How to fix MongoServerSelectionError: Server selection timed out after 30000 ms in MongoDB

# For local MongoDB
mongo --version
mongosh localhost:27017

# For Docker MongoDB
docker ps | grep mongo

# For MongoDB Atlas, check the cluster status in the web interface

# Local on Linux/Mac
mongod

# Local on Windows
"C:\Program Files\MongoDB\Server\bin\mongod.exe"

# Docker
docker run -d -p 27017:27017 mongo

// MongoDB Atlas format (recommended)
const uri = "mongodb+srv://username:[email protected]/database?retryWrites=true&w=majority";

// Local MongoDB format
const uri = "mongodb://localhost:27017/mydb";

// Docker MongoDB
const uri = "mongodb://mongo:27017/mydb";

# Test DNS resolution
nslookup mongodb.example.com
# or
dig mongodb.example.com

# Test port connectivity
telnet mongodb.example.com 27017
# or (Linux/Mac)
nc -zv mongodb.example.com 27017

# For MongoDB Atlas, verify whitelist IP
# Check MongoDB Atlas dashboard -> Network Access -> IP Whitelist
# Your application's IP must be whitelisted (0.0.0.0/0 allows all IPs, not recommended for production)

// Example with credentials
const uri = "mongodb+srv://myuser:[email protected]/mydatabase?retryWrites=true&w=majority";

// Mongoose
mongoose.connect(uri, {
  serverSelectionTimeoutMS: 5000, // 5 seconds instead of 30
});

// MongoDB Native Driver
const client = new MongoClient(uri, {
  serverSelectionTimeoutMS: 5000,
});

// Prisma
// Adjust the connection URL
// mongodb://localhost:27017?serverSelectionTimeoutMS=5000

# Connect to MongoDB and check replica set status
mongosh
> rs.status()

# Verify all nodes are healthy (StateStr: PRIMARY, SECONDARY, ARBITER)
# If topology shows "ReplicaSetNoPrimary", the replica set is in an election

// Must include replicaSet parameter
const uri = "mongodb://host1:27017,host2:27017,host3:27017/?replicaSet=rs0";

// Or for MongoDB Atlas (handles this automatically)
const uri = "mongodb+srv://user:[email protected]/db?retryWrites=true&w=majority";

// Mongoose
mongoose.set("debug", true);

// MongoDB Native Driver
const { MongoClient, Logger } = require("mongodb");
const logger = new Logger();
logger.setLevel("debug");

// Prisma
// Set environment variable
process.env.DEBUG = "prisma:*";