Circuit Breaker Pattern: How It Works and Implementation

Circuit Breaker Pattern

The Problem

When Service A calls Service B and B is unresponsive or consistently failing, A accumulates waiting threads and eventually crashes too (cascading failure).

Three Circuit Breaker States

Closed (normal) → failure threshold hit → Open (tripped)
       ↑         ←── reset timeout ──→  ↓
 Half-Open (probe) ← probe succeeds ←───
       ↓ probe fails
       → Open

State	Behavior
Closed	Normal requests; track failure rate
Open	Immediately return error (skip downstream call); give downstream time to recover
Half-Open	Allow a few probe requests; decide whether to recover

Implementation Tools

• Java: Resilience4j, Netflix Hystrix (deprecated)
• Go: gobreaker
• Service Mesh: Istio implements this at the sidecar level

Fallback Strategy

When circuit is open, execute a fallback:

• Return cached stale data
• Return a default value
• Return a friendly "service temporarily unavailable" message