {
                apc_delete($this->_prefix.'/'.$id);
                unset($ts[$id]);
            }
        }
        return apc_store($this->_prefix.'/TS', $ts);
    }
}