In one of the previous posts we wrote about session reliability. Today we will talk about “locking session data”. This is another session reliability topic and we will look at the problems that may occur in Symfony2 and how to solve them.
Session locking is when the web server thread acquires an exclusive lock on the session data to avoid concurrent access. Browsers use HTTP 1.1 keep-alive and would normally just use one open TCP connection and reuse that to get all dynamic content. When loading images (and other static content) the browser may decide to use multiple TCP connections (concurrent) to get the data as fast as possible. This also happens when using AJAX. This may (and will most likely) lead to different workers (threads) on the web server answering these concurrent requests concurrently.
Each of the requests may read the session data update and write it back. The last write wins, so some writes may get lost. This can be countered by applying session locking. The session lock will prevent race conditions from occurring and prevent any corrupted data appearing in the session. This can easily be understood by looking at the following two images.
The left image shows concurrent requests without session locking and the right shows concurrent requests with session locking. This is very well described in this post by Andy Bakun. Note that the above images are also from that post. Reading the Andy Bakun post allows you to truly understand the session locking problem (and the performance problems that AJAX may cause).
In Symfony2 one would normally use the NativeFileSessionHandler, which will just use the default PHP session handler. This works flawless in most cases. PHP uses “flock” to acquire an exclusive lock on the local filesystem. But when you scale out and run a server farm with multiple web servers you cannot use the local filesystem. You might be using a shared (NFS) filesystem and run into problems with “flock” (see the Linux NFS FAQs). If you use the database you may run into performance problems, escpecially when applying locking.
This leaves Memcache or Redis as options for session storage. These are fast key/value stores that can be used for session storage. Unfortunately the Symfony2 session storage implementations for Memcache (in Symfony) and Redis (in phpredis) do not implement session locking. This potentially leads to problems, especially when relying on AJAX calls as explained above. Note that other frameworks (like CakePHP) also do not implement session locking when using Memcache as session storage. Edit: This post has inspired the guys from SncRedisBundle and this Symfony2 bundle now supports session locking, which is totally awesome!
Custom save handlers
One can write “Custom Save Handlers” as described by the Symfony2 documentation:
Custom handlers are those which completely replace PHP’s built in session save handlers by providing six callback functions which PHP calls internally at various points in the session workflow. Symfony2 HttpFoundation provides some by default and these can easily serve as examples if you wish to write your own. — Symfony2 documentation
But you should be careful, since the examples do not implement session locking.
LswMemcacheBundle to the rescue
At LeaseWeb we love (to use) Memcache. Therefore, we have built session locking into our LswMemcacheBundle. It actually implements acquiring a “spin lock” with the timeout set to PHP’s “max_execution_time” (defaults to 30 seconds). The spin lock tries to acquire the lock every 150 ms (configurable). It will also hold the lock for a maximum time of the PHP “max_execution_time”. By using Memcache’s built-in key expire mechanism, we can ensure the lock is not held indefinitely.
This (spin-lock) implementation is a port of the session locking code from the memcached PECL module (written in C). Our bundle enables locking by default. If you want, you can disable the locking by setting the “locking” configuration parameter to “false” as described in the documentation.
This session locking code was also ported to SncRedisBundle and submitted as PR #109. LswMemcacheBundle is open-source and can be found on our GitHub account: