1. Core Concepts

MongoDB is a distributed, document-oriented NoSQL database system written in C++. It stores data in flexible, JSON-like documents where data structures consist of key-value pairs. This design allows for dynamic schemas, making it highly adaptable to evolving application requirements.

2. Comparison: Relational vs. Document

While relational databases like MySQL use rows and tables, MongoDB uses documents and collections. The mapping is as follows:

• Database: Both systems share this concept.
• Table (MySQL) vs. Collection (MongoDB): MongoDB organizes documents into collections.
• Row (MySQL) vs. Document (MongoDB): Data is represented as a self-contained object in MongoDB.
• Column (MySQL) vs. Field (MongoDB): MongoDB uses fields to store specific key-value pairs.

3. Use Cases

• Content Management: Storing articles with varying metadata.
• Log Aggregation: Handling high-volume, unstructured log data from IoT devices.
• E-commerce: Managing product catalogs where items have highly heterogeneous attributes.
• Gaming: Storing player profiles, inventories, and scores within nested documents.

4. Deployment and Configuration

To deploy a production-ready MongoDB instance, observe the following best practices:

• Disable Transparent Huge Pages (THP) and ensure proper memory limits in /etc/security/limits.conf.
• Create a dedicated system user (e.g., mongodb) and structure your directories for data, logs, and configuration.

Example YAML-based configuration snippet:

systemLog:
 destination: file
 path: "/var/log/mongodb/mongod.log"
 logAppend: true
storage:
 dbPath: "/var/lib/mongodb"
 journal:
   enabled: true
net:
 port: 27017
 bindIp: 127.0.0.1,192.168.1.10
security:
 authorization: enabled
   

5. Basic Operations

Interacting with MongoDB is typically done via the mongosh shell:

• Create/Switch Database: use my_app_db
• Insert Data: db.users.insertOne({ name: "Alice", role: "admin" })
• Query Data: db.users.find({ name: "Alice" }).pretty()
• Drop Collection: db.users.drop()

6. Security and Access Control

Authentication should be enabled in production. Always create a root user in the admin database to manage cluster-wide permissions:

use admin
db.createUser({
 user: "adminUser",
 pwd: "securePassword123",
 roles: [{ role: "root", db: "admin" }]
})
   

7. Replication Sets

MongoDB uses Replication Sets to ensure high availability. A typical set consists of one Primary node (for writes) and multiple Secondary nodes (for reads). If the Primary fails, the set performs an automated election to promote a new Primary.

To initialize a set in the shell:

rs.initiate({
 _id: "my_cluster",
 members: [
   { _id: 0, host: "node1:27017" },
   { _id: 1, host: "node2:27017" },
   { _id: 2, host: "node3:27017" }
 ]
})
   

8. Sharded Clusters

Sharding enables horizontal scaling by distributing data across multiple machines. It involves three components:

• Shards: Store the actual data subsets.
• Config Servers: Store metadata and cluster configuration.
• mongos: Act as a query router, directing requests to the appropriate shard.

9. Backup and Restoration

For routine maintenance, use the binary utilities mongodump and mongorestore, which operate on BSON files. For cross-platform migrations, use mongoexport and mongoimport to interact with JSON/CSV formats.

Example Backup Command:

mongodump --host localhost --port 27017 --username admin --authenticationDatabase admin --db my_db --out /backups/db_snapshot