Feature flags

When developing new features for Element, 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 Element 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.getValue("feature_cats")

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 true in the develop and nightly configs:

    "features": {
        "feature_cats": true
    },

Enabling by default on staging, app, and release

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

Note: The above will only enable the feature for https://app.element.io and official Element Desktop builds. It will not be enabled for self-hosted installed, custom desktop builds, etc. To cover these cases, change the setting's default in Settings.tsx to true.

Feature deployed successfully

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

If the feature is meant to be turned off/on by the user:

Remove isFeature from the setting
Change the default to true (if desired).
Remove the feature from the labs documentation
Celebrate! 🥳

If the feature is meant to be forced on (non-configurable):

Remove the setting
Remove all getValue lines that test for the feature.
Remove the feature from the labs documentation
If applicable, remove the feature state from develop, nightly, staging / app, and release configs
Celebrate! 🥳

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