How does a circuit breaker protect an API, and what must you decide before using one?

Use a circuit breaker around an unhealthy dependency

Explains closed, open, and half-open behavior, failure classification, recovery probes, and the limits of circuit breakers.

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TL;DR

While the circuit is closed, calls flow normally and relevant failures are counted. When failures or slow calls cross a threshold, the circuit opens and new calls fail quickly or use a defined fallback instead of waiting on the dependency. After a delay it becomes half-open and allows a small number of probe calls. Successful probes close the circuit; failed probes open it again. The policy must count only failures that indicate dependency health, not caller validation or expected business responses.

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Circuit states, failure thresholds, recovery probes, dependency scoping, fallback behavior, and observability.

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A circuit breaker stops repeatedly calling a dependency that is already failing, preserving local capacity and giving the dependency room to recover.
While the circuit is closed, calls flow normally and relevant failures are counted. When failures or slow calls cross a threshold, the circuit opens and new calls fail quickly or use a defined fallback instead of waiting on the dependency. After a delay it becomes half-open and allows a small number of probe calls. Successful probes close the circuit; failed probes open it again. The policy must count only failures that indicate dependency health, not caller validation or expected business responses.
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I would scope the breaker to the real failure boundary, such as one provider operation or endpoint, rather than one global switch for unrelated calls. The threshold needs enough volume to avoid opening on a single error while still reacting before saturation spreads. Half-open probes must be limited so every application instance does not flood the recovering dependency. Opening the circuit does not create a fallback automatically; the API still needs an honest cached, partial, queued, or unavailable response contract.

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Counting every non-success response as dependency failure.
Using one global breaker for unrelated operations.
Allowing unlimited half-open probes.
Assuming a breaker replaces timeouts or idempotency.
Returning stale fallback data without marking its limits.

How does a circuit breaker protect an API, and what must you decide before using one?

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