In PHP, sessions are by default stored in a file in a directory. Sessions can be specifically destroyed from within the code, for example when users logout explicitly, but frequently they do not. As a result session files tend to hang around, and cause the problem of how to clean them up. The standard way is to use PHP’s own garbage collection which is normally enabled out of the box. In this, we define constants that specify the maximum lifetime for the session, and essentially the probability of clean up.
To make things more interesting, Debian, out of the box doesn’t do garbage collection in this way. It has a cron job that regularly erases session files in the defined session directory. But, if like me, and many others, you put your session files in a different directory for each application to avoid clashes on namespaces for two applications running under the same browser from the same server, you have a problem. If you forget Debian’s behaviour the session files will just grow indefinitely. I had forgotten this issue and found over a year’s worth of session files in a directory recently.
Solving this problem is actually quite difficult to do optimally. I mean, I could create a cron job to mirror Debian’s own, but then I’d have to put the maximum lifetime in a cron job somewhere, out of the way, and difficult for the average sys admin I’m working with to find and deal with. (That is, away from the main configuration of the project). Or I could parse this value out of the main configuration. But this leads to another problem. For some users, a 30 minute maximum idle time is acceptable (although in my case where actually a suite of applications are being used as a single gestalt entity that can even be a problem), but for many of my administrator users you need huge idle times, since they are used to logging in first thing, and periodically working at the application through the day.
In the end I settled on changing our framework to make it easy to pass through garbage collection values. This makes an interface to the configuration really easy, but it doesn’t solve the problems of long session times that not all users need, and huge delays in garbage collection.
In my last article I talked about a Munin plugin for OPUS, but when you look at it you’ll see these kind of cliff fall drops, which are caused by the garbage collection finally kicking in and removing sessions where users have not explicitly logged out. Currently, every ten minutes, OPUS runs through its user database and finds users who are allegedly online but have no active session file and then marks them offline. Then it updates the file with the online user count that Munin reads.
I suspect eventually, I will write a more sophisticated script that actually kills sessions depending upon idle time and the user class, which would make for a more accurate picture here. Any brighter ideas gratefully accepted.