Use Feature Flags

Use feature flags

Gate behaviour behind a runtime flag instead of a redeploy or an environment branch. Examples NestJS/TS, neutral domain. principle → ▸ Example → ▸ Other stacks. Pairs with release-safety (flags are how you ramp + roll back a release) and restates the global Feature Flags & Observability rule.

Core principle

Gate rollouts on an explicit flag evaluated at runtime, never on NODE_ENV or a redeploy — and fail safe. A flag turns a feature on/off (or picks a variant) for a user/cohort without shipping code; if the flag service is unreachable or the flag is unknown, the code must fall back to a safe default (usually OFF), never crash.

1. Evaluate against a central flag service (with targeting context)

• A small shared client asks a central flag/config service to evaluate a flag, passing the targeting context — which service is asking, the flag name, and who for (user/session) plus optional properties — so the service can do % rollouts and cohort targeting centrally.

  // query → central service (message pattern / SDK); response is the decision
  const { isEnabled } = await this.flags.get({
    service: SERVICE_NAME, flagName: 'new-checkout',
    userId, sessionId, properties: { country, plan },   // targeting context for gradual rollout
  });
  if (isEnabled) { /* new path */ } else { /* current path */ }
  

• Keep flag names as constants, not inline strings; the service owns the rollout %/rules, the caller just passes context and branches. ▸ Other stacks: LaunchDarkly / Unleash / Flagsmith / OpenFeature SDKs — same shape: evaluate(flag, context) → decision. The central service + targeting context is the point, not the vendor.

2. Fail safe + cache

• Default OFF on any failure — unknown flag, timeout, or service down → return the safe default and log it; a flag-service outage must not break the feature path. (Wrap the call; never let it throw into the handler.)

  async isOn(flag: string, ctx: Ctx): Promise<boolean> {
    try { return (await this.flags.get({ flagName: flag, ...ctx })).isEnabled; }
    catch (e) { this.logger.warn('flag eval failed → default off', { flag, e }); return false; }
  }
  

• Cache the evaluation briefly (per-request, or a short TTL keyed by flag+context) so you don't hit the service on every check in a hot path. ▸ Other stacks: SDKs stream/cache flag rules locally; if you call a service, add your own short cache + default. Principle: availability of the flag service is never on the critical path.

2b. A flag is not a NODE_ENV branch (global rule)

Never if (process.env.NODE_ENV === 'production') to gate business logic — that's not toggleable, not targetable, and couples behaviour to where it runs. Use a flag/config value instead. Typical uses: pick a provider (e.g. which payment gateway), dark-launch a rewrite, enable a feature for a cohort, kill-switch a flaky integration.

3. Flag lifecycle — name it, own it, remove it

• A flag is temporary scaffolding. Give it a clear name, record an owner + a removal condition ("remove after new-checkout is 100% for 2 weeks"), and once fully ramped, delete the flag and the dead branch — stale flags rot into permanent hidden config. (This is exactly what release-safety ramps + the comment policy track.)
• Log every flag-driven branch + the fallback path (observability), so you can see which path ran. ▸ Other stacks: same discipline — flags have owners and expiry; a "flag debt" review removes ramped flags.

Verification

• Behaviour is gated on a named flag evaluated at runtime, not NODE_ENV or a redeploy.
• The eval passes targeting context (service + user/session + properties) for gradual rollout.
• Failure/unknown defaults OFF (wrapped, logged) and the result is cached off the hot path.
• Each flag has an owner + removal condition; ramped flags (and their dead branches) get deleted.

Related

• release-safety — flags are how you ramp a release and roll back without a deploy; comment policy for transition code.
• integrate-internal-services — the client that calls the central flag/config service (RPC + fail-safe).
• write-service-code §7 (log the fallback path) · global Feature Flags & Observability, Anti-patterns (no NODE_ENV gating).