Design a system for managing a distributed feature flag system.

Question

Anonymous · Accepted Answer

Requirements -

* Manage feature flags via a UI (Admin UI) - create, update, delete

* Access feature flags from services/applications

* Propagate changes in feature flag values across applications under 200ms,
   low-latency feature flag evaluation within applications

Assumptions -

* Distributed environment with multiple micro-services

Solution -

* An application named "feature-flag-management-service" would be exposing APIs
   to create/update feature flags.

* The feature flag entity/model would comprise of - flag-key, flag-name.
   flag-value, flag-metadata, updated-timestamp

* The APIs would be POST /feature-flags to create the flags, PUT
   /feature-flags/{flag-id} to update it, GET /feature-flags/{flag-id} to
   retrieve a specific flag definition.

* A new SDK would be created which will have operations to fetch the feature
   flag definition, but will primarily create a subscription for feature flag
   changes. This means the sdk will be consuming all the feature-flag-updated
   events and store the latest values of the flags needed by the application
   in-memory. This would allow fast evaluation of the flag values.

* An operational user can login to the Admin UI which will allow the creation
   and modification of the flags in the system. Upon every change, a new event
   will be emitted representing the current state of the feature flag.

* The "metadata" field in feature flag can contain additional details needed by
   the applications, like percentage rollout, or a user-id for example.

* In case of a failure of the APIs, the sdk will have no value or stale value
   of the feature flag. This can be managed by providing a fallback value for
   the respective feature flag.

* Assuming the availability is more important here than consistency of the
   system, we can tolerate certain degree of staleness of data.

* The changes made to feature flags will be audited. We can also protect the
   modification of the flags using a reasonable RBAC policy.

Design a system for managing a distributed feature flag system.

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