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HTTP pool and load balancer for node

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readme.md

poolee

HTTP pool and load balancer for node.

Example

var Pool = require("poolee")
var http = require("http")

var servers =
  ["127.0.0.1:8886"
  ,"127.0.0.1:8887"
  ,"127.0.0.1:8888"
  ,"127.0.0.1:8889"]

var postData = '{"name":"Danny Coates"}'

var pool = new Pool(http, servers, options)

pool.request(
  { method: "PUT"
  , path: "/users/me"
  }
, postData
, function (error, response, body) {
    if (error) {
      console.error(error.message)
      return
    }
    if(response.statusCode === 201) {
      console.log("put succeeded")
    }
    else {
      console.log(response.statusCode)
      console.log(body)
    }
  }
)

API

Pool

new

var Pool = require('poolee')
//...

var pool = new Pool(
  http                     // the http module to use (require('http') or require('https'))
  ,
  [ "127.0.0.1:1337"       // array of endpoints in "host:port" form
  , "127.0.0.1:1338"
  ]
  ,                        // options
  { maxPending: 1000       // maximum number of outstanding request to allow
  , maxSockets: 200        // max sockets per endpoint Agent
  , timeout: 60000         // request timeout in ms
  , resolution: 1000       // timeout check interval (see below)
  , ping: undefined        // health check url
  , pingTimeout: 2000      // ping timeout in ms
  , retryFilter: undefined // see below
  , retryDelay: 20         // see below
  , name: undefined        // optional string
  }
)
maxPending

Once this threshold is reached, requests will return an error to the callback as a signal to slow down the rate of requests.

resolution

Pending requests have their timeouts checked at this rate. If your timeout is 60000 and resolution is 1000, the request will timeout no later than 60999

retryFilter

All valid http responses aren't necessarily a "success". This function lets you check the response before calling the request callback. Returning a "truthy" value will retry the request.

For instance, we may want to always retry 500 responses by default:

options.retryFilter = function (
    options  // the request.options
  , response // the http response object
  , body     // the response body
  ) {
  return response.statusCode === 500
}

If the returned value is true the next attempt will be delayed using exponential backoff; if its Number it will delay the next attempt by that many ms (useful for Retry-After headers)

retryDelay

Pool uses exponential backoff when retrying requests. This value is a scaling factor of the time (ms) to wait. Here's how it works:

Math.random() * Math.pow(2, attemptNumber) * retryDelay

If retryDelay is 20, attemptNumber 1 (the first retry) will delay at most 40ms

ping

When an endpoint is unresponsive the pool will not use it for requests. The ping url gives a downed endpoint a way to rejoin the pool. If an endpoint is marked unhealthy and a ping url is given, the endpoint will make requests to its ping url until it gets a 200 response, based on the resolution time.

If the ping url is undefined, the endpoint will never be marked unhealthy.

pool.request

An http request. The pool sends the request to one of it's endpoints. If it fails, the pool may retry the request on other endpoints until it succeeds or reaches options.attempts number of tries. When data is a Stream, only 1 attempt will be made

Usage

The first argument may be a url path. If the callback has 3 arguments the full response body will be returned

pool.request('/users/me', function (error, response, body) {})

The first argument may be an options object. Here's the default values:

pool.request(
  { path: undefined        // the request path (required)
  , method: 'GET'
  , data: undefined        // request body, may be a string, buffer, or stream
  , headers: {}            // extra http headers to send
  , retryFilter: undefined // see below
  , attempts: pool.length  // or at least 2, at most 5
  , retryDelay: 20         // retries wait with exponential backoff times this number of ms
  , timeout: 60000         // ms to wait before timing out the request
  , encoding: 'utf8'       // response body encoding
  , stream: false          // stream instead of buffer response body
  }
  ,
  function (error, response, body) {}
)

The request body may be the second argument, instead of options.data (more useful with pool.post and pool.put)

pool.request(
  { path: '/foo' }
  , 'hi there'
  , function (error, response, body) {}
)

A callback with 2 arguments will stream the response and not buffer the response body.

pool.request('/foo', function (error, response) {
  response.pipe(somewhere)
})

pool.get

Just a synonym for request

pool.put

Same arguments as request that sets options.method = 'PUT'. Nice for putting :)

pool.put('/tweet/me', 'Hello World!', function (error, response) {})

pool.post

Same arguments as request that sets options.method = 'POST'

pool.del

Same arguments as request that sets options.method = 'DELETE'


Advanced

Make a pool

var pool = new Poolee(http, servers, options)

servers: array of strings formatted like 'ip:port'

options: defaults and explanations below

// options
{
  // number of pending requests allowed
  maxPending: 1000

  // ping path. (default = no ping checks)
, ping: null

, retryFilter: function (options, response) {
    // return true to reject response and retry
  }

  // number in milliseconds
, retryDelay: 20

  // optional string name
, name: null
}

Events emitted by pool:

pool.on('health', function(messageString) {
  // message string of of the form:
  // "127.0.0.1:8888 health: true"
  // or
  // "127.0.0.1:8888 health: false"
})

pool.on('timeout', function(url) {
  // where url is a ip+port+path combination for the timed-out request
})

pool.on('retrying', function(error) { })

pool.on('timing', function(time, options) {
  // `time`: the time the latest request took to complete
  // `options`: options used to send the request
})

Get a healthy node

var node = pool.get_node()

Attached to node:

// Counts of interest
// node.pending;
// node.successes
// node.failures
// node.requestRate

// node.ip;
// node.port;
// node.name = node.ip + ':' + node.port;

Events emitted by node

node.on('health', function(self) {
  // `self` has all the same properties as `node`
})

node.on('timeout', function (request) {
  // the request that timed out
})

Example

Note that this example should fail, because there won't be any nodes running. You can also see this code in examples/.

client.js

var Poolee = require('../')
  , http = require('http')
  , ms = require('ms') // converts a time to milliseconds
  , servers = [ '127.0.0.1:8080', '127.0.0.1:8081', '127.0.0.1:8082' ]
  , pool = null

  // healthiest node, populated later on
  , active_node = null

pool = new Poolee(http, servers)

pool.on('retrying', function(error) {
  console.log(error.message);
})

console.log('fib(40) = ...calculating on worker.js...');
pool.request(
  { method: 'GET'
  , path: '/'
  }
, function (error, response, body) {
    if (error) {
      console.error(error.message)
      return
    }
    if (response.statusCode === 200) {
      console.log(body)

    } else {
      console.log(response.statusCode)
      console.log(body)
    }
  }
)

Run this before the above script, then see what happens.

worker.js

// An http server that does things for you!
// Do not write your fib server this way, instead use
// https://gist.github.com/2018811 which this code is based on.
var http = require('http')
var PORT = process.argv[2]

function fib(n) {
  if (n < 2) {
    return 1
  } else {
    return fib(n - 2) + fib(n - 1)
  }
}

var server = http.createServer(function(req, res) {
  res.writeHead(200)
  res.end(fib(40) + "\n")
})
server.listen(PORT)
console.log("worker.js online at http://localhost:" + PORT)

To see a pool that is 100% healthy:

node ./worker.js 8080 &
node ./worker.js 8081 &
node ./worker.js 8082 &

echo "running client.js ..."
node ./client.js

Running tests

npm -g install mocha
mocha
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