Feature Toggles

I'm a fan of regular releasing. My background and experience leads me to release as regularly as possible. There are numerous benefits to regular releases; limited risk, slicker release processes and the ability to change as requirements evolve.

The problem with this concept is how can you release when features are not functionally complete?


If there is still work in progress, one solution to allow frequent releases is to use feature toggles. Feature toggles are simple conditional statements that are either enabled or disabled based on some condition.

This simple example shows a feature toggle for an "Edit User" feature. If the boolean condition is false, then we only show the "New User" feature and the "Admin" feature. This boolean value will be provided by various means, usually a configuration file. This means at certain points we can change this value in order to demonstrate the "Edit User" functionality. Our demo environment could have this enabled, while the live system would be disabled until the feature is fully complete.

If the feature to edit users took more than an ideal release cycle the code could still be released. As long as all the tests and other release checks pass there is no reason to defer this task. This is after all one of the benefits of continuous integration. Any consumer of this code base would always be working with up to date code, merge conflicts would be next to non existent. Our new code would be integrated regularly.

Ideally feature toggles live as high as possible in the dependency graph of your application. In most cases this would be the composition root of the application or within UI/presentation logic. This simplifies the addition of toggles, but you need to be careful that just because the UI hides a feature it is not truly disabled. In scenarios where security is a concern the feature toggles may need to live further down the stack.

It's best to remove feature toggles once the feature is complete otherwise they can become a maintenance burden. Is this feature enabled or disabled? Can we delete this code? These sort of questions can cause legacy code to live unquestioned. One way to aid in their removal is to add assertions to fail the build at a certain point in the future or include a toggle with built in date/time logic.

Feature Toggles help with demonstrating features, but they can be more complex. For risky features you may want to slowly ramp up the number of users who are exposed to the feature. In this case the actual toggle may perform some basic logic such as "one out of ten requests" enable the new feature. Overtime this ratio can be increased until the feature is fully enabled and proven.

Another technique to allow fast, regular releases is to rely on Branch by Abstraction. This works great when the toggles live in the composition root or the team have the ability to split work around features.