Implementing a Circuit Breaker in Rails

The Circuit Breaker pattern is a design pattern used in software development to improve the stability and resilience of a system. In a Rails application, using a circuit breaker can prevent your system from making unnecessary requests to external services when they are known to be failing. This pattern can be particularly useful when your Rails application interacts with external APIs, databases, or other microservices.

The Basics of the Circuit Breaker Pattern

Think of a circuit breaker in electrical systems. It trips or “breaks” the circuit when there’s too much current, preventing potential damage. Similarly, in software, a circuit breaker “trips” when certain conditions are met, usually after detecting that a particular service is unavailable or failing.

Here are the three primary states of a circuit breaker:

1. Closed: Everything is functioning as expected. Requests are made to the external service.
2. Open: The external service is down or unresponsive. Requests are not made; instead, failures are returned immediately.
3. Half-Open: After a predetermined amount of time, the circuit breaker allows some requests to check if the service has recovered. If it’s stable, the state returns to Closed; otherwise, it remains Open.

Implementing a Simple Circuit Breaker in Rails

Let’s design a simple circuit breaker without relying on any gems or plugins.

Model the Circuit Breaker

class CircuitBreaker
  attr_accessor :failure_count, :state, :last_failed_time

  FAILURE_THRESHOLD = 5
  HALF_OPEN_TIME = 60 # seconds

  def initialize
    @failure_count = 0
    @state = :closed
    @last_failed_time = nil
  end

  def request
    case state
    when :closed
      make_request
    when :open
      raise "Service Unavailable"
    when :half_open
      make_request if Time.now - last_failed_time >= HALF_OPEN_TIME
    end
  end

  private

  def make_request
    # Replace with your actual request to an external service
    response = ExternalService.call

    if response.success?
      reset
    else
      register_failure
    end
  rescue
    register_failure
    raise "Service Error"
  end

  def register_failure
    @failure_count += 1
    @last_failed_time = Time.now

    if failure_count >= FAILURE_THRESHOLD
      @state = :open
    end
  end

  def reset
    @state = :closed
    @failure_count = 0
  end
end

Using the Circuit Breaker

Whenever you want to make a request to the external service, you would call the request method on an instance of CircuitBreaker.

cb = CircuitBreaker.new
cb.request

Enhancements

While this is a basic implementation, in real-world scenarios, you may want to:

• Store the state of the circuit breaker in a distributed cache like Redis if you have multiple instances of your application.
• Add logging to monitor and alert when the circuit is open.
• Make the thresholds and timings configurable.

Conclusion

The Circuit Breaker pattern provides an elegant solution to handle failures and improve the resilience of your Rails application. With this simple implementation, you can begin to protect your application from cascading failures and ensure a better experience for your users.