Circuit breaker in JavaScript

Design Patterns

What happens behind the scenes when we type google.com in a browser?

A circuit breaker is a design pattern that helps to prevent cascading failures. Falling under the sustainable category, it is majorly used on the micro-services but can be implemented on the front-end side as well.

Imagine you are making an API call and the request keeps failing, rather than keep on bombarding the server, we can halt the request sending for a certain time. That is how a circuit breaker works.

Circuit breaker pattern flow in JavaScript We have to implement a function that will halt the operation for X amount of time if it fails for Y count.

This can implemented by forming a closure where we track the count of failure and time since the last failure and based on that decide if the function is available or not.

const circuitBreaker = (fn, failureCount, timeThreshold) => {
  let failures = 0;
  let timeSinceLastFailure = 0;
  let isClosed = false;
  
  return function(...args){
    // if service is closed
    if(isClosed){
      const diff = Date.now() - timeSinceLastFailure;
      
      // if the time since last failure has exceeded 
      // the time threshold
      // open the service
      if(diff > timeThreshold){
        isClosed = false;
      }
      // else throw error
      else{
        console.error("Service unavailable");
        return;
      }
    }
    
    // try running the function
    // if it passes reset the failure count
    try{
      const result = fn(...args);
      failures = 0;
      return result;
    }
    // if function throws error / fails
    // increase the failure count and 
    // timewhen it fails
    catch(error){
      failures++;
      timeSinceLastFailure = Date.now();
      if(failures >= failureCount){
        isClosed = true;  
      }
      
      console.log("Error");
    }
  }
}

To test the circuit-breaker function, we have implemented a function that will fail for first 3 times it is called. This will close the service for a given amount of time.

// test function
const testFunction = () => {
  let count = 0;
  
  return function(){
    count++;
    if(count < 4){
      throw "failed";
    }else{
      return "hello";
    }
  }
};
 
 
let t = testFunction();
let c = circuitBreaker(t, 3, 200);
 
c(); // "error"
c(); // "error"
c(); // "error"
 
// service is closed for 200 MS
c(); // "service unavailable" 
c(); // "service unavailable"
c(); // "service unavailable"
c(); // "service unavailable"
c(); // "service unavailable"
 
// service becomes available after 300ms
setTimeout(() => {console.log(c());}, 300); // "hello";