How do you implement a circuit breaker pattern when calling a downstream microservice in NestJS?

Q: How do you implement a circuit breaker pattern when calling a downstream microservice in NestJS?

// npm i opossum @types/opossum import CircuitBreaker from 'opossum'; @Injectable() export class UsersGatewayService implements OnModuleInit { private breaker: CircuitBreaker; constructor( @Inject('USERS_SERVICE') private client: ClientProxy, ) {} onModuleInit() { this.breaker = new CircuitBreaker( (id: number) => firstValueFrom( this.client.send ({ cmd: 'get_user' }, { id }), ), { timeout: 3000, // request considered failed after 3s errorThresholdPercentage: 50, // open after 50% failure rate resetTimeout: 10000, // try again after 10s (half-open) }, ); // Fallback — return safe default when circuit is open this.breaker.fallback((id: number) => ({ id, email: 'unknown@fallback.com', name: 'Unknown User', })); this.breaker.on('open', () => console.warn('Circuit open')); this.breaker.on('halfOpen', () => console.log('Circuit half-open — testing')); this.breaker.on('close', () => console.log('Circuit closed — recovered')); } async getUser(id: number): Promise { return this.breaker.fire(id) as Promise ; } } Closed — normal operation; requests flow through; failure rate is tracked. Open — circuit tripped; requests fail immediately with the fallback value; upstream is not called. Half-open — after resetTimeout; one test request is allowed through to check recovery. Pair circuit breakers with retry logic — but retries amplify load on an already-struggling service. Export circuit breaker metrics (open/close events) to your monitoring system for alerting.

Difficulty: 6/10

circuit breaker, NestJS interceptors, resilience

Use the opossum library to wrap calls to downstream services. Configure errorThresholdPercentage to open the circuit after a failure rate is exceeded and resetTimeout to attempt recovery. Register a fallback function that returns safe default data when the circuit is open so the application degrades gracefully.

Circuit breaker with opossum and fallback

Circuit breaker states and configuration:

1
Closed — normal operation; requests flow through; failure rate is tracked.
2

How do you implement a circuit breaker pattern when calling a downstream microservice in NestJS?

Difficulty: 6/10

circuit breaker, NestJS interceptors, resilience

Use the opossum library to wrap calls to downstream services. Configure errorThresholdPercentage to open the circuit after a failure rate is exceeded and resetTimeout to attempt recovery. Register a fallback function that returns safe default data when the circuit is open so the application degrades gracefully.

Circuit breaker with opossum and fallback

Circuit breaker states and configuration:

1
Closed — normal operation; requests flow through; failure rate is tracked.
2

How do you implement a circuit breaker pattern when calling a downstream microservice in NestJS?

How do you implement a circuit breaker pattern when calling a downstream microservice in NestJS?

Scenario Questions

Follow-up Questions