Mongodb

MongoDB Core Knowledge

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: mongodb for comprehensive documentation.

CRUD Operations

// Create
db.users.insertOne({
  name: "John",
  email: "john@example.com",
  createdAt: new Date()
});

// Read
db.users.find({ isActive: true })
  .sort({ createdAt: -1 })
  .limit(20);

db.users.findOne({ _id: ObjectId("...") });

// Update
db.users.updateOne(
  { _id: ObjectId("...") },
  { $set: { name: "Jane" } }
);

// Delete
db.users.deleteOne({ _id: ObjectId("...") });

Query Operators

// Comparison
{ age: { $gt: 18, $lt: 65 } }
{ status: { $in: ["active", "pending"] } }

// Logical
{ $and: [{ age: { $gt: 18 } }, { isActive: true }] }
{ $or: [{ status: "admin" }, { role: "moderator" }] }

// Array
{ tags: { $all: ["tech", "news"] } }
{ "scores.0": { $gt: 90 } }

Aggregation Pipeline

db.orders.aggregate([
  { $match: { status: "completed" } },
  { $group: {
      _id: "$userId",
      totalSpent: { $sum: "$amount" },
      orderCount: { $count: {} }
  }},
  { $sort: { totalSpent: -1 } },
  { $limit: 10 }
]);

Indexes

db.users.createIndex({ email: 1 }, { unique: true });
db.users.createIndex({ createdAt: -1 });
db.users.createIndex({ name: "text" }); // Text search

Production Readiness

Security Configuration

// Enable authentication (mongod.conf)
// security:
//   authorization: enabled

// Create admin user
use admin
db.createUser({
  user: "admin",
  pwd: "secure_password",
  roles: ["root"]
});

// Create application user with limited privileges
use mydb
db.createUser({
  user: "app_user",
  pwd: "app_password",
  roles: [
    { role: "readWrite", db: "mydb" }
  ]
});

// Create read-only user for reporting
db.createUser({
  user: "reporter",
  pwd: "reporter_password",
  roles: [{ role: "read", db: "mydb" }]
});

# mongod.conf - Security settings
security:
  authorization: enabled

net:
  ssl:
    mode: requireSSL
    PEMKeyFile: /path/to/mongodb.pem
    CAFile: /path/to/ca.pem

Connection with SSL

// Node.js connection with SSL
const { MongoClient } = require('mongodb');

const client = new MongoClient('mongodb://host:27017', {
  ssl: true,
  sslCA: fs.readFileSync('/path/to/ca.pem'),
  sslCert: fs.readFileSync('/path/to/client.pem'),
  sslKey: fs.readFileSync('/path/to/client.key'),
  authSource: 'admin',
});

Replica Set (High Availability)

// Initialize replica set
rs.initiate({
  _id: "myReplicaSet",
  members: [
    { _id: 0, host: "mongo1:27017", priority: 2 },
    { _id: 1, host: "mongo2:27017", priority: 1 },
    { _id: 2, host: "mongo3:27017", priority: 1 }
  ]
});

// Connection string for replica set
mongodb://mongo1:27017,mongo2:27017,mongo3:27017/mydb?replicaSet=myReplicaSet&readPreference=secondaryPreferred

Backup & Recovery

# mongodump backup
mongodump --uri="mongodb://user:pass@host:27017/mydb" \
  --out=/backup/$(date +%Y%m%d) \
  --gzip

# mongorestore
mongorestore --uri="mongodb://user:pass@host:27017/mydb" \
  --gzip /backup/20240115

# Continuous backup with oplog
mongodump --oplog --out=/backup/full

# Point-in-time recovery
mongorestore --oplogReplay --oplogLimit=1705315200 /backup/full

Performance Tuning

// Index best practices
db.collection.createIndex({ field: 1 }, { background: true });

// Compound indexes for common queries
db.orders.createIndex({ userId: 1, createdAt: -1 });

// TTL index for automatic expiration
db.sessions.createIndex({ createdAt: 1 }, { expireAfterSeconds: 3600 });

// Partial indexes for filtered queries
db.orders.createIndex(
  { status: 1 },
  { partialFilterExpression: { status: { $in: ["pending", "processing"] } } }
);

// Analyze query performance
db.orders.find({ userId: "123" }).explain("executionStats");

Monitoring Metrics

Monitoring Commands

// Server status
db.serverStatus();

// Current operations
db.currentOp({ "active": true, "secs_running": { "$gt": 5 } });

// Replication status
rs.status();

// Index usage stats
db.collection.aggregate([{ $indexStats: {} }]);

// Collection stats
db.collection.stats();

// Database profiler (slow queries)
db.setProfilingLevel(1, { slowms: 100 });
db.system.profile.find().sort({ ts: -1 }).limit(10);

Read/Write Concerns

// Write concern for durability
db.orders.insertOne(order, {
  writeConcern: { w: "majority", j: true, wtimeout: 5000 }
});

// Read concern for consistency
db.orders.find({ userId: "123" }).readConcern("majority");

// Read preference for scaling reads
db.orders.find().readPref("secondaryPreferred");

Sharding (Horizontal Scaling)

// Enable sharding on database
sh.enableSharding("mydb");

// Shard collection with hashed key
sh.shardCollection("mydb.orders", { _id: "hashed" });

// Shard collection with range key
sh.shardCollection("mydb.logs", { timestamp: 1 });

// Check sharding status
sh.status();

Checklist

• Authentication enabled
• TLS/SSL encryption enabled
• Least-privilege user accounts
• Replica set configured (3+ nodes)
• Regular mongodump backups
• Oplog size adequate for recovery window
• Indexes on query fields
• Query profiler enabled (slow queries)
• Write concern: majority + journal
• Connection pooling configured
• Monitoring alerts configured
• Sharding (if > 100GB or high throughput)

When NOT to Use This Skill

• Relational data with complex joins - Use postgresql or mysql for relational databases
• Transactions across multiple tables - Use SQL databases with ACID guarantees
• Full-text search - Use elasticsearch for advanced search features
• Caching - Use redis for in-memory caching
• Graph relationships - Consider Neo4j or graph databases

Anti-Patterns

Quick Troubleshooting

Reference Documentation

• Aggregation
• Indexes