Graceful shutdown handling in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when the server receives SIGINT in this Express app?

Consider this Express server code that listens for SIGINT to shut down gracefully. What will be the console output if you press Ctrl+C to stop the server?

Express

const express = require('express');
const app = express();
const server = app.listen(3000, () => console.log('Server started'));

process.on('SIGINT', () => {
  console.log('SIGINT received');
  server.close(() => {
    console.log('Server closed');
    process.exit(0);
  });
});

A SIGINT received\nServer started\nServer closed

B Server started\nServer closed\nSIGINT received

C Server started\nSIGINT received

D Server started\nSIGINT received\nServer closed

💡 Hint

Think about the order of events when SIGINT is caught and the server closes.

When Ctrl+C is pressed, SIGINT is caught first, logging 'SIGINT received'. Then server.close() stops the server and calls its callback, logging 'Server closed'. The process then exits.

📝 Syntax

intermediate

Which option correctly attaches a listener for graceful shutdown in Express?

You want to listen for the SIGTERM signal to gracefully close your Express server. Which code snippet is syntactically correct and will work as intended?

A process.on('SIGTERM', () => server.close(() => process.exit(0)));

B process.addListener('SIGTERM', server.close(() => process.exit(0)));

C process.on('SIGTERM', server.close(() => process.exit(0)));

D process.on('SIGTERM', () => { server.close; process.exit(0); });

💡 Hint

Remember that the listener callback should be a function, not the result of calling a function.

Option A correctly passes a function to process.on that calls server.close with a callback. Option A calls server.close immediately, not on signal. Option A passes the result of server.close instead of a function. Option A does not call server.close properly.

🔧 Debug

advanced

Why does this Express server not exit on SIGINT?

This Express server code listens for SIGINT but never exits. What is the cause?

Express

const express = require('express');
const app = express();
const server = app.listen(3000);

process.on('SIGINT', () => {
  console.log('SIGINT caught');
  server.close();
  console.log('Server closed');
});

A console.log blocks the event loop preventing exit

B SIGINT event is not emitted on Ctrl+C

C server.close() is asynchronous; process.exit() is missing so process stays alive

D app.listen() must be called inside SIGINT handler

💡 Hint

Think about what happens after server.close() and how the process knows to exit.

server.close() stops accepting new connections but is asynchronous. Without calling process.exit() or waiting for the close callback, the process stays alive.

❓ state_output

advanced

What is the value of 'connections' after this code runs?

This Express server tracks open connections to close them on shutdown. What is the value of the 'connections' Set after the code below runs and a client connects then disconnects?

Express

const express = require('express');
const app = express();
const server = app.listen(3000);
const connections = new Set();

server.on('connection', (conn) => {
  connections.add(conn);
  conn.on('close', () => connections.delete(conn));
});

// Simulate client connect and disconnect
const net = require('net');
const client = net.createConnection({ port: 3000 }, () => {
  client.end();
});

A undefined

B Set(0) {} (empty set)

C Set(1) { <Socket> } (one connection still present)

D null

💡 Hint

Think about when the 'close' event fires and what happens to the Set.

The connection is added on 'connection' event, then removed on 'close' event when the client disconnects. After disconnect, the Set is empty.

🧠 Conceptual

expert

Why is it important to track open connections during graceful shutdown in Express?

In Express, when shutting down gracefully, why should you track and close open connections before exiting the process?

A To prevent clients from hanging by ensuring all requests finish before exit

B To speed up server startup on next launch

C To automatically restart the server without manual intervention

D To free up memory used by the Express app immediately

💡 Hint

Think about user experience and network connections during shutdown.

Tracking open connections allows the server to finish handling requests and close connections cleanly, preventing clients from experiencing abrupt disconnections or hanging requests.