Handling worker crashes and restart in Node.js - Step-by-Step Execution

Concept Flow - Handling worker crashes and restart

Start Master Process

↓

Fork Worker Process

↓

Worker Runs

↓

Worker Crashes?

No→Continue Running

Yes↓

Master Detects Crash

↓

Master Forks New Worker

↓

Worker Runs

The master process starts and forks a worker. If the worker crashes, the master detects it and restarts a new worker, keeping the system running.

Execution Sample

Node.js

import cluster from 'cluster';
import os from 'os';

if (cluster.isPrimary) {
  cluster.fork();
  cluster.on('exit', (worker) => {
    console.log(`Worker ${worker.process.pid} died, restarting...`);
    cluster.fork();
  });
} else {
  // Worker code
  setTimeout(() => { process.exit(1); }, 1000); // Simulate crash
}

This code forks a worker that crashes after 1 second; the master detects the crash and restarts the worker.

Execution Table

Step	Action	Worker PID	Master Detects Crash	Master Action	Output
1	Master starts and forks worker	12345	No	None	Worker 12345 started
2	Worker runs normally	12345	No	None	Worker 12345 running
3	Worker crashes (exit code 1)	12345	Yes	Fork new worker	Worker 12345 died, restarting...
4	Master forks new worker	12346	No	None	Worker 12346 started
5	New worker runs	12346	No	None	Worker 12346 running
6	No more crashes	-	No	None	System stable
Exit	No crash detected	-	No	None	Execution stops

💡 No crash detected, system runs stable

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	Final
worker PID	none	12345	12345 (crashed)	12346	12346
master detects crash	false	false	true	false	false

Key Moments - 2 Insights

Why does the master fork a new worker after a crash?

Because the execution_table row 3 shows the master detects the worker crash and immediately forks a new worker to keep the system running.

Does the master fork multiple workers at start?

No, the master forks only one worker at start as shown in execution_table row 1; additional forks happen only after crashes.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what does the master do when the worker crashes?

A Forks a new worker

B Does nothing

C Shuts down the system

D Logs but does not restart

💡 Hint

See execution_table row 3 under 'Master Action' and 'Output'

At which step does the new worker start running after the crash?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Check execution_table rows 4 and 5 for worker start and running

If the worker never crashes, what happens to the master’s restart logic?

A It runs repeatedly

B It forks multiple workers at start

C It never runs

D It shuts down the master

💡 Hint

Look at variable_tracker 'master detects crash' staying false and execution_table exit note

Concept Snapshot

Node.js cluster master forks worker(s).
If a worker crashes, master detects 'exit' event.
Master forks a new worker to replace crashed one.
This keeps the app running continuously.
Use cluster.on('exit', ...) to handle crashes.
Restart logic runs only on worker crash.

Full Transcript

In Node.js cluster, the master process starts and forks a worker process. The worker runs its code. If the worker crashes, the master detects this via the 'exit' event. Upon detection, the master immediately forks a new worker to replace the crashed one. This cycle ensures the application keeps running even if workers fail. The example code forks one worker that crashes after one second. The master logs the crash and restarts the worker. Variables like worker PID and crash detection status change as the program runs. The master only forks new workers after crashes, not multiple at start. This pattern is essential for building resilient Node.js applications using cluster.