Optimising a slow build? You’re solving the wrong problem

At the time I left Songkick, it took 1.5 hours to run all the cukes and rspec ‘unit’ tests on the big ball of Rails. We were already parallelising over a few in-house VMs at the time to make this manageable, but it still took 20 minutes or so to get feedback. After I left, the team worked around this by getting more slave nodes from EC2, and the build time went down to under 10 minutes.

Then guess what happened?

They added more features to the product, more tests for those features, and the build time went up again. So they added more test slave nodes. In the end, I think the total build time was something like 15 hours. 15 fucking hours! You’re hardly going to run all of that on your laptop before you check in.

The moral of this story: if you optimise your build, all you’ll do is mask the problem. You haven’t changed the trajectory of your project, you’ve just deferred the inevitable.

The way Songkick solved this took real courage. First, they started with heart-to-heart conversations with their stakeholders about removing rarely-used features from the product. Those features were baggage, and once the product team saw what it was costing them to carry that baggage, they were persuaded to remove them.

Then, with a slimmed-down feature set, they then set about carving up their architecture, so that many of those slow end-to-end Cucumber scenarios became fast unit tests for simple, decoupled web service components. Now it takes them 15 seconds to run the tests on the main Rails app. That’s more like it!

So by all means, use tricks to optimise and speed up the feedback you get from your test suite. In the short term, it will definitely help. But realise that the real problem is your architecture: if your tests take too long, the code you’re testing has too many responsibilities. The sooner you start tackling this problem head-on, the sooner you can start enjoying the benefits.