MongoDB Architecture: A simplified analogy

Single MongoDB Instance:

• Library (MongoDB Instance):

  • Section (Database):

    • Shelf (Collection):

      • Books (Documents)

In a single MongoDB instance, you have one library with different sections (databases). Each section has shelves (collections), and each shelf contains books (documents).

MongoDB Replica Set:

• Library Chain (Replica Set):

  • Library 1 (Primary Node)

  • Library 2 (Secondary Node)

  • Library 3 (Secondary Node)

  • Each Library (Node) Contains:

    • Section (Database):

      • Shelf (Collection):

        • Books (Documents)

In a replica set, you have multiple copies of the same library (nodes). Each library/node has identical sections (databases), shelves (collections), and books (documents) to ensure data redundancy.

MongoDB Sharded Cluster:

• Library System (Sharded Cluster):

  • Central Library Catalogue (Config Server):

  • Librarian’s Desk (Query Router - mongos):

  • Branch Libraries (Shards):

    • Library Branch 1 (Shard/Replica Set 1):

    • Library Branch 2 (Shard/Replica Set 2):

    • Each Branch Library (Shard) Contains Partial:

      • Section (Database):

        • Shelf (Collection):

          • Books (Documents)

In a sharded cluster, the data (books) is distributed across multiple branch libraries (shards). Each branch library holds a portion of the total data and might be composed of a replica set to ensure data availability. The central library catalogue (config server) knows which books are in which branch library. The librarian’s desk (query router - mongos) directs the visitors (queries) to the correct branch library (shard) based on the catalogue information.

Practical Context:

• A Database Instance is like a single library, sufficient for small applications.

• A Replica Set is like having multiple identical libraries (or nodes) where if one is closed (fails), you can go to the other without losing any information, ensuring high availability.

• A Sharded Cluster is like having several branch libraries for a city, each storing different books to manage the high volume of books and visitors, enabling horizontal scaling