How to shut down express js gracefully while supporting keep-alive

expressjs-shutdown.js

// this is an excerpt of essential but non-working part extracted from local expressjs application ...

var app = require( "express" )();

var httpServer = require( "http" ).createServer( app );

httpServer.on( "connection", function( socket ) {
	socket.unref();
} );

httpServer.listen( CONFIG.port, CONFIG.ip );

app.use( function _gracefulShutdownSupporter( req, res, next ) {
	// mark connection to be actively serving request now
	req.connection.ref();

	// wrap genuine end() on response to clear mark afterwards
	res._genuineEnd = res.end;
	res.end = function() {
		var args = [], i, l, a, injected = false,
			socket = req.connection;

		for ( i = 0, l = arguments.length, args = []; i < l; i++ ) {
			a = arguments[i];

			if ( typeof a === "function" && !injected ) {
				injected = true;
				args[i] = (function( genuine ) {
					return function() {
						socket.unref();
						return genuine.apply( this, arguments );
					};
				})( a );
			} else {
				args[i] = a;
			}
		}

		if ( !injected ) {
			args.push( function() {
				socket.unref();
			} );
		}

		return res._genuineEnd.apply( this, args );
	};

	next();
} );

Shut down is performed like this:

var shuttingDown = false;

function _shutdownService() {
    if ( !shuttingDown ) {
        shuttingDown = true;

        var shutdowns = 0;

        if ( httpServer ) {
            console.log( "closing http listener" );
            shutdown( httpServer );
        }

        // [..]
    }

    function shutdown( service ) {
        shutdowns++;

        service.on( "close", _onServersClosed );
        service.close();
    }

    function _onServersClosed() {
        if ( --shutdowns === 0 ) {
            process.exit();
        }
    }
}

process.on( "SIGINT", _shutdownService );
process.on( "SIGTERM", _shutdownService );

if ( process.platform === "win32" ) {
    var rl = require( "readline" ).createInterface( {
        input: process.stdin,
        output: process.stdout
    } );

    rl.on( "SIGINT", function() {
        rl.close();
        process.emit( "SIGINT" );
    } );
}

Code has been tested on Win32 platform.

Shutdown works instantly unless having processed any request. It takes a time if shutdown was initiated shortly after having processed requests.

Tested behaviour using this script ...

var http = require( "http" );
var app = require( "express" )();

app.get( "/", function( req, res ) {
    req.connection.ref();

    res.end( "Ho!", function() {
        req.connection.unref();
    } );
} );

var server = http.createServer( app );
server.listen( 8080 );

var server2 = http.createServer( app );
server2.listen( 8081 );

function _shutdownService() {
    var c = 2;

    console.log( "service got SIGINT/SIGTERM, shutting down" );

    process.on( "beforeExit", function() { console.log( "exiting" ); } );

    server.on( "close", function _onServersClosed() {
        console.log( "listener 1 closed" );

        if ( !--c ) {
            process.exit();
        }
    } );

    server2.on( "close", function _onServersClosed() {
        console.log( "listener 2 closed" );

        if ( !--c ) {
            process.exit();
        }
    } );

    server.close();
    server2.close();
    console.log( "shut-down initiated" );
}

process.on( "SIGINT", _shutdownService );

if ( process.platform === "win32" ) {
    var rl = require( "readline" ).createInterface( {
        input: process.stdin,
        output: process.stdout
    } );

    rl.on( "SIGINT", function() {
        rl.close();
        process.emit( "SIGINT" );
    } );
}

Test includes fetching from port 8080 several times. Instant output on pressing Ctrl+C in console was:

service got SIGINT/SIGTERM, shutting down
shut-down initiated
listener 2 closed
exiting

Basically, it looks like success, but listener 1 - the used one - didn't get close event before exiting application.

Obviously, server is behaving differently (e.g. by not emitting "close" event) in case of using ref()/unref() on sockets.

Relying on a separate collection of used sockets and tracking their activity is working though:

var http = require( "http" );
var app = require( "express" )();

var sockets = {}, nextId = 1;

app.get( "/", function( req, res ) {
    var socket = req.connection,
        id = socket._socketsQueueId;

    if ( !id ) {
        id = socket._socketsQueueId = nextId++;
        sockets[id] = socket;
        socket.on( "close", function() {
            sockets[id] = undefined;
        } );
    }

    socket._currentlyActive = true;

    res.end( "Ho!", function() {
        socket._currentlyActive = false;
    } );
} );

var server = http.createServer( app );
server.listen( 8080 );

var server2 = http.createServer( app );
server2.listen( 8081 );

function _shutdownService() {
    var c = 2;

    console.log( "service got SIGINT/SIGTERM, shutting down" );

    process.on( "beforeExit", function() { console.log( "exiting" ); } );

    server.on( "close", function _onServersClosed() {
        console.log( "listener 1 closed, %d left", --c );

        if ( c === 0 ) {
            console.log( "trigger exit" );
            process.exit();
        }
    } );

    server2.on( "close", function _onServersClosed() {
        console.log( "listener 2 closed, %d left", --c );

        if ( c === 0 ) {
            console.log( "trigger exit" );
            process.exit();
        }
    } );

    Object.keys( sockets )
        .forEach( function( id ) {
            var socket = sockets[id];

            if ( socket && !socket._currentlyActive ) {
                socket.setTimeout( 100, function() {
                    socket.end();
                    socket.destroy();
                } );
            }
        } );

    server.close();
    server2.close();

    console.log( "shut-down initiated" );
}

process.on( "SIGINT", _shutdownService );

if ( process.platform === "win32" ) {
    var rl = require( "readline" ).createInterface( {
        input: process.stdin,
        output: process.stdout
    } );

    rl.on( "SIGINT", function() {
        rl.close();
        process.emit( "SIGINT" );
    } );
}

Though requesting on either port 8080 and 8081, the service stops instantly on pressing Ctrl+C in Windows terminal writing:

service got SIGINT/SIGTERM, shutting down
shut-down initiated
listener 2 closed, 1 left
listener 1 closed, 0 left
trigger exit

Will try to inject this solution into my initial application now.

As a final note, gracefully shutting down expressjs application (involving database connection) was working finally using the latest approach given above.