Flickr is somewhat unique in that it uses a code repository with no branches; everything is checked into head, and head is pushed to production several times a day. This works well for bug fixes that we want to go out immediately, but presents a problem when we’re working on a new feature that takes several months to complete. How do we solve that problem? With flags and flippers!

Feature Flags

Flags allow us to restrict features to certain environments, while still using the same code base on all servers. In our code, we’ll write something like this:

if ($cfg.enable_unicorn_polo) {
    // do something new and amazing here.
}
else {
    // do the current boring stuff.
}

We can then set the value of $cfg.enable_unicorn_polo on each environment (false for production, true for dev, etc.). This has the additional benefit of making new feature launches extremely easy: Instead of having to copy a bunch of new code over to the production servers, we simply change a single false to true, which enables the code that has already been on the servers for months.

Feature Flippers

Flags allows us to enable features on a per environment basis, but what if we wanted to be more granular and enable features on a per user basis? For that we use feature flippers. These allow us to turn on features that we are actively developing without being affected by the changes other developers are making. It also lets us turn individual features on and off for testing.

Here is what the Feature Flip page looks like:

Continuously Integrating

Feature flags and flippers mean we don’t have to do merges, and that all code (no matter how far it is from being released) is integrated as soon as it is committed. Deploys become smaller and more frequent; this leads to bugs that are easier to fix, since we can catch them earlier and the amount of changed code is minimized.

This style of development isn’t all rainbows and sunshine. We have to restrict it to the development team because occasionally things go horribly wrong; it’s easy to imagine code that’s in development going awry and corrupting all your data. Also, after launching a feature, we have to go back in the code base and remove the old version (maintaining separate versions of all features on Flickr would be a nightmare). But overall, we find it helps us develop new features faster and with fewer bugs.