Using cluster in an Expressjs app

I’m doing a little OJT on my first node project and, while I can stand up a simple server, the app is going to get hammered so using cluster seems like a good idea. I’ve cobbled together some code snippets that I’ve found in various searches (including SO), but the server won’t start. I’m sure my inexperience with node has me doing something stupid, but I don’t see it.

var express = require( 'express' );
var cluster = require( 'cluster' );
var path    = require( 'path' );

var cCPUs   = require( 'os' ).cpus().length;
var port    = 3000;
var root    = path.dirname( __dirname );

if( cluster.isMaster ) {
    for( var i = 0; i < cCPUs; i++ ) {
      cluster.fork();
    }

    cluster.on( 'death', function( worker ) {
      console.log( 'Worker ' + worker.pid + ' died.' );
    });
}
else {
    // eyes.inspect( process.env );
    console.log( 'Worker: %s', process.env.NODE_WORKER_ID );

    var app = express();
    var routes  = require( './routes' )( app );
    app
      .use( cluster.repl( root + 'cluster.repl' ) )
      .use( cluster.stats({ connections: true, requests: true }) )
      .use( cluster.reload( root ) )
      .listen( port );
}

RESULT:

TypeError: Object #<Cluster> has no method 'repl'

If I remove the use calls, the workers start up correctly, but process.env.NODE_WORKER_ID is undefined. Inspecting process.env shows me that it’s definitely not defined. Maybe the snippet I used was from an old version, but I’m not sure how to identify the worker thread in any other way.

If anyone can unscrambled whatever I’ve scrambled, I’d really appreciate it.