What is cluster2

cluster2 is a node.js (>= 0.6.x) compatible multi-process management module. This module grew out of our needs in operationalizing node.js for ql.io at eBay. Built on node's cluster, cluster2 adds several safeguards and utility functions to help support real-world production scenarios:

• Scriptable start, shutdown and stop flows
• Worker monitoring for process deaths
• Worker recycling
• Graceful shutdown
• Idle timeouts
• Validation hooks (for other tools to monitor cluster2 apps)
• Events for logging cluster activities
• Exit with error code when the port is busy to fail start scripts
• Disable monitor
• and more coming soon

Usage

Getting cluster2

npm install cluster2

Start a TCP Server

var Cluster = require('cluster2'),
    net = require('net');
var server = net.createServer(function (c) {
    c.on('end', function () {
        console.log('server disconnected');
    });
    c.write('hello\r\n');
    c.pipe(c);
});

var c = new Cluster({
    port: 3000,
    cluster: true
});

Start a HTTP Server

var Cluster = require('cluster2'),
    http = require('http');
var server = http.createServer(function (req, res) {
    res.writeHead(200);
    res.end('hello');
});
var c = new Cluster({
    port: 3000
});
c.listen(function(cb) {
    cb(server);
});

Start an Express Server

var Cluster = require('cluster2'),
    express = require('express');
var app = express.createServer();
app.get('/', function(req, res) {
    res.send('hello');
});

var c = new Cluster({
    port: 3000,
});
c.listen(function(cb) {
    cb(app);
});

Stop a Server

var Cluster = require('cluster2');
var c = new Cluster();
c.stop();

Gracefully Shutdown a Server

var Cluster = require('cluster2');
var c = new Cluster();
c.shutdown();

Options

Cluster2 takes the following options.

• cluster: When true starts a number of workers. Use false to start the server as a single process. Defaults to true.
• pids: A directory to write PID files for master and workers.
• port: Port number for the app, defaults to 3000.
• monPort: Port number for the monitor URL, defaults to 3001. Go to http://<localhost>:3001 to view application logs (whatever is written to a /logs dir), and npm dependencies.
• ecv: ECV stands for "extended content verification". This is an object with the following additional properties:
  • path: A path to serve a heart beat. See below.
  • monitor: A URI to check before emitting a valid heart beat signal
  • control: When true, allows clients to enable or disable the signal. See below. validator to validate the runtime health of the app. If found unhealthy, emits a disable
• noWorkers: Defaults to os.cpus().length.
• timeout: Idle socket timeout. Automatically ends incoming sockets if found idle for this duration. Defaults to 30 seconds.
• connThreshold: When the number of connections processed exceeds this numbers, recycle the worker process. This can help recover from slow leaks in your code or dependent modules.

Graceful Shutdown

The purpose of shutdown() is to let the server reject taking new connections, handle all pending requests and end the connecton so that no request dropped. In order to handling shutdown(), the server must handle close events as follows.

var serving = true;
var server = http.createServer(function (req, res) {
    if(!serving) {
        // Be nice and send a connection: close as otherwise the client may pump more requests
        // on the same connection
        res.writeHead(200, {
            'connection': 'close'
        });
    }
    res.writeHead(200);
    res.end('hello');
});
server.on('close', function() {
    serving = false;
})
var c = new Cluster({
    port: 3000,
    cluster: true
});

Completion of shutdown() does not necessarily mean that all worker processes are dead immediately. The workers may take a while to complete processing of current requests and exit. The shutdown() flow only guarantees that the server takes no new connections.

Cluster2 Events

Cluster2 is an EventEmitter and emits the following events.

• died: Emitted when a worker dies. This event is also emitted during normal shutdown() or stop().
• forked: Emitted when a new worker is forked.
• <signal>: Emitted when a worker receives a signal (such as SIGKILL, SIGTERM or SIGINT).

Here is an example that logs these events to the disk.

var Cluster = require('cluster2'),
    http = require('http');

var server = http.createServer(function (req, res) {
    res.writeHead(200);
    res.end('hello');
});
var c = new Cluster({
    cluster: true,
    port: 3000
});
c.on('died', function(pid) {
    console.log('Worker ' + pid + ' died');
});
c.on('forked', function(pid) {
    console.log('Worker ' + pid + ' forked');
});
c.on('SIGKILL', function() {
    console.log('Got SIGKILL');
});
c.on('SIGTERM', function(event) {
    console.log('Got SIGTERM - shutting down');
});
c.on('SIGINT', function() {
    console.log('Got SIGINT');
});
c.listen(function(cb) {
    cb(server);
});

Routing Traffic

It is fairly common for proxies or load balancers deployed in front of node clusters, and those proxies to use monitor URLs to detect the health of the cluster. Cluster2 includes a monitor at http://<host>:<port>/ecv. You can change this by setting the path property when initializing the cluster.

In case you want to take the node cluster out of rotation from the proxy/load balancer, you can do so by setting control to true when initializing the cluster. At runtime, you can send a POST request to http://<host>:<port>/ecv/disable. Once this is done, further requests to http://<host>:<port>/ecv will get a network error. You can bring the cluster back to rotation by sending a POST request to http://<host>:<port>/ecv/enable.

Since it will be potentially disastrous to let artibrary clients enable/disable traffic, you should configure your proxy/load balancer to prevent external traffic to /ecv*.

To test this, bring up an example

node examples/express/express-server.js

and send a GET request to http://localhost:3000/ecv and notice the response.

HTTP/1.1 200 OK
X-Powered-By: Cluster2
content-type: text/plain
since: Fri May 18 2012 09:49:32 GMT-0700 (PDT)
cache-control: no-cache
Connection: keep-alive
Transfer-Encoding: chunked

status=AVAILABLE&ServeTraffic=true&ip=127.0.0.1&hostname=somehost&port=3000&time=Fri May 18 2012 09:49:49 GMT-0700 (PDT)

To flip the monitor into a disabled state, send a POST request to http://localhost:3000/disable.

HTTP/1.1 204 No Content
X-Powered-By: Cluster2
since: Fri May 18 2012 09:54:25 GMT-0700 (PDT)
cache-control: no-cache
Connection: close

Subsequent GET requests to http://localhost:3000/ecv will return a response similar to the one below.

HTTP/1.1 400 Bad Request
X-Powered-By: Cluster2
content-type: text/plain
since: Fri May 18 2012 09:54:25 GMT-0700 (PDT)
cache-control: no-cache
Connection: close
Transfer-Encoding: chunked

status=DISABLED&ServeTraffic=false&ip=127.0.0.1&hostname=somehost&port=3000&time=Fri May 18 2012 09:55:17 GMT-0700 (PDT)

To flip the monitor back into an enabled state, send a POST request to http://localhost:3000/enable.