Graceful shutdown on errors in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a graceful shutdown in Node.js?

A graceful shutdown means stopping the Node.js app carefully by finishing ongoing tasks and closing resources before exiting, so no data is lost or corrupted.

beginner

Why should you handle errors during shutdown in Node.js?

Handling errors during shutdown helps avoid crashes and ensures the app cleans up resources like database connections or servers properly before stopping.

intermediate

Which Node.js events are commonly used to trigger graceful shutdown?

The 'SIGINT' and 'SIGTERM' signals are used to start graceful shutdown, and 'uncaughtException' or 'unhandledRejection' events handle unexpected errors.

intermediate

What is the role of process.exit() in graceful shutdown?

process.exit() stops the Node.js process. In graceful shutdown, it is called only after all cleanup tasks finish to avoid abrupt termination.

intermediate

How can you ensure database connections close during a graceful shutdown?

You call the database client's close or disconnect method inside the shutdown handler to close connections before the app exits.

Which event should you listen to for catching unexpected errors in Node.js?

A 'close'

B 'exit'

C 'SIGINT'

D 'uncaughtException'

The 'uncaughtException' event catches errors not handled anywhere else in the code.

What does a graceful shutdown NOT do?

A Finish ongoing requests

B Close open resources

C Immediately kill the process

D Log shutdown steps

Graceful shutdown avoids immediately killing the process to prevent data loss.

Which signal is commonly sent to a Node.js app to request shutdown?

A SIGINT

B SIGKILL

C SIGSTOP

D SIGCONT

SIGINT is sent when you press Ctrl+C to stop the app.

What should you do before calling process.exit() during shutdown?

A Ignore errors

B Close all open connections

C Restart the server

D Do nothing

Closing connections ensures no data is lost before exiting.

How can you handle promise rejections during shutdown?

A Listen to 'unhandledRejection' event

B Use try-catch only

C Ignore them

D Restart the app immediately

'unhandledRejection' catches promises that fail without a catch.

Explain how to implement a graceful shutdown in a Node.js app when an error occurs.

Think about stopping the app carefully without losing data.

Why is it important to handle errors during the shutdown process in Node.js?

Consider what happens if the app stops suddenly.

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