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Choose a fallback separately for every feature flag: decide which behavior leaves your application in an acceptable state when the flag service cannot supply a value. There is no universally safe default—not even false. Keep startup and core paths working in a degraded mode where possible, and verify the actual behavior of your SDK and version during an outage.
Choose the fallback by the consequence of each flag
Start with the behavior a flag controls, then consider what happens if that behavior is enabled or disabled while the service is unavailable. A flag might gate a new user-facing feature, a critical operation, or an emergency protection; the safe outcome depends on which one it is. Record the acceptable degraded behavior for each flag rather than applying one boolean default across the application.
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For a multivariate flag, LaunchDarkly recommends using the off variation for the control or baseline behavior. If an off variation is not configured, its API documentation says the SDK serves the code fallback. That baseline is a useful starting point only when it is actually safe for the outage scenario in your application.
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Know which value the application will use
A code fallback and a cached last-known value are different. A code fallback is the value supplied by the application when evaluation cannot return a flag value. A cached value is a previously received decision; it may preserve recent targeting during an outage, but it can become stale after the control plane changes.
Behavior can also differ between a process that was connected before an outage and one that initializes during it. LaunchDarkly documents that already-connected SDK instances continue using locally cached flag data. A newly initialized instance may fail to connect and use code-supplied fallback values until connectivity returns. These are LaunchDarkly-specific documented behaviors, not guarantees for other SDKs: LaunchDarkly guidance on network resilience.
A cache is useful only if it has data. For example, a Relay Proxy that was already running can serve its in-memory cache during an upstream outage, but one that starts without upstream connectivity has no initial flag data to provide. Client-side bootstrapping may use local storage for returning users or server-rendered flag values, depending on the deployment. Persistent stores can allow server-side SDKs to read cached state, while cache TTL may leave instances out of sync. Decide explicitly whether stale data is acceptable for each flag.
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More caching or another provider can improve availability, but each option changes cold-start behavior, staleness, dependencies, and operational work. Compare the options against the same outage cases instead of treating any one of them as a complete fallback strategy.
| Approach | Cold start and outage behavior | Trade-off to assess |
|---|---|---|
| Code fallback | Provides the evaluation’s fallback when a usable flag value is unavailable. | Available without previously fetched flag data, but may not preserve recent targeting. Choose a safe value per flag. |
| In-memory last-known state | An already-connected process may continue using data it has cached locally; a fresh process may have no such data. | Can be stale, and behavior differs between warm and cold starts. |
| Persistent feature store | May let server-side SDKs read cached flag state after restart. | Adds a storage dependency; TTL can leave instances out of sync. |
| Relay Proxy | An already-running proxy can serve its in-memory cache during an upstream outage; a proxy starting without upstream data cannot. | Introduces a proxy to operate and does not solve an empty-cache cold start. |
| Client-side bootstrapping | May provide initial values from local storage for returning users or from server-rendered values, depending on deployment. | Availability depends on the chosen bootstrap source and whether it has usable data. |
| Provider chain | Depends on the provider strategy and which providers succeed; a default may be returned if all fail. | Requires clear understanding of strategy, error handling, and provider dependencies. |
When assessing any approach, ask how long cached decisions can lag behind a change, what happens with an empty cache, which processes or users remain able to evaluate, which storage or network components must stay healthy, and whether operators can tell an outage from a configuration or runtime problem.
Make provider behavior and errors observable
Do not infer provider-chain semantics from the phrase “fallback provider.” OpenFeature documents that when no provider is set, its evaluation API returns the supplied default. Its PHP SDK’s FirstSuccessfulStrategy tries providers in sequence and returns the first successful result; if all fail, it returns a default and aggregated errors. The ComparisonStrategy checks whether providers agree; it is not an error-recovery fallback. See the OpenFeature provider concepts and PHP SDK documentation.
OpenFeature’s provider specification describes initialization work such as HTTP requests and worker startup. A provider that fails to become ready should report abnormal execution; irrecoverable errors such as bad credentials or invalid configuration should use the PROVIDER_FATAL error code. Status reporting helps operators diagnose the problem, but it does not make the application’s per-flag fallback decision for them: OpenFeature provider specification.
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Keep enough status and error information to distinguish a service outage from bad credentials, invalid configuration, or local runtime failures. Increasing a startup wait alone is not a substitute for a safe evaluation path. For the LaunchDarkly Python SDK specifically, its support article says the initialization wait defaults to five seconds and warns that increasing it may delay startup without fixing the underlying cause. Do not generalize that number to other SDKs or versions: LaunchDarkly Python initialization-timeout guidance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Implement and test the degraded path
- Pass a typed fallback at each evaluation call. Use the API and value type required by the SDK instead of relying on an implicit global default.
- Document the decision per flag. Note the controlled behavior, the acceptable outage outcome, and when the fallback should be reviewed as the feature changes.
- Test with SDK network access blocked. Confirm that the application reaches its intended degraded state without critical errors, including during startup.
- Exercise distinct failure cases. Verify cold start, reconnect, stale cache, missing flag, and provider error behavior for the specific SDK and version in use.
- Check operational signals. Confirm that logs or status information let responders distinguish a service outage from bad credentials, invalid configuration, and local runtime failures.
Keep application startup and essential paths available when the control plane is down unless the application’s own safety requirements demand stopping. A longer initialization wait can postpone the failure without making a value available; test the fallback path rather than relying on waiting for recovery. LaunchDarkly’s broader resilience guidance describes persistent stores, Relay Proxy, daemon mode, and bootstrapping as options whose usefulness depends on deployment and cache state.
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