Circuit Breaker

Level: Advanced 60–90 min

Concepts: Design PatternsStateMockingBoundariesEdge Cases

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Implement the circuit breaker pattern to protect against cascading failures when calling unreliable external services.

Requirements

A circuit breaker has three states: Closed, Open, and Half-Open.

1. Closed (normal operation)

   • Requests pass through to the underlying service
   • Track consecutive failures
   • When failures reach the failure threshold, transition to Open
   • Successes reset the failure counter

2. Open (failing fast)

   • All requests fail immediately without calling the service
   • After the reset timeout expires, transition to Half-Open

3. Half-Open (testing recovery)

   • Allow a single request through to the service
   • If it succeeds, transition to Closed and reset counters
   • If it fails, transition back to Open and restart the timeout

Configuration:

• failureThreshold — number of consecutive failures before opening (default: 3)
• resetTimeout — seconds to wait before transitioning from Open to Half-Open (default: 30)

The circuit breaker should accept an injectable clock for testability.

Test Cases

Given a circuit breaker with threshold=3 and timeout=30s:

Step	State	Action	Service Result	Circuit Breaker Result	New State
1	Closed	call	success	success	Closed
2	Closed	call	failure	failure	Closed (1 fail)
3	Closed	call	failure	failure	Closed (2 fails)
4	Closed	call	failure	failure	Open (3 fails = threshold)
5	Open	call	—	fail immediately	Open
6	Open +30s	call	success	success	Closed
7	Open +30s	call	failure	failure	Open (restart timeout)

Additional cases:

• Success in Closed state resets failure count to 0
• Multiple successes after failures keep the counter at 0
• Open state does not call the underlying service at all
• Half-Open allows exactly one request through

Bonus

• Add an onStateChange callback that fires when the state transitions
• Add getState() to inspect the current state
• Add getMetrics() returning success count, failure count, and rejection count
• Implement a half-open quota — allow N requests through in half-open instead of just 1
• Add exponential backoff — double the reset timeout after each Open→Half-Open→Open cycle