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cluster.isMaster

Learn master/worker distinction with cluster.isMaster.

cluster.isMaster

cluster.isMaster returns true in the master process and false in a worker. This flag lets the same code tell whether it runs as master or worker.

A typical pattern: if (cluster.isMaster) { cluster.fork(); }. The master forks and returns; the worker starts the server. This prevents an infinite fork loop.

cluster.isWorker

cluster.isWorker is the inverse of cluster.isMaster: it returns true in a worker and false in the master. Sometimes isWorker reads more naturally.

For example: if (cluster.isWorker) { startServer(); }. Together the two flags make the execution context explicit.

Practice Log the isMaster value

Import the cluster module. Log cluster.isMaster with console.log.

Starter code

index.js
// Import the cluster module

// log the isMaster value

Sample solution

index.js
const cluster = require("cluster");
console.log(cluster.isMaster);

Expected output

true

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What does cluster.isMaster return in the master?

  • false
  • undefined
  • null
  • true Correct answer

Explanation: isMaster returns true in the master process.

2 Match each piece to its role

Term Meaning
cluster.isMaster Main process?
cluster.isWorker Worker?
true Main process
false Worker process

3 Complete the flag name

Question 
if (cluster.____) { cluster.fork(); }
Answer 
if (cluster.isMaster) { cluster.fork(); }

Explanation: cluster.isMaster returns true in the master.

All lessons in Advanced

1 Clustering

  1. cluster module
  2. cluster.fork
  3. Worker IPC
  4. cluster.isMaster
  5. Graceful reload
  6. Summary - Clustering

2 Worker threads

  1. worker_threads
  2. Worker
  3. postMessage
  4. MessageChannel
  5. SharedArrayBuffer
  6. Summary - Worker threads

3 Performance

  1. perf_hooks
  2. Event loop latency
  3. Memory usage
  4. CPU profiling
  5. Optimization
  6. Summary - Performance

4 Security

  1. Helmet
  2. CORS
  3. Rate limiting
  4. Input validation
  5. JWT
  6. Summary - Security

5 MongoDB

  1. Mongoose
  2. Schema
  3. Model
  4. Query
  5. Aggregate
  6. Summary - MongoDB

6 SQL database

  1. pg module
  2. Pool
  3. Parameterized query
  4. Transactions
  5. Sequelize
  6. Summary - SQL database

7 WebSocket

  1. ws module
  2. WebSocket server
  3. Connection
  4. Message
  5. Broadcast
  6. Summary - WebSocket

8 Realtime

  1. Socket.io
  2. io.on
  3. emit and on
  4. Rooms
  5. Namespaces
  6. Summary - Realtime

9 Microservice basics

  1. What are microservices?
  2. Service communication
  3. API gateway
  4. Service discovery
  5. Circuit breaker
  6. Summary - Microservices

10 Message queue

  1. What are message queues?
  2. Bull
  3. Producer
  4. Consumer
  5. Retry and backoff
  6. Summary - Message queue

11 Logging

  1. Winston
  2. Log levels
  3. Transports
  4. Structured logging
  5. Pino
  6. Summary - Logging

12 Configuration

  1. dotenv
  2. Environment variables
  3. Config hierarchy
  4. Validation
  5. Secrets management
  6. Summary - Configuration

13 Advanced testing

  1. Integration tests
  2. Supertest
  3. Test database
  4. Coverage
  5. E2E tests
  6. Summary - Advanced testing

14 Advanced final

  1. Project design
  2. Express + DB
  3. WebSocket
  4. Security
  5. Putting it together