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| 1 | +# Feature flags |
| 2 | + |
| 3 | +When developing new features for Riot, we use feature flags to give us more |
| 4 | +flexibility and control over when and where those features are enabled. |
| 5 | + |
| 6 | +For example, flags make the following things possible: |
| 7 | + |
| 8 | +* Extended testing of a feature via labs on develop |
| 9 | +* Enabling features when ready instead of the first moment the code is released |
| 10 | +* Testing a feature with a specific set of users (by enabling only on a specific |
| 11 | + Riot instance) |
| 12 | + |
| 13 | +The size of the feature controlled by a feature flag may vary widely: it could |
| 14 | +be a large project like reactions or a smaller change to an existing algorithm. |
| 15 | +A large project might use several feature flags if it's useful to control the |
| 16 | +deployment of different portions independently. |
| 17 | + |
| 18 | +Everyone involved in a feature (engineering, design, product, reviewers) should |
| 19 | +think about its deployment plan up front as best as possible so we can have the |
| 20 | +right feature flags in place from the start. |
| 21 | + |
| 22 | +## Interaction with spec process |
| 23 | + |
| 24 | +Historically, we have often used feature flags to guard client features that |
| 25 | +depend on unstable spec features. Unfortunately, there was never clear agreement |
| 26 | +about how long such a flag should live for, when it should be removed, etc. |
| 27 | + |
| 28 | +Under the [new spec |
| 29 | +process](https://github.com/matrix-org/matrix-doc/pull/2324), server-side |
| 30 | +unstable features can be used by clients and enabled by default as long as |
| 31 | +clients commit to doing the associated clean up work once a feature stabilises. |
| 32 | + |
| 33 | +## Starting work on a feature |
| 34 | + |
| 35 | +When starting work on a feature, we should create a matching feature flag: |
| 36 | + |
| 37 | +1. Add a new |
| 38 | + [setting](https://github.com/matrix-org/matrix-react-sdk/blob/develop/src/settings/Settings.js) |
| 39 | + of the form: |
| 40 | +```js |
| 41 | + "feature_cats": { |
| 42 | + isFeature: true, |
| 43 | + displayName: _td("Adds cats everywhere"), |
| 44 | + supportedLevels: LEVELS_FEATURE, |
| 45 | + default: false, |
| 46 | + }, |
| 47 | +``` |
| 48 | +2. Check whether the feature is enabled as appropriate: |
| 49 | +```js |
| 50 | + SettingsStore.isFeatureEnabled("feature_cats") |
| 51 | +``` |
| 52 | +3. Add the feature to the [set of labs on develop](https://github.com/vector-im/riot-web/blob/develop/riot.im/develop/config.json): |
| 53 | +```json |
| 54 | + "features": { |
| 55 | + "feature_cats": "labs" |
| 56 | + }, |
| 57 | +``` |
| 58 | +4. Document the feature in the [labs documentation](https://github.com/vector-im/riot-web/blob/develop/docs/labs.md) |
| 59 | + |
| 60 | +With these steps completed, the feature is disabled by default, but can be |
| 61 | +enabled on develop by interested users for testing. |
| 62 | + |
| 63 | +Different features may have different deployment plans for when to enable where. |
| 64 | +The following lists a few common options. |
| 65 | + |
| 66 | +## Enabling by default on develop |
| 67 | + |
| 68 | +Set the feature to `enable` in the [develop config](https://github.com/vector-im/riot-web/blob/develop/riot.im/develop/config.json): |
| 69 | + |
| 70 | +```json |
| 71 | + "features": { |
| 72 | + "feature_cats": "enable" |
| 73 | + }, |
| 74 | +``` |
| 75 | + |
| 76 | +## Enabling by default on staging and app |
| 77 | + |
| 78 | +Set the feature to `enable` in the [app |
| 79 | +config](https://github.com/vector-im/riot-web/blob/develop/riot.im/app/config.json). |
| 80 | + |
| 81 | +## Feature deployed successfully |
| 82 | + |
| 83 | +Once we're confident that a feature is working well, we should remove the flag: |
| 84 | + |
| 85 | +1. Remove the [setting](https://github.com/matrix-org/matrix-react-sdk/blob/develop/src/settings/Settings.js) |
| 86 | +2. Remove all `isFeatureEnabled` lines that test for the feature's setting |
| 87 | +3. Remove the feature from the [labs documentation](https://github.com/vector-im/riot-web/blob/develop/docs/labs.md) |
| 88 | +4. Remove feature state from |
| 89 | + [develop](https://github.com/vector-im/riot-web/blob/develop/riot.im/develop/config.json) |
| 90 | + and [app](https://github.com/vector-im/riot-web/blob/develop/riot.im/app/config.json) |
| 91 | + configs |
| 92 | +5. Celebrate! 🥳 |
| 93 | + |
| 94 | +## Convert to a regular setting (optional) |
| 95 | + |
| 96 | +Sometimes we decide a feature should always be user-controllable as a setting |
| 97 | +even after it has been fully deployed. In that case, we would craft a new, |
| 98 | +regular setting: |
| 99 | + |
| 100 | +1. Remove the feature flag from |
| 101 | + [settings](https://github.com/matrix-org/matrix-react-sdk/blob/develop/src/settings/Settings.js) |
| 102 | + and add a regular setting with the appropriate levels for your feature |
| 103 | +2. Replace the `isFeatureEnabled` lines with `getValue` or similar calls |
| 104 | + according to the [settings |
| 105 | + docs](https://github.com/matrix-org/matrix-react-sdk/blob/develop/docs/settings.md) |
| 106 | + (checking carefully, as we may want a different mix of code paths when the |
| 107 | + feature is always present but gated by a setting) |
| 108 | +3. Remove the feature from the [labs documentation](https://github.com/vector-im/riot-web/blob/develop/docs/labs.md) |
| 109 | +4. Remove feature state from |
| 110 | + [develop](https://github.com/vector-im/riot-web/blob/develop/riot.im/develop/config.json) |
| 111 | + and [app](https://github.com/vector-im/riot-web/blob/develop/riot.im/app/config.json) |
| 112 | + configs |
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