Implementing a circuit breaker pattern with promises

26 Oct 2023

In modern distributed systems, failures are inevitable. When a service or resource becomes unavailable or unreliable, it can have a cascading effect on the overall system performance. To handle such scenarios, the Circuit Breaker Pattern comes to the rescue. It provides a way to handle and recover from failures gracefully, while still allowing the system to function and protect against further failures.

In this blog post, we’ll explore how to implement a Circuit Breaker Pattern using promises, a popular asynchronous programming pattern in many programming languages.

What is the Circuit Breaker Pattern?

The Circuit Breaker Pattern is a design pattern that adds resiliency to the system by wrapping calls to external services or resources with a circuit breaker logic. It monitors the state of the external service and determines when to break the circuit and prevent any further calls to the service.

When the circuit breaker is open, any subsequent calls to the service will be blocked or redirected to an alternative response, such as a fallback or cached data. This prevents overloading the service and cascading failures.

The circuit breaker can periodically attempt to close and allow calls to the service again, thus providing a mechanism for self-recovery. It can also detect the health of the service by monitoring failures or timeouts and adjust its state accordingly.

Implementing the Circuit Breaker Pattern with Promises

To implement the Circuit Breaker Pattern using promises, we’ll define the following states for the circuit breaker:

Closed: The circuit breaker allows calls to the external service, monitoring for failures or timeouts.
Open: The circuit breaker rejects all calls to the external service and redirects them to an alternative response.
Half-Open: After a certain time, the circuit breaker allows a limited number of calls to the external service to test its health.

Let’s look at an example implementation of a circuit breaker using promises in JavaScript:

class CircuitBreaker {
  constructor(service) {
    this.service = service;
    this.state = 'closed';
  }

  async callService() {
    if (this.state === 'closed') {
      try {
        const result = await this.service();
        // Process the result from the service
        this.state = 'closed';
        return result;
      } catch (error) {
        this.state = 'open';
        // Handle the error or redirect to fallback
        throw error;
      }
    } else {
      // Circuit breaker is open or half-open, redirect to fallback or cached data
      throw new Error('Service unavailable');
    }
  }

  // Method to close the circuit breaker and reset its state
  close() {
    this.state = 'closed';
  }
}

In the example above, we define a CircuitBreaker class that accepts a service function as a parameter. The callService method is responsible for handling the circuit breaker logic.

When the circuit breaker is in the closed state, the callService method calls the actual service function and awaits its result. If the service call succeeds, the result is returned, and the circuit breaker remains in the closed state. If the service call fails, the circuit breaker transitions to the open state, and the error is propagated.

If the circuit breaker is in the open or half-open state, the callService method throws an error indicating that the service is unavailable. The client code can handle this error by redirecting to a fallback or cached data.

The close method is provided to reset the circuit breaker to the closed state, allowing calls to the service again.

Conclusion

Implementing the Circuit Breaker Pattern with promises provides a powerful tool for handling failures in distributed systems. By monitoring and controlling the flow of calls to external services, we can gracefully handle failures, prevent further damage, and recover when the service becomes available again.

Promises allow us to write asynchronous code in a more concise and manageable way, making it easier to implement complex patterns like the Circuit Breaker Pattern.

By adopting this pattern, we can improve the resilience and availability of our systems, ensuring a better user experience and mitigating the impact of failures.

References

#circuitbreaker #promises