HLD: Circuit Breaker Explained to Prevent Failures

What is a Circuit Breaker?

A Circuit Breaker is a resilience pattern used in software architecture, especially in distributed systems. Its main purpose is to monitor calls to a service (or dependency) and temporarily stop normal calls when the failure rate crosses a certain threshold. This helps in preventing cascading failures and protects the overall system.

Why Use a Circuit Breaker?

• Prevent Resource Exhaustion: When a service fails, repeated calls can consume resources unnecessarily.
• Improve System Stability: It allows the system to recover instead of getting overwhelmed by continuous failures.
• Control Failure Impact: By stopping calls to a failing service, it prevents the failure from affecting other parts of the system.

The Three States of a Circuit Breaker

1. CLOSED: Normal traffic flows to the dependency. The circuit breaker monitors the responses and tracks failures.
2. OPEN: When failures exceed the configured threshold, the circuit breaker opens. Calls to the failing service are short-circuited, and the system may return a fast error or a fallback response.
3. HALF-OPEN: After a set period, the circuit breaker allows a limited number of requests to test if the service has recovered. If these requests succeed, the circuit breaker returns to the CLOSED state. If they fail, it goes back to OPEN.

Example in Real Life

Consider an e-commerce platform where the Order Service relies on a Payment Service. If the Payment Service starts failing, the Circuit Breaker will stop sending requests to it, allowing the system to recover without exhausting resources.

Circuit Breaker vs. Other Patterns

• Timeout: Stops waiting for a response.
• Retry: Attempts to call the service again.
• Circuit Breaker: Prevents repeated calls to an unhealthy service.

These patterns can be used together, but care must be taken to avoid amplifying outages through excessive retries.

Key Takeaways

• A Circuit Breaker does not fix the underlying issue; it just contains the failure.
• Understanding the states (CLOSED, OPEN, HALF-OPEN) is crucial for effective implementation.
• It is an essential concept in system design interviews, especially for roles in backend engineering and architecture.