Implement a circuitBreaker function

Apr 03, 2024

Circuit Breaker is a Javascript design pattern used to handle faults and failures in distributed systems. It is particulary helpful in scenario when external resources might be unavilable and we want to prevent reaching out to them continously and save resources.

You can learn more about the circuit breaker pattern on Wikipedia.

We’ll recreate a very small utility function called createCircuitBreaker which takes a function and prevents calling the function if it fails for a threshold count. We can retry reaching the service again after a certain back-off cooling period. When the service in unreachable, we can return a message saying “service unavailable”.

1. Basic implementation

function createCircuitBreaker(service, failureThreshold, cooldownTime) {
  let failureCount = 0;
  let lastFailureTime = 0;
  let isAvailable = true;

  function circuitBreakerFnImpl(...args) {
    // when service is not available we check if we can open the service again
    if (!isAvailable) {
      const now = Date.now();
      const diff = now - lastFailureTime;

      if (diff < cooldownTime) {
        return {
          message: 'service unavailable',
          isAvailable,
          result: undefined
        };
      } else {
        // we have waited the cooldown period since last failure
        isAvailable = true;
      }
    }

    try {
      const res = service.call(this, ...args);

      // we reset failures whenever service is reached without any errors
      failureCount = 0;

      return {
        message: undefined,
        isAvailable,
        result: res
      };
    } catch (error) {
      // when service throws any error, we check the if it has met the failure threshold
      failureCount++;
      lastFailureTime = Date.now();

      // if met the threshold, we close the service for a cool down time
      if (failureCount >= failureThreshold) {
        isAvailable = false;
      }

      return {
        isAvailable,
        result: undefined,
        message: 'error'
      };
    }
  }

  return circuitBreakerFnImpl;
}

// Usage:
let count = 0;
function incCout() {
  count += 1;

  if (count > 2 && count <= 5) {
    throw new Error('Count in unavailable range');
  }
  return count;
}

const breaker = createCircuitBreaker(incCout, 3, 1000);

// { message: undefined, isAvailable: true, result: 1 }
console.log(breaker());

// { message: undefined, isAvailable: true, result: 2 }
console.log(breaker());

// { message: 'error', isAvailable: true, result: undefined }
console.log(breaker());

// { message: 'error', isAvailable: true, result: undefined }
console.log(breaker());

// { message: 'error', isAvailable: false, result: undefined }
console.log(breaker());

// { message: 'service unavailable', isAvailable: false, result: undefined }
console.log(breaker());

// { message: 'service unavailable', isAvailable: false, result: undefined }
console.log(breaker());

// { message: 'service unavailable', isAvailable: false, result: undefined }
console.log(breaker());

setTimeout(() => {
  // { message: undefined, isAvailable: true, result: 6 }
  console.log(breaker());
}, 1200);