MongoDB Replication & Sharding

Master distributed MongoDB architectures.

Quick Start

Replica Set Setup

# Start mongod instances with replica set config
mongod --replSet rs0 --port 27017
mongod --replSet rs0 --port 27018
mongod --replSet rs0 --port 27019

# Initiate replica set
mongo mongodb://localhost:27017
> rs.initiate({
    _id: 'rs0',
    members: [
      { _id: 0, host: 'localhost:27017', priority: 1 },
      { _id: 1, host: 'localhost:27018', priority: 0.5 },
      { _id: 2, host: 'localhost:27019', priority: 0 }
    ]
  })

# Check replica set status
> rs.status()

Replication Concepts

Primary: Accepts reads and writes
Secondary: Replicates from primary, serves reads only
Arbiter: Participates in elections, no data

Write Concerns

// Unacknowledged
await collection.insertOne(doc, { writeConcern: { w: 0 } })

// Acknowledged (single node)
await collection.insertOne(doc, { writeConcern: { w: 1 } })

// Majority
await collection.insertOne(doc, { writeConcern: { w: 'majority' } })

// Majority with timeout
await collection.insertOne(doc, {
  writeConcern: { w: 'majority', wtimeout: 5000 }
})

Read Preferences

// Read from primary only (default)
find().setReadPreference('primary')

// Read from primary, failover to secondary
find().setReadPreference('primaryPreferred')

// Read from secondary if available
find().setReadPreference('secondary')

// Read from secondary, failover to primary
find().setReadPreference('secondaryPreferred')

// Read from nearest node
find().setReadPreference('nearest')

Sharding

Enable Sharding

# Start config servers
mongod --configsvr --dbpath /data/config0 --port 27019
mongod --configsvr --dbpath /data/config1 --port 27020
mongod --configsvr --dbpath /data/config2 --port 27021

# Start mongos router
mongos --configdb localhost:27019,localhost:27020,localhost:27021

# Start shard servers
mongod --shardsvr --dbpath /data/shard0 --port 27017
mongod --shardsvr --dbpath /data/shard1 --port 27018

Shard Key Design

// Enable sharding on database
sh.enableSharding('myapp')

// Shard collection with key
sh.shardCollection('myapp.users', { email: 1 })

// Hash-based sharding (better distribution)
sh.shardCollection('myapp.logs', { userId: 'hashed' })

// Compound shard key
sh.shardCollection('myapp.events', { tenantId: 1, timestamp: 1 })

Check Sharding Status

// Get sharding status
sh.status()

// Get shard information
db.adminCommand({ listShards: 1 })

// Get chunk distribution
sh.balancerStatus()

Failover & High Availability

Replica Set Elections

// Trigger election (step down)
rs.stepDown()

// View election status
rs.status()

// Check oplog
db.oplog.rs.find().sort({ ts: -1 }).limit(5)

Monitor Replication Lag

// Check replica set members lag
rs.status().members

// Monitor oplog length
db.oplog.rs.find().limit(1).sort({ $natural: -1 })

Transactions (MongoDB 4.0+)

// Multi-document transaction
const session = client.startSession();
try {
  await session.withTransaction(async () => {
    await users.insertOne({ name: 'John' }, { session });
    await accounts.insertOne({ userId: '...', balance: 100 }, { session });
  });
} finally {
  await session.endSession();
}

Python Examples

from pymongo import MongoClient

# Connect to replica set
client = MongoClient('mongodb://localhost:27017,localhost:27018,localhost:27019/?replicaSet=rs0')

# Write concern
collection.insert_one(
    {'name': 'John'},
    write_concern=WriteConcern(w='majority')
)

# Read preference
collection.find().with_options(
    read_preference=ReadPreference.SECONDARY
)

Monitoring

✅ Check replica set status regularly
✅ Monitor replication lag
✅ Watch chunk migration progress
✅ Monitor oplog size
✅ Alert on member failures
✅ Track failover events
✅ Monitor balancer activity
✅ Check data distribution