API Failover

Overview

API Failover is the controlled switch of API traffic from an unhealthy primary endpoint or provider to an alternate path. It is used to maintain availability when a dependency, region, network route, or provider is degraded. It differs from simple retry, which repeats an attempt but may continue targeting the same failing destination.

A typical implementation works as follows: A health or request policy detects failure, checks whether the operation is safe to repeat, selects an alternate endpoint, and preserves identifiers and state. Observability for API Failover should record operation, version, principal, request ID, latency, retry count, error code, and final state without exposing secrets.

Common risks include inconsistent provider data and duplicate side effects. Important failure modes include inconsistent provider data, duplicate side effects, stale configuration, failback flapping, and simultaneous overload. These failures can make failover more damaging than waiting.

Core controls require teams to define health criteria, use idempotency, verify data compatibility, apply circuit breakers, test failover and failback, and reconcile uncertain operations. Data handled by API Failover should use documented types, units, timestamp rules, nullable fields, and enum behavior to prevent silent interpretation errors.

Operational evidence should include operation ID, primary and alternate endpoint, trigger, state before switch, attempts, outcome, and reconciliation result. API Failover should be documented alongside RPC Endpoint, Retry Policy, and API Compatibility. The API Failover contract should define identifiers, validation, authorization, status semantics, errors, limits, and version behavior at the same boundary.

Testing API Failover should cover malformed input, denied access, rate limits, timeouts, duplicates, asynchronous completion, and version changes.