I use a cool little web app called ThinkUp to keep track of stuff I post to Twitter and Facebook. I use the self-hosted version, running on my own server, and I’ve had it running for a year or so and never had a problem.
This weekend, I went to login to see if ThinkUp would show me anything interesting. Except I couldn’t log in. Every time I tried to login, it would just kick me back to the login screen. Clearly something had gone wrong. I watched the login requests via Developer Tools in Safari and Chrome and noticed that I was not getting a PHP session cookie. That’s certainly odd—setting a session cookie is pretty straight forward and I’ve never seen it fail.
As is typical in this sort of issue, I debugged it ass backwards. I spent an hour or so writing test scripts, changing permissions on session directories, and changing session settings before realizing I was debugging things entirely wrong.
My stack looks something like this:
nginx -> varnish -> apache2
I realized that I should start by looking to see if the request to Apache2 was getting the cookie headers back. I ran a quick curl command, and sure enough, the cookie headers were there when talking directly to Apache2. Logically, I then ran the same curl command, changing it to talk to Varnish. Sure enough, the cookie headers were gone.
Finally, I’d figured out where my cookies were getting eaten (haw haw).
Diving into the Varnish config, it was pretty quickly obvious what had happened. When adding Varnish caching to support this here blog, I added this line,
which basically says “get rid of the cookie header we’re sending back to the user”, which allows us to cache more stuff. Of course, that was getting set far too liberally, dropping the PHP session cookie, and making it so I couldn’t login. A couple of tweaks and restart later, and all was well.
This sort of thing happens to me somewhat frequently. I muck around with some settings on my server, and everything works great for my blog or my static site, but I forget about other things I have running, and a few weeks later, I notice they’re broken, but now I have no idea why. It’s a pretty good case for using a tool like doing, to log things I do (that aren’t necessary driven by my OmniFocus to-do list) so that I don’t spend hours debugging my self-inflicted problems.