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How to Branch by Abstraction with Feature Flags

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Branching by abstraction is a pattern used for making large-scale changes gradually, while simultaneously continuing to release your application. This is an important part of trunk-based development and is critical for creating a continuous integration and continuous delivery (CI/CD) pipeline.

In a non-CI/CD situation, the standard way to make large-scale changes is to use feature branches in version control, but this isn’t compatible with continuous integration (because code on a branch isn’t integrated). So how is it possible to keep integrating code while a large new feature is being built, or a significant amount of code is being refactored? How can you satisfy the necessary constraints of a) ensuring that other members of your development team can continue to work on other code which is dependent on the code being changed, and b) that the whole codebase is releasable at any time? And how can you ever be sure that you’re releasing safely?

The Branch by Abstraction Process

For the sake of example, let’s say that you’re refactoring a significant portion of the code in the backend of an application. There is a decent amount of client-facing code that is dependent on the existing, old implementation, and you cannot risk breakage, nor can you release the entire set of changes at once.

In order to manage your updates in a continuous integration environment, you’d traditionally want to implement branching by abstraction via the following process:

  1. Identify the legacy system to be replaced
  2. Build an abstraction layer to allow continued communication between the systems that are being replaced and the entities requesting that service
  3. Methodically build the replacement system, linking each rebuilt feature into the abstraction layer as they get finished. Note that at this point all traffic, relying on both new and old systems, will be passing through the abstraction layer
  4. As code in the old systems become obsolete, delete it
  5. Once all of the old code has been removed, you can dismantle the abstraction layer
Branch by abstraction

There are many variations on this process: for example, it may not be possible to swap out each part of the new implementation individually, it might need to all be swapped over all at once. Martin Fowler points out that, “in the simplest case you build the entire abstraction layer, refactor everything to use it, build the new implementation, and then flick the switch. But there’s various ways to break it up. You may not build the whole abstraction layer, just a subset of functionality, migrate that and then do another hunk of functionality (providing new and old can co-exist.) Otherwise, you may shift some calling code onto the abstraction and have that implemented both ways before you move the rest.”

In any case, the abstraction layer allows an easy transition between one implementation and another by allowing both to coexist in the same system.

Benefits and Drawbacks of Branching by Abstraction

In addition to the central benefit of being able to migrate large features easily in continuous integration, there are a few side benefits of branching by abstraction. Because your release schedule is completely decoupled from your architectural changes, pausing and resuming migration is easy and cheap, because the new implementation is guarded by the system. Aka, when your leadership team comes up with a high priority feature, or CS needs a bug fix to ship ASAP you can stop and resume with relative ease. On a standard feature branch, it’s possible to pause a migration, but it’s more difficult to resume.

Looking for additional benefits? How about this… Because you’re only interfacing with the abstraction layer, the breadth of merge conflict you could run into is restricted to that abstraction layer. Without the abstraction layer, your entire codebase refactor would be sitting in a feature branch for potentially quite some time, earning itself a variety of merge conflicts that you may never be able to fully resolve.

Branching by abstraction is not always the best option, though. For example, in situations where the customer can choose for themselves when they upgrade their version of your software, it won’t work because the entire system must be upgraded at once.

Branch by Abstraction with Feature Flags

So, you need the utility of branching by abstraction in your modern CI/CD release pipeline? Enter feature flags. Feature flags (sometimes referred to as feature toggles) can enhance your abstraction layer. Your development teams can rebuild legacy features behind flags, and when you’re ready you can safely flip your entire application over to use the new code, deleting your feature flag and deprecating the legacy code you replaced simultaneously.

How would this work in practice?

  1. Build an abstraction layer to allow continued communication between the systems that are being replaced and the entities requesting that service
  2. Begin building the replacement system, linking it into the abstraction layer to provide a (hopefully) seamless transition to the rebuilt features. Feature flags here function as the toggle for user access to your legacy or new code, as you’re ready. With feature flags, you could easily allow for a limited release of the new features to protect against critical bugs or to gauge user response
  3. As code in the old systems become obsolete, delete it
  4. Once all of the old code has been removed, you can dismantle the abstraction layer

One clear benefit of deploying feature flags as you migrate from legacy systems is the ability to test in production and thus release safely. When adding the complexity of an abstraction layer to your legacy codebase, you can never be sure your staging environment is a close enough match to catch any breaking issues. Feature flags allow you to safely test in production and know that when you release you won’t need to immediately rollback.

Learn More About Feature Flags, Testing in Production, and CI/CD

As you can see, branching by abstraction (with or without feature flags) is a significant improvement over the days of long-lived feature branches. If you’d like to learn more about feature flags, or other modern CI/CD patterns, we encourage you to read on:

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