Build a circuit breaker

Hammering a service that is already failing makes the outage worse and makes your UI slow, since every call waits for a timeout. A circuit breaker trips after a run of failures and fails fast instead, then lets a single trial call through later to see if the service is back.

Implement createBreaker(task, options) where options may contain failureThreshold (default 3) and resetTimeout (default 1000). It returns a function call(...args) that forwards its arguments to task, plus a call.state() method returning "closed", "open" or "half-open". It starts closed. A fulfilment resets the consecutive-failure count to zero. Once failureThreshold consecutive failures happen, the breaker opens and records the time: while open, call rejects with new Error("Circuit is open") without invoking task. Once resetTimeout milliseconds have passed since it opened, state() reports "half-open" and the next call does invoke task: if it fulfils the breaker closes and the count resets, and if it rejects the breaker opens again with a fresh timestamp.

What it has to do

• Forward arguments to task and resolve with its value while closed.
• Open after failureThreshold consecutive failures.
• Reject with an Error reading Circuit is open without calling task while open.
• Reset the failure count on any success.
• Report half-open after resetTimeout, and close again on a successful trial call.

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