---
layout: pattern
title: Circuit Breaker
folder: circuit-breaker
permalink: /patterns/circuit-breaker/
categories: Behavioral
language: en
tags:
  - Performance
  - Decoupling
  - Cloud distributed
---

## Intent

Handle costly remote service calls in such a way that the failure of a single service/component 
cannot bring the whole application down, and we can reconnect to the service as soon as possible.

## Explanation

Real world example

> Imagine a web application that has both local files/images and remote services that are used for 
> fetching data. These remote services may be either healthy and responsive at times, or may become 
> slow and unresponsive at some point of time due to variety of reasons. So if one of the remote 
> services is slow or not responding successfully, our application will try to fetch response from 
> the remote service using multiple threads/processes, soon all of them will hang (also called 
> [thread starvation](https://en.wikipedia.org/wiki/Starvation_(computer_science))) causing our entire web application to crash. We should be able to detect 
> this situation and show the user an appropriate message so that he/she can explore other parts of 
> the app unaffected by the remote service failure. Meanwhile, the other services that are working 
> normally, should keep functioning unaffected by this failure.

In plain words

> Circuit Breaker allows graceful handling of failed remote services. It's especially useful when 
> all parts of our application are highly decoupled from each other, and failure of one component 
> doesn't mean the other parts will stop working.

Wikipedia says

> Circuit breaker is a design pattern used in modern software development. It is used to detect 
> failures and encapsulates the logic of preventing a failure from constantly recurring, during 
> maintenance, temporary external system failure or unexpected system difficulties.

## Programmatic Example

So, how does this all come together? With the above example in mind we will imitate the 
functionality in a simple example. A monitoring service mimics the web app and makes both local and 
remote calls.

The service architecture is as follows:

![alt text](./etc/ServiceDiagram.png "Service Diagram")

In terms of code, the end user application is:

```java
@Slf4j
public class App {

  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);

  /**
   * Program entry point.
   *
   * @param args command line args
   */
  public static void main(String[] args) {

    var serverStartTime = System.nanoTime();

    var delayedService = new DelayedRemoteService(serverStartTime, 5);
    var delayedServiceCircuitBreaker = new DefaultCircuitBreaker(delayedService, 3000, 2,
        2000 * 1000 * 1000);

    var quickService = new QuickRemoteService();
    var quickServiceCircuitBreaker = new DefaultCircuitBreaker(quickService, 3000, 2,
        2000 * 1000 * 1000);

    //Create an object of monitoring service which makes both local and remote calls
    var monitoringService = new MonitoringService(delayedServiceCircuitBreaker,
        quickServiceCircuitBreaker);

    //Fetch response from local resource
    LOGGER.info(monitoringService.localResourceResponse());

    //Fetch response from delayed service 2 times, to meet the failure threshold
    LOGGER.info(monitoringService.delayedServiceResponse());
    LOGGER.info(monitoringService.delayedServiceResponse());

    //Fetch current state of delayed service circuit breaker after crossing failure threshold limit
    //which is OPEN now
    LOGGER.info(delayedServiceCircuitBreaker.getState());

    //Meanwhile, the delayed service is down, fetch response from the healthy quick service
    LOGGER.info(monitoringService.quickServiceResponse());
    LOGGER.info(quickServiceCircuitBreaker.getState());

    //Wait for the delayed service to become responsive
    try {
      LOGGER.info("Waiting for delayed service to become responsive");
      Thread.sleep(5000);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    //Check the state of delayed circuit breaker, should be HALF_OPEN
    LOGGER.info(delayedServiceCircuitBreaker.getState());

    //Fetch response from delayed service, which should be healthy by now
    LOGGER.info(monitoringService.delayedServiceResponse());
    //As successful response is fetched, it should be CLOSED again.
    LOGGER.info(delayedServiceCircuitBreaker.getState());
  }
}
```

The monitoring service: 

```java
public class MonitoringService {

  private final CircuitBreaker delayedService;

  private final CircuitBreaker quickService;

  public MonitoringService(CircuitBreaker delayedService, CircuitBreaker quickService) {
    this.delayedService = delayedService;
    this.quickService = quickService;
  }

  //Assumption: Local service won't fail, no need to wrap it in a circuit breaker logic
  public String localResourceResponse() {
    return "Local Service is working";
  }

  /**
   * Fetch response from the delayed service (with some simulated startup time).
   *
   * @return response string
   */
  public String delayedServiceResponse() {
    try {
      return this.delayedService.attemptRequest();
    } catch (RemoteServiceException e) {
      return e.getMessage();
    }
  }

  /**
   * Fetches response from a healthy service without any failure.
   *
   * @return response string
   */
  public String quickServiceResponse() {
    try {
      return this.quickService.attemptRequest();
    } catch (RemoteServiceException e) {
      return e.getMessage();
    }
  }
}
```
As it can be seen, it does the call to get local resources directly, but it wraps the call to 
remote (costly) service in a circuit breaker object, which prevents faults as follows:

```java
public class DefaultCircuitBreaker implements CircuitBreaker {

  private final long timeout;
  private final long retryTimePeriod;
  private final RemoteService service;
  long lastFailureTime;
  private String lastFailureResponse;
  int failureCount;
  private final int failureThreshold;
  private State state;
  private final long futureTime = 1000 * 1000 * 1000 * 1000;

  /**
   * Constructor to create an instance of Circuit Breaker.
   *
   * @param timeout          Timeout for the API request. Not necessary for this simple example
   * @param failureThreshold Number of failures we receive from the depended service before changing
   *                         state to 'OPEN'
   * @param retryTimePeriod  Time period after which a new request is made to remote service for
   *                         status check.
   */
  DefaultCircuitBreaker(RemoteService serviceToCall, long timeout, int failureThreshold,
      long retryTimePeriod) {
    this.service = serviceToCall;
    // We start in a closed state hoping that everything is fine
    this.state = State.CLOSED;
    this.failureThreshold = failureThreshold;
    // Timeout for the API request.
    // Used to break the calls made to remote resource if it exceeds the limit
    this.timeout = timeout;
    this.retryTimePeriod = retryTimePeriod;
    //An absurd amount of time in future which basically indicates the last failure never happened
    this.lastFailureTime = System.nanoTime() + futureTime;
    this.failureCount = 0;
  }

  // Reset everything to defaults
  @Override
  public void recordSuccess() {
    this.failureCount = 0;
    this.lastFailureTime = System.nanoTime() + futureTime;
    this.state = State.CLOSED;
  }

  @Override
  public void recordFailure(String response) {
    failureCount = failureCount + 1;
    this.lastFailureTime = System.nanoTime();
    // Cache the failure response for returning on open state
    this.lastFailureResponse = response;
  }

  // Evaluate the current state based on failureThreshold, failureCount and lastFailureTime.
  protected void evaluateState() {
    if (failureCount >= failureThreshold) { //Then something is wrong with remote service
      if ((System.nanoTime() - lastFailureTime) > retryTimePeriod) {
        //We have waited long enough and should try checking if service is up
        state = State.HALF_OPEN;
      } else {
        //Service would still probably be down
        state = State.OPEN;
      }
    } else {
      //Everything is working fine
      state = State.CLOSED;
    }
  }

  @Override
  public String getState() {
    evaluateState();
    return state.name();
  }

  /**
   * Break the circuit beforehand if it is known service is down Or connect the circuit manually if
   * service comes online before expected.
   *
   * @param state State at which circuit is in
   */
  @Override
  public void setState(State state) {
    this.state = state;
    switch (state) {
      case OPEN:
        this.failureCount = failureThreshold;
        this.lastFailureTime = System.nanoTime();
        break;
      case HALF_OPEN:
        this.failureCount = failureThreshold;
        this.lastFailureTime = System.nanoTime() - retryTimePeriod;
        break;
      default:
        this.failureCount = 0;
    }
  }

  /**
   * Executes service call.
   *
   * @return Value from the remote resource, stale response or a custom exception
   */
  @Override
  public String attemptRequest() throws RemoteServiceException {
    evaluateState();
    if (state == State.OPEN) {
      // return cached response if the circuit is in OPEN state
      return this.lastFailureResponse;
    } else {
      // Make the API request if the circuit is not OPEN
      try {
        //In a real application, this would be run in a thread and the timeout
        //parameter of the circuit breaker would be utilized to know if service
        //is working. Here, we simulate that based on server response itself
        var response = service.call();
        // Yay!! the API responded fine. Let's reset everything.
        recordSuccess();
        return response;
      } catch (RemoteServiceException ex) {
        recordFailure(ex.getMessage());
        throw ex;
      }
    }
  }
}
```

How does the above pattern prevent failures? Let's understand via this finite state machine 
implemented by it.

![alt text](./etc/StateDiagram.png "State Diagram")

- We initialize the Circuit Breaker object with certain parameters: `timeout`, `failureThreshold` and `retryTimePeriod` which help determine how resilient the API is.
- Initially, we are in the `closed` state and nos remote calls to the API have occurred.
- Every time the call succeeds, we reset the state to as it was in the beginning.
- If the number of failures cross a certain threshold, we move to the `open` state, which acts just like an open circuit and prevents remote service calls from being made, thus saving resources. (Here, we return the response called ```stale response from API```)
- Once we exceed the retry timeout period, we move to the `half-open` state and make another call to the remote service again to check if the service is working so that we can serve fresh content. A failure sets it back to `open` state and another attempt is made after retry timeout period, while a success sets it to `closed` state so that everything starts working normally again. 

## Class diagram

![alt text](./etc/circuit-breaker.urm.png "Circuit Breaker class diagram")

## Applicability

Use the Circuit Breaker pattern when

- Building a fault-tolerant application where failure of some services shouldn't bring the entire application down.
- Building a continuously running (always-on) application, so that its components can be upgraded without shutting it down entirely.

## Related Patterns

- [Retry Pattern](https://github.com/iluwatar/java-design-patterns/tree/master/retry)

## Real world examples

* [Spring Circuit Breaker module](https://spring.io/guides/gs/circuit-breaker)
* [Netflix Hystrix API](https://github.com/Netflix/Hystrix)

## Credits

* [Understanding Circuit Breaker Pattern](https://itnext.io/understand-circuitbreaker-design-pattern-with-simple-practical-example-92a752615b42)
* [Martin Fowler on Circuit Breaker](https://martinfowler.com/bliki/CircuitBreaker.html)
* [Fault tolerance in a high volume, distributed system](https://medium.com/netflix-techblog/fault-tolerance-in-a-high-volume-distributed-system-91ab4faae74a)
* [Circuit Breaker pattern](https://docs.microsoft.com/en-us/azure/architecture/patterns/circuit-breaker)
