How do you check if the current process is a master or worker in Node.js?

Difficulty: 4/10

cluster module, process roles, scaling

You can use cluster.isPrimary (or the deprecated cluster.isMaster) to check if the current process is the master, and cluster.isWorker to check if it is a worker process.

The cluster module exposes boolean properties that let you branch your code logic based on whether the current process is the primary/master or a worker. This is the foundational pattern used in every cluster-based Node.js application.

Checking Process Type

Important Notes

1
cluster.isPrimary is the modern property introduced in Node.js v16.0.0
2
cluster.isMaster is deprecated but still functional in older codebases
3
cluster.isWorker is the complement of cluster.isPrimary
4
process.pid gives the OS-level process ID for both master and workers

Scenario Questions

0-2 years experience

1We need a simple log that prints 'master' or 'worker' when the script starts. How would you write that check?
2If you accidentally placed code that should only run in the master inside a worker, what would you see at runtime?

2-5 years experience

1Your service crashes intermittently only when run with cluster. How would you determine whether the crash originates in the master or a worker?

Follow-up Questions

What would happen if you called cluster.fork() inside a worker process?
How does the value of cluster.isMaster change after a fork?
Can you rely on environment variables like NODE_UNIQUE_ID to distinguish master from worker?