brakes

brakes

Brakes is a circuit breaker library for Node. A circuit breaker provides latency and fault protection for distributed systems. Brakes will monitor your outgoing requests, and will trip an internal circuit if it begins to detect that the remote service is failing. Circuit protection allows you to redirect requests to sane fallbacks, and back-off the downstream services so they can recover. This module is largely based on Netflix's Hystrix

Requires Node 4.2.0 or higher

http://martinfowler.com/bliki/CircuitBreaker.html

https://github.com/Netflix/Hystrix/wiki/How-it-Works

https://en.wikipedia.org/wiki/Circuit_breaker_design_pattern

---

• Bluebird and Promisify
• Examples
  • Promise
  • Callback
  • Fallback
  • Health Check
  • Slave Circuits
• Methods
• Events
• Configuration
• Stats
• Hystrix Dashboard
• Development
• Change Log

Bluebird and Promisify

Bluebird

This module utilizes bluebird promises. For more on the features of bluebird visit their site: http://bluebirdjs.com/. Brakes uses bluebird over native promises in order to provide more feature rich promises and because bluebird offers performance comparable to that of raw callbacks.

Promisify

If you pass an async function that relies on callback, brakes will promisify it into a bluebird promise. If you pass a promise to brakes, it will use that promise as is.

Note: brakes will only detect async callback functions that use callbacks with one of the following names: cb, callback, callback_, or done.

Examples

Promise

  function promiseCall(foo){
    return new Promise((resolve, reject) =>{
      if (foo) resolve(foo);
      else reject(foo);
    });
  }

  const brake = new Brakes(promiseCall, {timeout: 150});

  brake.exec('bar')
    .then((result) =>{
      console.log(`result: ${result}`);
    })
    .catch(err =>{
      console.error(`error: ${err}`);
    });

Callback

  function asyncCall(foo, cb){
    if (foo) cb(null, foo);
    else cb(new Error(foo));
  }

  const brake = new Brakes(asyncCall, {timeout: 150});

  brake.exec('bar')
    .then((result) =>{
      console.log(`result: ${result}`);
    })
    .catch(err =>{
      console.error(`error: ${err}`);
    });

Fallback

  function promiseCall(foo){
    return new Promise((resolve, reject) =>{
      if (foo) resolve(foo);
      else reject(foo);
    });
  }

  function fallbackCall(foo){
    return new Promise((resolve, reject) =>{
      resolve('I always succeed');
    });
  }

  const brake = new Brakes(promiseCall, {timeout: 150});

  brake.fallback(fallbackCall)

  brake.exec(false)
    .then((result) =>{
      console.log(`result: ${result}`);
    })
    .catch(err =>{
      console.error(`error: ${err}`);
    });

Health check

  function promiseCall(foo){
    return new Promise((resolve, reject) =>{
      if (foo) resolve(foo);
      else reject(foo);
    });
  }

  function fallbackCall(foo){
    return new Promise((resolve, reject) =>{
      resolve('I always succeed');
    });
  }

  function healthCheckCall(foo){
    return new Promise((resolve, reject) => {
      //this will return 20% true, 80% false
      if (Math.random() > 0.8){
        resolve('Health check success');      
      } else {
        reject('Health check failed');
      }
    });
  }

  const brake = new Brakes(promiseCall, {timeout: 150});

  brake.fallback(fallbackCall);

  brake.healthCheck(healthCheckCall);

  brake.exec(false)
    .then((result) =>{
      console.log(`result: ${result}`);
    })
    .catch(err =>{
      console.error(`error: ${err}`);
    });

Slave Circuits

Brakes exposes the ability to create a master Brakes instance that can then contain slaveCircuits that all report to a central stats object. This allows all slaveCircuits to be tripped at the same time when the overall health of all the slaveCircuits crosses a defined threshold.

See examples/slave-circuit.js for a more complete example.

function promiseCall(foo){
  return new Promise((resolve, reject) =>{
    if (foo) resolve(foo);
    else reject(foo);
  });
}

const brake = new Brakes({
  timeout: 150,
  fallback: () => Promise.resolve('Response from fallback'),
});

const slaveCircuit1 = brake.slaveCircuit(promiseCall);
const slaveCircuit2 = brake.slaveCircuit(promiseCall);

slaveCircuit1.exec('bar')
  .then((result) =>{
    console.log(`result: ${result}`);
  })
  .catch(err =>{
    console.error(`error: ${err}`);
  });

// all stats are reported through the main brakes instance
brake.on('snapshot', snapshot => {
  console.log(`Stats received -> ${snapshot}`);
});

Demonstration

For a terminal based demonstration:

General Demo npm install && node examples/example1.js

Fallback Demo npm install && node examples/fallback-example.js

Health check Demo npm install && node examples/healthCheck-example.js

Hystrix Stream Demo npm install && node examples/hystrix-example.js

Slave Circuits npm install && node examples/slave-circuit.js

Methods

Method	Argument(s)	Returns	Description
getGlobalStats	N/A	globalStats	Returns a reference to the global stats tracker
static getGlobalStats	N/A	globalStats	Returns a reference to the global stats tracker
exec	N/A	Promise	Executes the circuit
fallback	function (must return promise or accept callback)	N/A	Registers a fallback function for the circuit
healthCheck	function (must return promise or accept callback)	N/A	Registers a health check function for the circuit
slaveCircuit	function (required), function (optional), opts (optional)	N/A	Create a new slave circuit that rolls up into the main circuit it is created under.
on	eventName, function	N/A	Register an event listener
destroy	N/A	N/A	Removes all listeners and deregisters with global stats tracker.
isOpen	N/A	boolean	Returns true if circuit is open

Events

Every brake is an instance of EventEmitter that provides the following events:

• exec: Event on request start
• failure: Event on request failure
• success: Event on request success
• timeout: Event on request timeout
• circuitClosed: Event fired when circuit is closed
• circuitOpen: Event fired when circuit is open
• snapshot: Event fired on stats snapshot
• healthCheckFailed: Event fired on failure of each health check execution

Configuration

Available configuration options.

• name: string to use for name of circuit. This is mostly used for reporting on stats.
• group: string to use for group of circuit. This is mostly used for reporting on stats.
• bucketSpan: time in ms that a specific bucket should remain active
• statInterval: interval in ms that brakes should emit a snapshot event
• percentiles: array<number> that defines the percentile levels that should be calculated on the stats object (i.e. 0.9 for 90th percentile)
• bucketNum: # of buckets to retain in a rolling window
• circuitDuration: time in ms that a circuit should remain broken
• waitThreshold: number of requests to wait before testing circuit health
• threshold: % threshold for successful calls. If the % of successful calls dips below this threshold the circuit will break
• timeout: time in ms before a service call will timeout
• isFailure: function that returns true if an error should be considered a failure (receives the error object returned by your command.) This allows for non-critical errors to be ignored by the circuit breaker
• healthCheckInterval: time in ms interval between each execution of health check function
• healthCheck: function to call for the health check (can be defined also with calling healthCheck function)
• fallback: function to call for fallback (can be defined also with calling fallback function)
• isPromise: boolean to opt out of check for callback in function. This affects the passed in function, health check and fallback
• isFunction: boolean to opt out of check for callback, always promisifying in function. This affects the passed in function, health check and fallback
• modifyError: modifies the error message by adding circuit name. default is true.

Stats

Based on the opts.statInterval an event will be fired at regular intervals that contains a snapshot of the running state of the application.

// ...
  brake.on('snapshot', snapshot => {
    console.log(`Stats received -> ${snapshot}`);
  });
// ...

Example Stats Object

{ name: 'defaultBrake',
  group: 'defaultBrakeGroup',
  time: 1463297869298,
  circuitDuration: 15000,
  threshold: 0.5,
  waitThreshold: 100,
  stats:
   { failed: 0,
     timedOut: 0,
     total: 249,
     shortCircuited: 0,
     latencyMean: 100,
     successful: 249,
     percentiles:
      { '0': 100,
        '1': 102,
        '0.25': 100,
        '0.5': 100,
        '0.75': 101,
        '0.9': 101,
        '0.95': 102,
        '0.99': 102,
        '0.995': 102 }
    }
  }

Global Stats Stream

Brakes automatically tracks all created instances of brakes and provides a global stats stream for easy consumption and reporting on all brakes instances. These streams will aggregate all stat events into one single stream.

const globalStats = Brakes.getGlobalStats();

globalStats.getRawStream().on('data', (stats) =>{
  console.log('received global stats ->', stats);
});

Hystrix Dashboard

Using the global stats stream with a special transform, brakes makes it incredibly easy to generate a SSE stream that is compliant with the hystrix dashboard and turbine.

Example:

const globalStats = Brakes.getGlobalStats();

/*
Create SSE Hystrix compliant Server
*/
http.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/event-stream;charset=UTF-8');
  res.setHeader('Cache-Control', 'no-cache, no-store, max-age=0, must-revalidate');
  res.setHeader('Pragma', 'no-cache');
  globalStats.getHystrixStream().pipe(res);
}).listen(8081, () => {
  console.log('---------------------');
  console.log('Hystrix Stream now live at localhost:8081/hystrix.stream');
  console.log('---------------------');
});

To aid in testing it might be useful to have a local instance of the hystrix dashboard running:

docker run -d -p 8080:8080 -p 9002:9002 --name hystrix-dashboard mlabouardy/hystrix-dashboard:latest

Additional Reading: Hystrix Metrics Event Stream, Turbine, Hystrix Dashboard

---

Development

We gladly welcome pull requests and code contributions. To develop brakes locally clone the repo and use the following commands to aid in development:

npm install
npm run test
npm run test:lint
npm run coverage

Change Log

See: https://github.com/awolden/brakes/releases