⚡ MongoDB Replication

# Terminal 1 - Start first server
mongod --replSet myReplica --port 27017 --dbpath /data/db1

# Terminal 2 - Start second server  
mongod --replSet myReplica --port 27018 --dbpath /data/db2

# Terminal 3 - Start third server
mongod --replSet myReplica --port 27019 --dbpath /data/db3

# Note: All use same replica set name "myReplica"

# Connect to first server
mongosh --port 27017

# Initialize replica set (run this ONCE)
rs.initiate({
  _id: "myReplica",
  members: [
    { _id: 0, host: "localhost:27017" },
    { _id: 1, host: "localhost:27018" },
    { _id: 2, host: "localhost:27019" }
  ]
})

# Wait ~10 seconds, then check:
rs.status()

# You'll see:
# • One PRIMARY
# • Two SECONDARYs
# 🎉 Replica set is ready!

const { MongoClient } = require('mongodb');

// Connection string lists ALL servers
const uri = "mongodb://localhost:27017,localhost:27018,localhost:27019/mydb?replicaSet=myReplica";

const client = new MongoClient(uri);
await client.connect();

// Write (automatically goes to PRIMARY)
await db.users.insertOne({ name: "Alice" });

// Read (goes to PRIMARY by default)
const users = await db.users.find().toArray();

// That's it! Driver handles everything:
// ✅ Finds PRIMARY automatically
// ✅ Routes writes to PRIMARY
// ✅ Handles failover automatically
// ✅ Reconnects if PRIMARY changes

Minimum: 3 servers

Why 3? You need a majority to elect a new PRIMARY:
• With 3 servers: Need 2 votes (majority) ✅
• With 2 servers: Need 2 votes, but if 1 dies, only 1 vote left (no majority) ❌

Recommended: 3 for small apps, 5+ for critical production apps

NO data loss if you use write concern "majority"

This tells MongoDB: "Don't say 'success' until most servers have the data"

// Safe write (waits for majority)
await db.users.insertOne(
  { name: "Alice" },
  { writeConcern: { w: "majority" } }
);

// MongoDB waits until 2 out of 3 servers have the data
// Even if PRIMARY crashes right after, data is safe! ✅