imclab/levee

A circuit-breaker pattern implementation with fallback support.

Levee

A circuit breaker implementation based heavily on ryanfitz's node-circuitbreaker. More information about the circuitbreaker pattern can be found in the akka documentation.

Basic Usage

'use strict';

var Levee = require('levee');
var Wreck = require('wreck');

var options, circuit;

options = {
    maxFailures: 5,
    timeout: 60000,
    resetTimeout: 30000
};

circuit = Levee.createBreaker(Wreck.get, options);
circuit.run('http://www.google.com', function (err, req, payload) {
    // If the service fails or timeouts occur 5 consecutive times,
    // the breaker opens, fast failing subsequent requests.
    console.log(err || payload);
});

Advanced Usage

function fallback(url, callback) {
    callback(null, null, new Buffer('The requested website is not available. Please try again later.'));
}

circuit = Levee.createBreaker(service, options);
circuit.fallback = Levee.createBreaker(fallback, options);

circuit.on('timeout', function () {
    console.log('Request timed out.');
});

circuit.on('failure', function (err) {
    console.log('Request failed.', err);
});

circuit.run('http://www.google.com', function (err, req, payload) {
    // If the service fails or timeouts occur 5 consecutive times,
    // the breaker opens, fast failing subsequent requests.
    console.log(err || payload);
});

var stats, fbStats;
stats = Levee.createStats(circuit);
fbStats = Levee.createStats(circuit.fallback);

// Print stats every 5 seconds.
setInterval(function () {
    console.log(stats.snapshot());
    console.log(fbStats.snapshot());
}, 5000);

API

new Breaker(command [, options])

Creates a new Breaker instance with the following arguments:

• command - an object with a property named execute with value being a function using the signature: function (context, callback) where:
  • context - Any context needed to execute the desired behavior.
  • callback - A callback function with the signature function (err, [arg1, arg2, ...])

var Levee = require('levee');

var breaker = new Levee.Breaker({ execute: fn }, options);

createBreaker(command [, options])

An alternative method for creating Breaker instances with the following arguments:

• command - either function or an object with a property named execute with value being a function using the signature: function (context, callback) where:
  • context - Any context needed to execute the desired behavior.
  • callback - A callback function with the signature function (err, [arg1, arg2, ...])

var Levee = require('levee');

function doStuff(context, callback) {
    callback(null, 'ok');
}

var breaker = Levee.createBreaker(fn, options);

new Stats(breaker)

Create a new Stats instance with the following argument:

• breaker - a Breaker instance

var Levee = require('levee');

var breaker = new Levee.Stats(breaker);

createStats(breaker)

An alternative method for creating a new Stats instance with the following argument:

• breaker - a Breaker instance

var Levee = require('levee');

var breaker = Levee.createStats(breaker);

Breaker

new Levee.Breaker(command, options) or Levee.createBreaker(command, options)

Options

timeout

the amount of time to allow an operation to run before terminating with an error.

maxFailures

the number of failures allowed before the Breaker enters the open state.

resetTimeout

the amount of time to wait before switch the Breaker from the open to half_open state to attempt recovery.

Properties

fallback

a Breaker instance to fallback to in the case of the Breaker entering the open state.

Methods

run(context, callback)

Executes the wrapped functionality within the circuit breaker functionality with the arguments:

• context - any context to be provided to the implementation.
• callback - the callback to be fired upon completion with the signature function (err, [param1, param2, ...])

Stats

new Levee.Stats(breaker) or Levee.createStats(breaker)

A simple data aggregation object.

Methods

increment(name)

Increment a named counter.

• name - the label of the counter to increment.

decrement(name)

Decrement a named counter.

• name - the label of the counter to decrement.

sample(name, value)

Take a sample of a given value.

• name - the label of the sample being recorded.
• value - the sample value being recorded.

snapshot()

Get the current state of the current Stats instance. Returns an object with the following properties:

• counts - A map of names to current count values.
• samples - A map of names to current sample averages and counts, in the form of: { average: 0, count, 0 }

reset()

Resets all counts and samples.

resetCounts([name])

Reset counts for the provided name. If no name is provided, resets all counts.

• name - the label of the count to reset.

resetSamples([name])

Reset samples for the provided name. If no name is provided, resets all samples.

• name - the label of the sample to reset.