Mongodb

MongoDB

Administer, optimize, and secure MongoDB NoSQL databases in development and production environments.

When to Use

• You need a document-oriented database with flexible schemas.
• Your data is semi-structured or heavily nested (JSON-like documents).
• You need horizontal scaling through sharding.
• Your application benefits from rich querying and aggregation pipelines.

Prerequisites

• Linux server (Debian/Ubuntu or RHEL-based) or Docker.
• Root or sudo access for package installation.
• MongoDB 7.x recommended for production (6.x still supported).

Installation and Setup

# Debian / Ubuntu — MongoDB 7
curl -fsSL https://www.mongodb.org/static/pgp/server-7.0.asc | \
  sudo gpg -o /usr/share/keyrings/mongodb-server-7.0.gpg --dearmor
echo "deb [ signed-by=/usr/share/keyrings/mongodb-server-7.0.gpg ] \
  https://repo.mongodb.org/apt/ubuntu jammy/mongodb-org/7.0 multiverse" | \
  sudo tee /etc/apt/sources.list.d/mongodb-org-7.0.list
sudo apt update
sudo apt install -y mongodb-org

# Start and enable
sudo systemctl enable --now mongod

# Verify
mongosh --eval "db.version()"

Initial User Setup

// Connect without auth first
// mongosh

use admin

// Create admin user
db.createUser({
  user: "admin",
  pwd: "strong_admin_password",
  roles: [
    { role: "userAdminAnyDatabase", db: "admin" },
    { role: "readWriteAnyDatabase", db: "admin" },
    { role: "clusterAdmin", db: "admin" }
  ]
})

// Create an application-scoped user
use mydb
db.createUser({
  user: "myapp",
  pwd: "strong_app_password",
  roles: [{ role: "readWrite", db: "mydb" }]
})

Enable authentication in /etc/mongod.conf:

security:
  authorization: enabled

sudo systemctl restart mongod
# Now connect with credentials
mongosh -u myapp -p strong_app_password --authenticationDatabase mydb

mongosh Commands Reference

// Show databases and collections
show dbs
use mydb
show collections

// Insert documents
db.users.insertOne({ name: "Alice", email: "[email protected]", age: 30 })
db.users.insertMany([
  { name: "Bob", email: "[email protected]", age: 25 },
  { name: "Carol", email: "[email protected]", age: 35 }
])

// Query documents
db.users.find({ age: { $gte: 25 } }).sort({ name: 1 }).limit(10)
db.users.findOne({ email: "[email protected]" })
db.users.countDocuments({ age: { $gte: 30 } })

// Update
db.users.updateOne(
  { email: "[email protected]" },
  { $set: { age: 31 }, $currentDate: { updatedAt: true } }
)
db.users.updateMany(
  { age: { $lt: 30 } },
  { $set: { tier: "junior" } }
)

// Delete
db.users.deleteOne({ email: "[email protected]" })
db.users.deleteMany({ tier: "junior" })

Indexing

// Single-field index
db.users.createIndex({ email: 1 }, { unique: true })

// Compound index
db.orders.createIndex({ userId: 1, createdAt: -1 })

// Text index for search
db.articles.createIndex({ title: "text", body: "text" })
db.articles.find({ $text: { $search: "mongodb scaling" } })

// TTL index — auto-delete documents after 30 days
db.sessions.createIndex({ createdAt: 1 }, { expireAfterSeconds: 2592000 })

// List indexes
db.users.getIndexes()

// Drop an index
db.users.dropIndex("email_1")

// Explain a query to verify index usage
db.orders.find({ userId: 42 }).explain("executionStats")

Aggregation Pipeline Examples

// Revenue per status
db.orders.aggregate([
  { $group: {
      _id: "$status",
      totalRevenue: { $sum: "$total" },
      count: { $sum: 1 }
  }},
  { $sort: { totalRevenue: -1 } }
])

// Top 5 customers by order value (with a join)
db.orders.aggregate([
  { $group: {
      _id: "$userId",
      spent: { $sum: "$total" },
      orderCount: { $sum: 1 }
  }},
  { $sort: { spent: -1 } },
  { $limit: 5 },
  { $lookup: {
      from: "users",
      localField: "_id",
      foreignField: "_id",
      as: "user"
  }},
  { $unwind: "$user" },
  { $project: {
      _id: 0,
      name: "$user.name",
      email: "$user.email",
      spent: 1,
      orderCount: 1
  }}
])

// Daily signup trend
db.users.aggregate([
  { $group: {
      _id: { $dateToString: { format: "%Y-%m-%d", date: "$createdAt" } },
      signups: { $sum: 1 }
  }},
  { $sort: { _id: 1 } },
  { $limit: 30 }
])

Replica Set Setup

A replica set requires a minimum of three members (or two data-bearing nodes plus an arbiter).

Configuration File for Each Member

# /etc/mongod.conf (adjust port and dbPath per member)
storage:
  dbPath: /var/lib/mongodb
net:
  port: 27017
  bindIp: 0.0.0.0
replication:
  replSetName: rs0
security:
  authorization: enabled
  keyFile: /etc/mongodb-keyfile

# Generate a shared keyfile for internal auth
openssl rand -base64 756 > /etc/mongodb-keyfile
chmod 400 /etc/mongodb-keyfile
chown mongodb:mongodb /etc/mongodb-keyfile
# Copy this file to all replica set members

Initialize the Replica Set

// Connect to the first member
// mongosh --port 27017

rs.initiate({
  _id: "rs0",
  members: [
    { _id: 0, host: "mongo1:27017", priority: 2 },
    { _id: 1, host: "mongo2:27017", priority: 1 },
    { _id: 2, host: "mongo3:27017", priority: 1 }
  ]
})

// Check status
rs.status()

// View replication lag per member
rs.printReplicationInfo()
rs.printSecondaryReplicationInfo()

Backup and Restore

# Full dump of all databases
mongodump --uri="mongodb://admin:secret@localhost:27017" --out=/backups/full_$(date +%F)

# Single database
mongodump --uri="mongodb://myapp:secret@localhost:27017/mydb" --out=/backups/mydb_$(date +%F)

# Compressed dump
mongodump --uri="mongodb://admin:secret@localhost:27017" --gzip --out=/backups/gz_$(date +%F)

# Restore all databases
mongorestore --uri="mongodb://admin:secret@localhost:27017" /backups/full_2025-01-15/

# Restore a single database, dropping existing data first
mongorestore --uri="mongodb://admin:secret@localhost:27017" \
  --drop --db mydb /backups/mydb_2025-01-15/mydb/

# Restore compressed dump
mongorestore --uri="mongodb://admin:secret@localhost:27017" --gzip /backups/gz_2025-01-15/

Docker Compose Setup

# docker-compose.yml
version: "3.9"

services:
  mongo1:
    image: mongo:7
    restart: unless-stopped
    ports:
      - "27017:27017"
    environment:
      MONGO_INITDB_ROOT_USERNAME: admin
      MONGO_INITDB_ROOT_PASSWORD: secret
    volumes:
      - mongo1_data:/data/db
      - ./mongo-keyfile:/etc/mongodb-keyfile:ro
    command: >
      mongod
        --replSet rs0
        --keyFile /etc/mongodb-keyfile
        --bind_ip_all
    healthcheck:
      test: ["CMD", "mongosh", "--eval", "db.adminCommand('ping')"]
      interval: 10s
      timeout: 5s
      retries: 5

  mongo2:
    image: mongo:7
    restart: unless-stopped
    volumes:
      - mongo2_data:/data/db
      - ./mongo-keyfile:/etc/mongodb-keyfile:ro
    command: >
      mongod
        --replSet rs0
        --keyFile /etc/mongodb-keyfile
        --bind_ip_all

  mongo3:
    image: mongo:7
    restart: unless-stopped
    volumes:
      - mongo3_data:/data/db
      - ./mongo-keyfile:/etc/mongodb-keyfile:ro
    command: >
      mongod
        --replSet rs0
        --keyFile /etc/mongodb-keyfile
        --bind_ip_all

  mongo-init:
    image: mongo:7
    restart: "no"
    depends_on:
      mongo1:
        condition: service_healthy
    entrypoint: >
      mongosh --host mongo1 -u admin -p secret --authenticationDatabase admin --eval '
        rs.initiate({
          _id: "rs0",
          members: [
            { _id: 0, host: "mongo1:27017", priority: 2 },
            { _id: 1, host: "mongo2:27017", priority: 1 },
            { _id: 2, host: "mongo3:27017", priority: 1 }
          ]
        })
      '

volumes:
  mongo1_data:
  mongo2_data:
  mongo3_data:

# Generate keyfile before starting
openssl rand -base64 756 > mongo-keyfile
chmod 400 mongo-keyfile

docker compose up -d

# Connect
mongosh "mongodb://admin:[email protected]:27017/?replicaSet=rs0&authSource=admin"

Monitoring Queries

// Server status summary
db.serverStatus().connections
db.serverStatus().opcounters

// Current operations (look for long-running queries)
db.currentOp({ secs_running: { $gte: 5 } })

// Collection stats
db.orders.stats()

// Index sizes
db.orders.stats().indexSizes

// Profiler — log slow queries (> 100ms)
db.setProfilingLevel(1, { slowms: 100 })
db.system.profile.find().sort({ ts: -1 }).limit(5)

// Replica set lag
rs.printSecondaryReplicationInfo()

Configuration Tuning

# /etc/mongod.conf — production recommendations
storage:
  dbPath: /var/lib/mongodb
  journal:
    enabled: true
  wiredTiger:
    engineConfig:
      cacheSizeGB: 4          # ~50% of RAM, leave rest for OS cache
    collectionConfig:
      blockCompressor: snappy
net:
  port: 27017
  bindIp: 0.0.0.0
  maxIncomingConnections: 500
operationProfiling:
  mode: slowOp
  slowOpThresholdMs: 100

Troubleshooting

Symptom	Likely Cause	Fix
COLLSCAN in explain output	Missing index on queried field	Create an appropriate index
Replica member stuck in RECOVERING	Oplog window exceeded	Resync by removing data and restarting the member
too many open files	OS file descriptor limit too low	Set ulimit -n 65535 in service file
High memory usage	WiredTiger cache too large	Reduce cacheSizeGB in config
Slow aggregation pipelines	No index on $match stage fields	Add index; place $match as early as possible in pipeline
Authentication failure	Wrong authenticationDatabase	Specify --authenticationDatabase admin for admin users

Related Skills

• redis - Caching layer in front of MongoDB
• database-backups - Automated backup strategies
• postgresql - Alternative relational database