All 15 Clustering & Worker Threads questions
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Node.js › Clustering & Worker Threads

What is the difference in behavior between the 'primary' and 'worker' role in Node's cluster module regarding which code paths execute?

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Answer

A script using the cluster module is re-executed by every forked worker as well as the primary -- code branches on cluster.isPrimary (true only in the original process), typically containing only forking/management logic, while cluster.isWorker guards the actual application logic, ensuring the primary never also runs a full server copy.
if (cluster.isPrimary) {
  for (let i = 0; i < 4; i++) cluster.fork();
  cluster.on('exit', () => cluster.fork());
} else {
  require('./app').listen(3000);
}
Real-world example A misconfigured cluster setup that started the server inside both branches got an 'address already in use' error, resolved by keeping server-starting code only in the isWorker branch.

Common follow-ups: Why does having the primary only manage workers (rather than also serving requests) improve reliability?;How many workers does the primary count as when calculating total request capacity?

Advanced Node.js;Error Handling
All 15 Clustering & Worker Threads questions