Circuit Breaker

systemdesign

Circuit Breaker Pattern (Design Patterns for Microservices) | by Hasitha Subhashana | Geek Culture | Medium
Release It Design and Deploy Production-Ready Software, Chapter 5: Stability Pattern - Michael T. Nygard

Circuit Breakers prevent Cascading Failures. (💡Like SQL Cascade)

Circuit breaker is a mechanism that stops a client repeatedly attempting an operation that is likely to fail.

If a service is failing repeatedly, the circuit “breaks” the connection temporarily to stop more failures and allow the system to recover.

With respect to downstream services, a circuit breaker calculates the number of requests that failed within a recent interval. If an error thresh- old is exceeded, the client stops calling downstream services. Sometime later, the client attempts a limited number of requests. If they are successful, the client assumes that the failure is resolved and resumes sending requests without restrictions.

stateDiagram-v2
    [*] --> Closed
    Closed --> Open: Failures exceed threshold
    Open --> HalfOpen: After timeout
    HalfOpen --> Closed: If next call succeeds
    HalfOpen --> Open: If next call fails

    Open --> Closed: Manual reset (optional)

Example Scenario

1. Service A calls Service B.
2. Service B is slow or down.
3. Circuit breaker detects repeated timeouts/errors.
4. Circuit opens — now Service A instantly fails requests to B (or uses fallback).
5. After some time, a few requests go through (half-open).
6. If they succeed, circuit closes again.

Circuit Breakers Implementation

How is it implemented ?

• A common database holds the settings for each breaker.
• Services, before making call to others, checks the config
  • Cache the config to avoid checking the DB

Using Libraries in Production

If you want more powerful and configurable circuit breakers, use:

Language	Library/Tool	Notes
Java	Resilience4j	Lightweight, modern
.NET	Polly	Powerful retry/circuit-breaker policies
Node.js	opossum	Full-featured circuit breaker lib
Service Mesh	Istio, Envoy	Circuit breaker at infra level (no code needed)