hardSystem Design

How do you implement a circuit breaker for unreliable downstream services?

Q: How do you implement a circuit breaker for unreliable downstream services?

**TL;DR.** A **circuit breaker** tracks downstream failures and, once they exceed a threshold, **"opens"** to **fail fast** for a cooldown instead of hammering a dead dependency. After the cooldown it goes **"half-open"** to test recovery with a trial request. This prevents resource exhaustion and **cascading failures**. CLOSED calls pass fails > N OPEN <text class='d-sub' x='232' y='98' text-ancho

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Understand the problem

Failing fast to protect a service from cascading failures.

nodejsresiliencecircuit-breakerreliability

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Wrapping a call with opossum

import CircuitBreaker from 'opossum';

const breaker = new CircuitBreaker(callPaymentApi, {
  timeout: 3000,                 // a call over 3s counts as a failure
  errorThresholdPercentage: 50,  // open at 50% errors
  resetTimeout: 10_000,          // cooldown before half-open
});
breaker.fallback(() => ({ status: 'queued' }));   // while OPEN

const result = await breaker.fire(order);

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