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One of the most common arguments in favor of FeatureBranch is that it provides a mechanism for pending features that take longer than a single release cycle. Imagine you are releasing into production every two weeks, but need to build a feature that's going to take three months to complete. How do you use Continuous Integration to keep everyone working on the mainline without revealing a half-implemented feature on your releases? We run into this issue quite a lot and feature toggles are a handy tool to deal with it.

(I've seen a lot of names for this concept tossed around: feature bits, flags, flippers, switches and the like. There seems no generally accepted name yet.)

The basic idea is to have a configuration file that defines a bunch of toggles for various features you have pending. The running application then uses these toggles in order to decide whether or not to show the new feature.

Most of these decisions occur in the user-interface of the application. So if you are building a web application using jsp, you may use a set of jsp tags to surround any user-interface parts of a pending feature.

    <toggle name="petSurvey">
      <p>Take our new <a href = 'petSurvey'>pet survey</a></p>
    </toggle>

The implementation of the toggle tag then just passes through the content if the toggle is set to on, and skips it otherwise. Other UI technologies will use different details, but the basic notion of wrapping the pending elements is the same.

This technique works best when the number of entry points to the new feature are small. There could be many screens in the pet survey feature, but if there's only one link on the home page that gets you there, then that's the only element that needs to be protected with the toggle tag. Therefore it's best to limit the entry points as much as you can, but for some features that will be difficult and you'll need lots of toggle tags.

Some features may be like introducing a new pricing algorithm, where there might be no user-interface elements. Here the test of the toggle would be in the application code, it could be as crude as a conditional test, or something more sophisticated like a strategy wired through dependency injection.

Most feature toggles I've heard about are set at run-time, but I've also seen cases set at build time. The advantage of a build time toggle is that none of the new feature's code gets compiled into the released executable, although that's rarely much of an advantage. Run-time toggles make it easier to set up your build pipelines and to run tests with various configurations of features. It also facilitates canary releasing, A/B testing, and makes it easier to roll-back should a new feature misbehave in production.

One danger with feature toggles is an accidental exposure, when someone forgets to wrap the UI feature in a toggle tag. This is awkward to test, since it's difficult to form a test that nothing that should be hidden is visible without calling out the individual elements - which are likely to be forgotten at the same time.

A common question we hear about feature toggles concerns testing - does using feature toggles mean a combinatorial explosion of tests? I think this concern is a red herring. Using feature toggles doesn't mean you have to do any more testing that you need to do with feature branches, it just makes it easier to run the alternatives. The key is to focus testing on the combination of pending features that you expect to deploy together.

It's very important to retire the toggles once the pending features have bedded down in production. This involves removing the definitions on the configuration file and all the code that uses them. Otherwise you will get a pile of toggles that nobody can remember how to use. In one memorable example I heard of, it required making a special recompilation of the linux kernel to handle enough command line switches.

Feature toggles can be used for permanent variable configuration too, such as different versions of a software for different contexts. This is a different usage to handling pending features but most of the implementation is the same. If you use feature toggles for other scenarios too, it's wise to clearly separate the pending feature case from the permanent cases.

To probe further...

• An InfoQ presentation by Erik Sowa and Rob Loh
• Flags and flippers at flickr