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CircuitBreaker

The Circuit Breaker design pattern is used to increase application stability, improve response time, and prevent the application from making constant failing requests. This library provides the tools needed to bring Circuit Breaker logic to your Swift applications.

Circuit Breaker Diagram

Contents

Swift version

The latest version of CircuitBreaker works with the 4.0.3 and newer version of the Swift binaries. You can download this version of the Swift binaries by following this link.

Installation

To leverage the CircuitBreaker package in your Swift application, you should specify a dependency for it in your Package.swift file:

 import PackageDescription

 let package = Package(
     name: "MySwiftProject",

     ...

     dependencies: [
         // Swift 4
         .package(url: "https://github.com/IBM-Swift/CircuitBreaker.git", .upToNextMajor(from: "5.0.0")),
         ...

     ])

Usage

The CircuitBreaker state is based on timeouts and user defined failures (quite useful when the function you are circuit breaking makes an asynchronous call).

  1. Define a fallback function with the signature (BreakerError, (fallbackArg1, fallbackArg2,...)) -> Void:
func myFallback(err: BreakerError, msg: String) {
    // The fallback will be called if one of the below occurs:
    //  1. The request does not return before the specified timeout
    //  2. CircuitBreaker is currently in Open state and set to fail fast.
    //  3. There was an error in the user's called context function (networking error, etc.)
    Log.verbose("Error: \(error)")
    Log.verbose("Message: \(msg)")
}
  1. Extend BreakerError by defining your own error handling to be used in your context function.
extension BreakerError {
    public static let encodingURLError = BreakerError(reason: "URL could not be created")
    public static let networkingError = BreakerError(reason: "There was an error, while sending the request")
}
  1. Create a context function for the logic you intend to circuit break (this allows you to alert the CircuitBreaker of a failure or a success). Please note that a context function receives an Invocation object as its parameter. An instance of the Invocation class states 1) the parameter types that must be passed to the context function, 2) the return type from the execution of the context function, and 3) parameter type used as the second argument for the fallback closure:
func myContextFunction(invocation: Invocation<(String), String>) {
  let requestParam = invocation.commandArgs
  // Create HTTP request
  guard let url = URL(string: "http://myserver.net/path/\(requestParam)") else {
    // Something went wrong...

    ...

    invocation.notifyFailure(error: .encodingURLError)
  }

  var req = URLRequest(url: url)
  req.httpMethod = "GET"
  req.allHTTPHeaderFields = ["Content-Type": "application/json"]
  let session = URLSession.shared

  // Perform Request
  session.dataTask(with: req) { result, res, err in
    guard let result = result else {
      // Failed getting a result from the server

      ...

      invocation.notifyFailure(error: .networkingError)
      return
    }

    // Convert results to a JSON object
    let json = JSON(data: result)
    // Process JSON data

    ...

    invocation.notifySuccess()
  }.resume()
}
  1. Create a CircuitBreaker instance for each context function (e.g. endpoint) you wish to circuit break:
let breaker = CircuitBreaker(command: myContextFunction, fallback: myFallback)
* Must specify the fallback function and the endpoint to circuit break
* Optional configurations include: timeout, resetTimeout, maxFailures, rollingWindow, and bulkhead
  1. Invoke the call to the endpoint by calling the CircuitBreaker run() method. You should pass the corresponding arguments for the context command and fallback closures. In this sample, myContextFunction takes a string as its parameter while myFallback takes a string as its second parameter:
let id: String = ...
breaker.run(commandArgs: id, fallbackArgs: "Something went wrong.")
Full Implementation:
...
extension BreakerError {
    public static let encodingURLError = BreakerError(reason: "URL could not be created")
    public static let networkingError = BreakerError(reason: "There was an error, while sending the request")
}

func myFallback(err: BreakerError, msg: String) {
    // The fallback will be called if one of the below occurs:
    //  1. The request does not return before the specified timeout
    //  2. CircuitBreaker is currently in Open state and set to fail fast.
    //  3. There was an error in the user's called context function (networking error, etc.)
    Log.verbose("Error: \(error)")
    Log.verbose("Message: \(msg)")
}

func myContextFunction(invocation: Invocation<(String), String>) {
  let requestParam = invocation.commandArgs
  // Create HTTP request
  guard let url = URL(string: "http://mysever.net/path/\(requestParam)") else {
    // Something went wrong...

    ...

    invocation.notifyFailure(error: .encodingURLError)
  }

  var req = URLRequest(url: url)
  req.httpMethod = "GET"
  req.allHTTPHeaderFields = ["Content-Type": "application/json"]
  let session = URLSession.shared

  // Perform Request
  session.dataTask(with: req) { result, res, err in
    guard let result = result else {
      // Failed getting a result from the server

      ...

      invocation.notifyFailure(error: .networkingError)
      return
    }

    // Convert results to a JSON object
    let json = JSON(data: result)
    // Process JSON data

    ...

    invocation.notifySuccess()
  }.resume()
}

let breaker = CircuitBreaker(command: myContextFunction, fallback: myFallback)

let id: String = ...
breaker.run(commandArgs: id, fallbackArgs: "Something went wrong.")

...

API

CircuitBreaker

Constructor

CircuitBreaker(timeout: Int = 1000, resetTimeout: Int = 60000, maxFailures: Int = 5, rollingWindow: Int = 10000, bulkhead: Int = 0, command: @escaping AnyContextFunction<A>, fallback: @escaping AnyFallback<C>)

Constructor parameters

  • timeout Amount in milliseconds that your function should complete before the invocation is considered a failure. Default is set to 1000 milliseconds.
  • resetTimeout Amount in milliseconds to wait before setting to halfopen state. Default is set to 60000 milliseconds.
  • maxFailures Number of failures allowed within rollingWindow before setting state to open. Default is set to 5.
  • rollingWindow Time window in milliseconds where the maximum number of failures must occur to trip the circuit. For instance, say maxFailures is 5 and rollingWindow is 10000 milliseconds. In such case, for the circuit to trip, 5 invocation failures must occur in a time window of 10 seconds, even if these failures are not consecutive. Default is set to 10000 milliseconds.
  • bulkhead Number of the limit of concurrent requests running at one time. Default is set to 0, which is equivalent to not using the bulkheading feature.
  • fallback Function user specifies to signal timeout or fastFail completion. Required format: (BreakerError, (fallbackArg1, fallbackArg2,...)) -> Void
  • command Contextual function to circuit break, which allows user defined failures (the context provides an indirect reference to the corresponding circuit breaker instance).

Stats

Tracked Stats:

  • Total Requests
  • Concurrent Requests
  • Rejected Requests
  • Successful Responses
  • Average Execution Response Time
  • Average Total Response Time
  • Failed Responses
  • Total Timeouts
  • Total Latency
  • Total Execution Latency
  • Hystrix Compliant Snapshot
...
// Create CircuitBreaker
let breaker = CircuitBreaker(command: myFunction, fallback: myFallback)

// Invoke breaker call
breaker.run(commandArgs: (a: 10, b: 20), fallbackArgs: "Something went wrong.")

// Log Stats snapshot
breaker.snapshot()

// Hystrix compliant snapshot
let snapshot = breaker.snapshot
...

Observing stats

The CircuitBreaker library provides an interface for observing new CircuitBreaker instances in order to register and track stat changes. In the initialization of a CircuitBreaker instance, the linked monitors are notified of its instantiation allowing them to begin tracking the instance's stats. The CircuitBreaker instance exposes a Hystrix compliant stat snapshot to the monitor which can then be processed accordingly.


/// Initialize stat monitors
let monitor1 = SwiftMetrics()
let monitor2 = ...
...
let monitorN = ...

/// Register monitors
CircuitBreaker.addMonitor(monitor1)
CircuitBreaker.addMonitor(monitor2)
...
CircuitBreaker.addMonitor(monitorN)

// Create instances of CircuitBreaker
let circuit1 = CircuitBreaker()
let circuit2 = CircuitBreaker()
...
let circuitN = CircuitBreaker()

As mentioned above, the initializer takes care of notifying each one of the monitors of the new CircuitBreaker instance.

License

This Swift package is licensed under Apache 2.0. Full license text is available in LICENSE.

Github

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Dependencies

Releases

5.0.0 - Jan 9, 2018

What's New

  • Provides a new interface for monitoring circuit breaker instances in addition to expanded stat tracking capabilities

4.0.0 - Dec 12, 2017

Changes

  • Removes basic usage api.
  • Now calls fallback method on command failures in addition to timeout errors and when failing fast.
  • Introduces custom error handling for command functions.
  • Addresses potential race condition.

3.0.0 - Nov 14, 2017

Changes

  • Full migration to Swift 4.0.2

2.0.1 - Oct 2, 2017

What's New

  • Support for Swift 4

2.0.0 - Jul 7, 2017

  • Adjusted order of parameters for CircuitBreaker init and run method.
  • Renaming of wrapper function
  • Readme and unit test updates