CodeIgniter Output Caching

This week I had the privilege of doing load testing on an aws setup. The goal of the test was to support 25k concurrent users and was designed to ramp up over a period of a few minutes. The first round of testing failed pretty quick and I only had a select number of large queries cached. So the db spiked and was the bottle neck. During that test we was only able to reach 10k concurrents.

For the second round of testing I decided to just setup output caching before I even loaded the db in a MY_Controller. I set the number of minutes to something relatively high and was ready to commence the load test. Unfortunately this test maxed out the db again and I was honestly scratching my head on just why that would be the case.

Since this didn’t make any sense to me I decided I better figure just what is going on. I got my local system setup the exact same way, enabled mysql query log, and then proceded to test and see what was going on. Queries was being ran even after the cache file was written. At this point it was way late so I decided I better sleep on it and try and think about what I was missing.

The next morning I got to work ran the same tests and got the same results. Starting researching and then realized it was just my ajax calls that was generating the queries. This had me puzzled because the ajax controller extended the MY_Controller so it should be cached as well. Turns out I was wrong and missed the most important note in the user guide:

Warning: Because of the way CodeIgniter stores content for output, caching will only work if you are generating display for your controller with a view.

The more I thought about it the more sense that makes since the Loader ties into the output library but I wasn’t thinking clearly at 3am. Looked at my ajax controller and sure enough I was just returning a json string.

return json_encode($vars);

The quick fix was to change the return to the following:

return $this->output->set_output(json_encode($vars));

My hope is this will help others if they ever run into the same situation. Unfortunately the load testing was only scheduled for two rounds but I feel confident by fixing that it would have passed with flying colors.

So the moral of the story. READ THE MANUAL 🙂