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Feature flags in development of GitLab

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NOTE: The content below covers feature flags used by GitLab to deploy its own features, which is not the same as the feature flags offered as part of the product.

This page defines the process for the feature flags lifecycle at GitLab. Technical information for GitLab developers and code contributors can be found in the developer documentation.

When to use feature flags

The deployment of features behind feature flags is required for changes that could affect GitLab's availability of existing functionalities. If your changes only affect the new feature you're creating, use your judgment to determine if you need one or not. Such changes include:

  1. New features in high traffic areas (e.g., a new merge request widget, a new option in issues/epics, a new CI functionality).
  2. Complex security or performance improvements that may require additional testing in production (e.g., rewriting complex queries, reusing complex finders in new ways, destructive database updates or changes to frequently used API endpoints).
  3. Invasive changes to the user interface (e.g., introducing a new navigation bar, removal of a sidebar, significant UI changes in issues or MR interface).
  4. Introducing dependencies on third-party services (e.g., adding support for importing projects).
  5. Changes to features that can cause data corruption or cause data loss (e.g., features that process repository data or user-uploaded content).
  6. Changes to permissions that will add restrictions (e.g., requiring Owner permissions instead of Developer)

Situations where you might consider not using a feature flag:

  1. Adding a new API endpoint.
  2. Introducing new features in low traffic areas (e.g., adding new export functionality in the admin area, group settings area, or project settings area).
  3. Non-invasive frontend changes (e.g., slightly changing the color of a button, or moving a UI element in a low traffic area).

In all cases, if you're working on the changes like these, ask yourself:

Why do I need to add a feature flag? If I don't add one, what options do I have to control the impact on application reliability and user experience?

For perspective on why we limit our use of feature flags please watch the video Feature flags only when needed.

In case you are uncertain whether feature flag is necessary, please ask about this early in your merge request review process, and reviewers will likely provide you with an answer.

The benefits of feature flags

It may seem like feature flags are configuration, which goes against our convention-over-configuration principle. However, configuration is by definition something that is user-manageable. Feature flags are not intended to be user-editable. Instead, they are intended as a tool for Engineers and Site Reliability Engineers to use to de-risk their changes. Feature flags are the shim that gets us to Continuous Delivery with our monorepo and without having to deploy the entire codebase on every change. Feature flags are created to ensure that we can safely rollout our work on our terms. If we use Feature Flags as a configuration, we are doing it wrong and are indeed in violation of our principles. If something needs to be configured, we should intentionally make it configuration from the first moment.

Some of the benefits of using development-type feature flags are:

  1. It enables Continuous Delivery for GitLab.com.
  2. It significantly reduces Mean-Time-To-Recovery.
  3. It helps engineers to monitor and reduce the impact of their changes gradually, at any scale, allowing us to be more metrics-driven and execute good DevOps practices, shifting some responsibility "left".
  4. Controlled feature rollout timing: without feature flags, we would need to wait until a specific deployment was complete (which at GitLab could be at any time).
  5. Increased psychological safety: when a feature flag is used, an engineer has the confidence that if anything goes wrong they can quickly disable the code and minimize the impact of a change that might be risky.
  6. Improved throughput: when a change is less risky because a flag exists, theoretical tests about scalability can potentially become unnecessary or less important. This allows an engineer to potentially test a feature on a small project, monitor the impact, and proceed. The alternative would be to build complex benchmarks locally, or on staging, or on another GitLab deployment, which has a large impact on the time it can take to build and release a feature.

The cost of feature flags

When reading about the process, you can think that deploying a feature behind a feature flag adds a lot of work. Fortunately, this is not the case, and we'll show you why. For this example, we'll specify the cost of the work to do as a number, ranging from 0 to infinity. The greater the number, the more expensive the work is. The cost does not translate to time, it's just a way of measuring complexity of one change relative to another.

Let's say we are building a new feature, and we have determined that the cost of this is 10. We have also determined that the cost of adding a feature flag check in a variety of places is 1. If we do not use feature flags, and our feature works as intended, our total cost is 10. This, however, is the best-case scenario. Optimizing for the best-case scenario is guaranteed to lead to trouble, whereas optimizing for the worst-case scenario is almost always better.

To illustrate this, let's say our feature causes an outage, and there's no immediate way to resolve it. This means we'd have to take the following steps to resolve the outage:

  1. Revert the release.
  2. Perform any cleanups that might be necessary, depending on the changes that were made.
  3. Revert the commit, ensuring the main branch remains stable. This is especially necessary if solving the problem takes days or even weeks.
  4. Pick the revert commit into the appropriate stable branches, ensuring we don't block any future releases until the problem is resolved.

As history has shown, these steps are time-consuming, complex, often involve many developers, and, the worst of all: our users will have a bad experience using GitLab.com until the problem is resolved.

Now let's say that all of this has an associated cost of 10. This means that, in the worst-case scenario, which we should optimize for, our total cost is now 20.

If we had used a feature flag, things would have been very different. We don't need to revert a release, and because feature flags are disabled by default, we don't need to revert and pick any Git commits. In fact, all we have to do is disable the feature, and in the worst case, perform a cleanup. Let's say that the cost of this is 2. In this case, our best case cost is 11: 10 to build the feature, and 1 to add the feature flag. The worst case cost is now 13:

  • 10 to build the feature.
  • 1 to add the feature flag.
  • 2 to disable and clean up.

Here we can see that, in the best-case scenario, the work necessary to resolve the issue is only a tiny bit bigger when compared to not using a feature flag. Meanwhile, the process of reverting our changes was significantly and reliably cheaper.

In other words, feature flags do not slow down the development process. Instead, they speed up the process as managing incidents now becomes much easier. As continuous deployments are easier to perform, the time to iterate on a feature is reduced even further, as you no longer need to wait weeks before your changes are available on GitLab.com.

How to use Feature Flags

For an overview about starting with feature flags in GitLab development, follow the training template.

Before using feature flags, make sure to read the information on this page and the following development guides:

  1. Developing with feature flags: Learn about the types of feature flags, their definition and validation, how to create them, frontend and backend details, and other information.
  2. Documenting features deployed behind feature flags: How to document features deployed behind feature flags according to their state, and how to update the docs when their state changes.
  3. Controlling feature flags: Learn the process for deploying a new feature, enabling it on GitLab.com, communicating the change, logging, and cleaning up.
  4. Feature flags and the change management process: Learn whether your flag requires use of the change management process or not.
  5. Checking on the status of your teams feature flags: See if your team has too many, or stale feature flags. See how your feature flags have been toggled over time.
  6. Conducting an experiment with feature flags: Learn how the Growth department uses experiment feature flags.

For information about how the user will interact with features behind flags, see:

  1. How GitLab administrators can enable and disable features behind flags: An explanation for GitLab administrators about how they can enable or disable GitLab features behind feature flags.
  2. What "features deployed behind flags" means to the GitLab user: An explanation for GitLab users regarding how certain features might not be available to them until they are enabled.

Feature Flag Lifecycle

Planning

  1. Evaluate if you need a feature flag:
    1. Process for evaluation.
    2. You, as an engineer, come up with a proposed solution to the issue you're working on and decide whether you'll implement it behind a feature flag.
  2. Decide how you will implement the feature flag and its rollout according to:
    1. Choose the feature flag type.
    2. Decide on the definition and plan out the YAML.
    3. Decide on what type of actor to bind your feature flag to (if any).
    4. Think about where your feature flag should toggle in the code.
  3. Create a feature flag definition in code and make a follow-up rollout issue:
    1. Create a feature flag.
    2. Create a feature flag rollout issue using the template (if necessary).
    3. Ping the EM/PM on the feature flag rollout issue to get it scheduled/planned/refined.
    4. EM and Engineer work together to finalize the rollout plan for the feature flag. Not all steps in the feature flag rollout template are mandatory for every feature flag.

Development

  1. Add the feature flag into the code for the backend, frontend, and in tests:
    1. Develop with the feature flag in your vertical slice.
    2. Include the feature flag in tests to check its behavior when enabled and disabled.
    3. Flip the feature flag locally to ensure it works.
  2. Add the feature behind the flag to the codebase through an MR following the implementation process.
  3. Continue to iterate towards issue completion, using the feature flag to test logical slices:
    1. You may use your feature flag in more than one MR. Continue to iterate until the issue is complete.
  4. Ensure you follow the documentation guidelines for feature flags and keep them up-to-date according to the feature flag state.
  5. When slices are deemed complete enough (keeping in mind low level of shame), proceed to the rollout process.
    1. Some teams may choose to close the feature issue here once it is complete, other teams may want to wait until after the rollout process is finished. If you close your feature issues after the code is present in the default branch, you should close the issue here.

Rollout

  1. Finalize the rollout plan:
    1. Following the rollout guidelines determine a plan for rolling out your feature flag.
  2. Begin rollout plan:
    1. Rollout plans vary flag to flag, take the steps you’ve outlined in your feature flag rollout issue.
  3. Removing the flag, cleaning up, and feature announcement:
    1. Follow the feature flag clean up process.
    2. Ensure that the feature flag documentation process has been followed.
    3. Ensure the flag is removed from the code and from the database with the chatops feature flag removal command.
    4. If your team closes the feature issue after the feature flag has been removed, it should be closed here.
    5. If the originating issue will be included in the Release Post coordinate with Engineering Manager, Product Manager, Product Designer, Product Marketing Manager, and Technical writer to include the Release Post Item

Flowchart

graph TD A[Start] B{Evaluate need} C[Decide on implementation] D[Create flag / create rollout issue] E[End] F[Add flag and first iteration into code] G[MR into codebase] H[Iterate on issue] I{Feature issue complete?} J[Finalize rollout plan] K[Begin rollout] L[Complete rollout] M[Remove flag] N[Close feature issue] O[Close rollout issue] A --> B; B ----> |No| E; B --> |Yes| C; C --> D; D --> F; F --> G; G --> H; H --> I; I --> |No| H I --> |Yes| J; I -. *Optional* Yes .-> N; J --> K; K --> L; L --> M; M --> E; M --> O; M -. *Optional* Yes .-> N

Including a feature behind feature flag in the final release

In order to build a final release and present the feature for self-managed users, the feature flag should be at least defaulted to on. If the feature is deemed stable and there is confidence that removing the feature flag is safe, consider removing the feature flag altogether. It's strongly recommended that the feature flag is enabled globally on production for at least one day before making this decision. Unexpected bugs are sometimes discovered during this period.

The process for enabling features that are disabled by default can take 5-6 days from when the merge request is first reviewed to when the change is deployed to GitLab.com. However, it is recommended to allow 10-14 days for this activity to account for unforeseen problems.

Feature flags must be documented according to their state (enabled/disabled), and when the state changes, docs must be updated accordingly.

Changing the default state or removing the feature flag has to be done before the release date, at least 3-4 working days before, in order for the change to be included in the final self-managed release.

Note: consider that, by removing the flag, the feature will become available on GitLab.com shortly after the merge request was merged.

In addition to this, the feature behind feature flag should:

While rare, release managers may decide to reject picking or revert a change in a stable branch, even when feature flags are used. This might be necessary if the changes are deemed problematic, too invasive, or there simply isn't enough time to properly measure how the changes behave on GitLab.com.

Community contributions

Feature flag rollouts require access to internal GitLab systems. Because of this restriction, a community contribution will require assistance from a GitLab team member to rollout the feature flag.

During the planning phase it is essential that the EM and PM are pinged by the MR Coach or a GitLab Team member, who executes the review, on the feature flag rollout issue to get it scheduled/planned/refined. Also they should tag the feature flag rollout issue in the final MR using the feature flag, so that the EM and PM are aware that the rollout will be required soon.

Dashboard & Metrics

For more information on the lifecycle of a feature flag, such as how many are introduced per release, how long they exist in the application, or how long they've been enabled, see this dashboard (internal only). We also carry 7 days worth of data in Kibana (internal only) and a GitLab project feature-flag-log holds the same indefinitely for feature flags that become disabled or enabled.

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