node.js - How Do I Shut Down My Express Server Gracefully When Its Process Is Killed?

Question

When running my Express application in production, I want to shut down the server gracefully when its process is killed (i.e. a SIGTERM or SIGINT is sent).

Here is a simplified version of my code:

const express = require('express');

const app = express();

app.get('/', (req, res) => res.json({ ping: true }));

const server = app.listen(3000, () => console.log('Running…'));

setInterval(() => server.getConnections(
    (err, connections) => console.log(`${connections} connections currently open`)
), 1000);

process.on('SIGTERM', shutDown);
process.on('SIGINT', shutDown);

function shutDown() {
    console.log('Received kill signal, shutting down gracefully');
    server.close(() => {
        console.log('Closed out remaining connections');
        process.exit(0);
    });

    setTimeout(() => {
        console.error('Could not close connections in time, forcefully shutting down');
        process.exit(1);
    }, 10000);
}

When I run it and call the URL http://localhost:3000/ in a browser, the log statement in the setInterval function will keep printing “1 connection currently open” until I actually close the browser window. Even closing the tab will keep the connection open, apparently.

So wenn I kill my server by hitting Ctrl+C, it will run into the timeout and print “Could not close connections” after 10 seconds, all the while continuing to print “1 connection open”.

Only if I close the browser window before killing the process I get the “closed out remaining connections” message.

What am I missing here? What is the proper way to shut down an Express server gracefully?

Answer 1

In case anyone is interested, I found a solution myself (would love to hear feedback in comments).

I added a listener for connections opening on the server, storing references to those connections in an array. When the connections are closed, they are removed from the array.

When the server is killed, each of the connection is closed by calling its end methods. For some browsers (e.g. Chrome), this is not enough, so after a timeout, I call destroy on each connection.

const express = require('express');

const app = express();

app.get('/', (req, res) => res.json({ ping: true }));

const server = app.listen(3000, () => console.log('Running…'));

setInterval(() => server.getConnections(
    (err, connections) => console.log(`${connections} connections currently open`)
), 1000);

process.on('SIGTERM', shutDown);
process.on('SIGINT', shutDown);

let connections = [];

server.on('connection', connection => {
    connections.push(connection);
    connection.on('close', () => connections = connections.filter(curr => curr !== connection));
});

function shutDown() {
    console.log('Received kill signal, shutting down gracefully');
    server.close(() => {
        console.log('Closed out remaining connections');
        process.exit(0);
    });

    setTimeout(() => {
        console.error('Could not close connections in time, forcefully shutting down');
        process.exit(1);
    }, 10000);

    connections.forEach(curr => curr.end());
    setTimeout(() => connections.forEach(curr => curr.destroy()), 5000);
}