In this tutorial, we will see how to implement a circuit breaker pattern in JavaScript. This was a question asked in Atlassian’s front-end interview.
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.
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";
Also see,