CircuitBreaker

Implementation of the Circuit Breaker design pattern in Swift. Circuit Breakers are most useful for networking where remote calls can fail or time out at any moment. When configured thresholds are reached, the circuit breaker "trips" and thus prevents putting unnecessary load on the server until the breaker resets itself after a timeout. In addition, this implementation provides convenience features such as monitoring call timeouts and supporting retry logic with exponential backoff.

Requirements

>= iOS 8, Xcode 7, Swift 2.0

Installation

You can either use Carthage and then import the framework or simply copy the file CircuitBreaker.swift to your project.

Usage

let circuitBreaker = CircuitBreaker(
  timeout: 10.0,
  maxRetries: 2,
  timeBetweenRetries: 2.0,
  exponentialBackoff: true,
  resetTimeout: 2.0 
)

circuitBreaker.call = { [weak self] circuitBreaker in
  self?.someRemoteService.call { data, error in
    if let error = error {
      circuitBreaker.failure(error)
    } else {
      circuitBreaker.success()
    }
  }
}

circuitBreaker.didTrip = { circuitBreaker, error in 
  print("didTrip", error)
}

circuitBreaker.execute()

After initializing a CircuitBreaker instance, a closure is registered that is executed when the execute method is called or when the call is automatically retried after recording an error with the failure method. When the threshold is reached after maxRetries, the actual call is not performed but instead the didTrip closure is triggered. The breaker is now in the Open state. When resetTimeout has passed, the breaker is in the HalfOpen state. Another call to execute now sets the breaker back to Closed as soon as the success method is called.

Configuration

CircuitBreakercan be configured with five parameters:

• timeout: Timeout in seconds (per call). When no failure or success was recorded within timeout, the call is automatically retried.
• maxRetries: The maximum number of retries before the breaker trips.
• timeBetweenRetries: Time in seconds between retries. Used as base when exponentialBackoff is true.
• exponentialBackoff: Increases the time between retries (when timeBetweenRetries > 1.0) exponentially when set to true.
• resetTimeout: Time in seconds after which the breaker is reset when it was "tripped".

Public Interface

The main public interface consists of four methods and two properties for registering closures:

• execute(): Executes the closure registered via the call property.
• success(): Records a successful call and resets the circuit breaker. Call this method when your (asynchronous) method call was successful.
• failure(error: ErrorType? = nil): Records a failure with an optional error conforming to ErrorType. Executing this method may trigger the call closure again for retrying or it may trip the breaker by triggering the closure registered via didTrip. didTrip also receives the last error provided by failure() as second parameter.
• reset(): Stops all timeouts and resets the breaker to the default state. Note: Call this method when you need to cancel your call and deallocate the circuit breaker.

Additionally, two read-only properties expose the state of the circuit breaker:

• state: Returns one of .Closed, .HalfOpen or Open.
• failureCount: Returns the current count of failure calls.

Example

See the main ViewController.swift for a basic example that performs calls against a test service and logs the circuit breaker state.

Tests

See CircuitBreakerTests.swift for a couple of unit tests.

Author

fe9lix

License

CircuitBreaker is available under the MIT license. See the LICENSE file for more info.