Feature flags

When developing new features for Riot, we use feature flags to give us more flexibility and control over when and where those features are enabled.

For example, flags make the following things possible:

Extended testing of a feature via labs on develop
Enabling features when ready instead of the first moment the code is released
Testing a feature with a specific set of users (by enabling only on a specific Riot instance)

The size of the feature controlled by a feature flag may vary widely: it could be a large project like reactions or a smaller change to an existing algorithm. A large project might use several feature flags if it's useful to control the deployment of different portions independently.

Everyone involved in a feature (engineering, design, product, reviewers) should think about its deployment plan up front as best as possible so we can have the right feature flags in place from the start.

Interaction with spec process

Historically, we have often used feature flags to guard client features that depend on unstable spec features. Unfortunately, there was never clear agreement about how long such a flag should live for, when it should be removed, etc.

Under the new spec process, server-side unstable features can be used by clients and enabled by default as long as clients commit to doing the associated clean up work once a feature stabilises.

Starting work on a feature

When starting work on a feature, we should create a matching feature flag:

Add a new setting of the form:

    "feature_cats": {
        isFeature: true,
        displayName: _td("Adds cats everywhere"),
        supportedLevels: LEVELS_FEATURE,
        default: false,
    },

Check whether the feature is enabled as appropriate:

    SettingsStore.isFeatureEnabled("feature_cats")

Add the feature to the set of labs on develop and nightly:

    "features": {
        "feature_cats": "labs"
    },

Document the feature in the labs documentation

With these steps completed, the feature is disabled by default, but can be enabled on develop and nightly by interested users for testing.

Different features may have different deployment plans for when to enable where. The following lists a few common options.

Enabling by default on develop and nightly

Set the feature to enable in the develop and nightly configs:

    "features": {
        "feature_cats": "enable"
    },

Enabling by default on staging, app, and release

Set the feature to enable in the staging / app and release configs.

Warning: While this does mean the feature is enabled by default for https://riot.im and official Riot Desktop builds, it will not be enabled by default for self-hosted installs, custom desktop builds, etc. To cover those cases as well, the best options at the moment are converting to a regular setting defaulted on or to remove the flag. Simply enabling the existing flag by default in Settings.js does not work currently.

Feature deployed successfully

Once we're confident that a feature is working well, we should remove the flag:

Remove the setting
Remove all isFeatureEnabled lines that test for the feature's setting
Remove the feature from the labs documentation
Remove feature state from develop, nightly, staging / app, and release configs
Celebrate! 🥳

Convert to a regular setting (optional)

Sometimes we decide a feature should always be user-controllable as a setting even after it has been fully deployed. In that case, we would craft a new, regular setting:

Remove the feature flag from settings and add a regular setting with the appropriate levels for your feature
Replace the isFeatureEnabled lines with getValue or similar calls according to the settings docs (checking carefully, as we may want a different mix of code paths when the feature is always present but gated by a setting)
Remove the feature from the labs documentation
Remove feature state from develop, nightly, staging / app, and release configs

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