Fill 0003 — Circuit Breaker

A compact, testable model of the circuit breaker pattern, with a CLI to simulate state transitions.

Code: lcrc-experiments/fills/0003-circuit-breaker

Accelerator

A circuit breaker prevents cascading failures by failing fast when a downstream dependency is persistently unhealthy. Instead of accumulating queues and timeouts, traffic is stopped, then cautiously reintroduced after a cooldown.

MAPS

• States: closed → open → half-open.
• Open threshold: consecutive failures trip the breaker.
• Cooldown: time window where calls are rejected (fail-fast).
• Half-open: trial calls; enough successes close the breaker, a failure reopens it.
• Deliverable: a small Python package + CLI + unit tests.

BOB

References worth reading:

• Martin Fowler — Circuit Breaker
• Google SRE Book — Handling Overload

CMS

CLI example:

lcrc-breaker simulate --cooldown 2 --failure-threshold 3 --success-threshold 2 --dt 0.1 --seq ok,fail,fail,fail,call,wait:2.0,ok,ok --json

LCRCb

• Open is fail-fast until cooldown expires.
• Half-open success threshold closes; failure reopens.
• Time is injected (deterministic tests, no real sleeps).