What is a circuit breaker and when should you use one?

Timeouts and retries help a single request, but when a dependency is down, every request still waits and consumes a connection or thread. A circuit breaker tracks failures and opens the circuit, so callers get an immediate error or fallback while the dependency recovers.

if (breaker.state === "open") {
  if (Date.now() < breaker.openedUntil) {
    throw new CircuitOpenError("dependency unavailable");
  }
  breaker.state = "half-open";
}

const result = await callDependency();
breaker.recordSuccess();

Design details

• Count failures over a window, not a single error
• Use a cooldown before the half-open probe
• Allow only a small number of probe requests while half-open
• Scope the breaker per dependency and tenant where needed
• Provide a fallback, cached response or degraded mode
• Emit metrics for state changes and rejected calls

A breaker is not a substitute for a timeout. Every call still needs a deadline, because a slow dependency can hold resources even if it eventually succeeds. Combine the breaker with retries carefully: retries inside an open circuit just multiply failures.