Graceful shutdown on errors in Node.js

Introduction

Graceful shutdown helps your Node.js app close safely when errors happen. It stops new work, finishes current tasks, and cleans up before exiting.

When your server encounters an unexpected error and needs to stop without losing data.

When you want to close database connections properly before the app exits.

When you want to notify users or log important info before shutting down.

When you want to avoid crashing your app abruptly and causing bad user experience.

Syntax

Node.js

process.on('uncaughtException', (err) => {
  // handle error
  // clean up resources
  process.exit(1);
});

process.on('SIGTERM', () => {
  // clean up and exit
  process.exit(0);
});

process.on listens for system signals or errors.

Always clean up resources like database connections before exiting.

Examples

Catches unexpected errors and logs them before exiting with error code 1.

Node.js

process.on('uncaughtException', (err) => {
  console.error('Error caught:', err);
  // close server or DB here
  process.exit(1);
});

Handles Ctrl+C (SIGINT) to shut down gracefully.

Node.js

process.on('SIGINT', () => {
  console.log('SIGINT received, shutting down...');
  // clean up
  process.exit(0);
});

Example of async cleanup before exiting on SIGTERM signal.

Node.js

async function shutdown() {
  await db.close();
  console.log('Database closed');
  process.exit(0);
}

process.on('SIGTERM', shutdown);

Sample Program

This Node.js server listens on port 3000. If you visit /error, it throws an error. The uncaughtException handler catches it, logs the message, closes the server, then exits with code 1. Pressing Ctrl+C triggers SIGINT to close the server gracefully and exit with code 0.

Node.js

import http from 'node:http';

const server = http.createServer((req, res) => {
  if (req.url === '/error') {
    throw new Error('Simulated error');
  }
  res.end('Hello World');
});

server.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

process.on('uncaughtException', (err) => {
  console.error('Uncaught Exception:', err.message);
  server.close(() => {
    console.log('Server closed');
    process.exit(1);
  });
});

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

Important Notes

Never ignore errors; always handle them to avoid crashes.

Use server.close() to stop accepting new connections before exit.

Exiting with code 0 means success; non-zero means error.

Summary

Graceful shutdown stops new work and cleans up before exiting.

Use process.on to catch errors and signals.

Always close servers and resources before exiting.

Practice

1. What is the main purpose of implementing a graceful shutdown in a Node.js application?

easy

A. To restart the server automatically after an error

B. To log all incoming requests for debugging

C. To increase the server's performance under load

D. To stop accepting new requests and clean up resources before exiting

Solution

1. Step 1: Understand graceful shutdown concept

   Graceful shutdown means stopping new work and cleaning up before the app exits.

2. Step 2: Identify the main goal

   The goal is to avoid abrupt termination by closing servers and resources properly.

3. Final Answer:

   To stop accepting new requests and clean up resources before exiting -> Option D

4. Quick Check:

   Graceful shutdown = stop new work + cleanup [OK]

Hint: Remember: graceful shutdown means clean exit, not restart [OK]

Common Mistakes:

• Confusing graceful shutdown with automatic restart
• Thinking it improves performance directly
• Assuming it only logs requests

2. Which of the following is the correct way to listen for an uncaught exception to trigger graceful shutdown in Node.js?

easy

A. process.on('uncaughtException', handler)

B. process.addListener('error', handler)

C. process.listen('uncaughtException', handler)

D. process.catch('uncaughtException', handler)

Solution

1. Step 1: Recall Node.js event listening syntax

   Node.js uses process.on(event, handler) to listen for events.

2. Step 2: Identify the correct event for uncaught exceptions

   The event name is 'uncaughtException', so process.on('uncaughtException', handler) is correct.

3. Final Answer:

   process.on('uncaughtException', handler) -> Option A

4. Quick Check:

   Use process.on for events like uncaughtException [OK]

Hint: Use process.on(event, handler) for error events [OK]

Common Mistakes:

• Using process.catch instead of process.on
• Using wrong event names like 'error' for uncaughtException
• Confusing listen with on

3. Consider this Node.js code snippet handling graceful shutdown:

const server = require('http').createServer();
process.on('SIGINT', () => {
  console.log('Shutdown signal received');
  server.close(() => {
    console.log('Server closed');
    process.exit(0);
  });
});
server.listen(3000, () => console.log('Server running'));

What will be the output if you press Ctrl+C in the terminal?

medium

A. Shutdown signal received\nServer running\nServer closed

B. Server running\nShutdown signal received\nServer closed

C. Server running\nServer closed\nShutdown signal received

D. Server closed\nShutdown signal received\nServer running

Solution

1. Step 1: Understand server start and signal handling order

   The server logs 'Server running' when it starts listening. On Ctrl+C, SIGINT triggers the handler.

2. Step 2: Trace the shutdown logs

   On SIGINT, it logs 'Shutdown signal received', then closes the server, then logs 'Server closed'.

3. Final Answer:

   Server running\nShutdown signal received\nServer closed -> Option B

4. Quick Check:

   Startup log first, then SIGINT logs in order [OK]

Hint: Remember: server.listen logs first, then signal handler logs [OK]

Common Mistakes:

• Mixing order of logs on startup and shutdown
• Assuming server closes before signal logs
• Ignoring asynchronous server.close callback

4. Given this code snippet for graceful shutdown, what is the main problem?

const server = require('http').createServer();
process.on('uncaughtException', (err) => {
  console.error('Error:', err);
  process.exit(1);
});
server.listen(3000);

medium

A. It listens on the wrong port

B. It does not log the error properly

C. It exits immediately without closing the server

D. It uses the wrong event name for errors

Solution

1. Step 1: Analyze error handling behavior

   The handler logs the error but calls process.exit(1) immediately.

2. Step 2: Check for graceful shutdown steps

   It does not close the server before exiting, which can cause abrupt termination.

3. Final Answer:

   It exits immediately without closing the server -> Option C

4. Quick Check:

   Graceful shutdown requires closing server before exit [OK]

Hint: Always close servers before calling process.exit [OK]

Common Mistakes:

• Exiting without cleanup
• Ignoring server.close in error handlers
• Assuming logging is enough

5. You want to implement a graceful shutdown that handles both SIGTERM and uncaughtException events, ensuring the server closes before exit. Which code snippet correctly achieves this?

hard

A. process.on('SIGTERM', () => { server.close(() => process.exit(0)); }); process.on('uncaughtException', (err) => { console.error(err); server.close(() => process.exit(1)); });

B. process.on('SIGTERM', () => { server.close(); }); process.on('uncaughtException', () => { process.exit(1); });

C. process.on('SIGTERM', () => { process.exit(0); }); process.on('uncaughtException', () => { process.exit(1); });

D. process.on('SIGTERM', () => { console.log('Terminated'); }); process.on('uncaughtException', (err) => { console.error(err); });

Solution

1. Step 1: Check handling of SIGTERM

   process.on('SIGTERM', () => { server.close(() => process.exit(0)); }); process.on('uncaughtException', (err) => { console.error(err); server.close(() => process.exit(1)); }); closes the server and then exits, which is correct for graceful shutdown.

2. Step 2: Check handling of uncaughtException

   process.on('SIGTERM', () => { server.close(() => process.exit(0)); }); process.on('uncaughtException', (err) => { console.error(err); server.close(() => process.exit(1)); }); logs the error, closes the server, then exits with error code, ensuring cleanup.

3. Final Answer:

   process.on('SIGTERM', () => { server.close(() => process.exit(0)); }); process.on('uncaughtException', (err) => { console.error(err); server.close(() => process.exit(1)); }); -> Option A

4. Quick Check:

   Close server before exit on signals and errors [OK]

Hint: Always close server before exit on signals and errors [OK]

Common Mistakes:

• Exiting immediately without closing server
• Not logging errors on uncaughtException
• Ignoring asynchronous server.close callback