MongoDB Document Model: Collections, Schemas, And Index Discipline

**MongoDB** stores **BSON** documents in **collections**. Unlike rigid SQL schemas, document shape can vary per document, but production teams still enforce schemas via validation rules, ODM models, or migration scripts.

Queries use a rich filter language, aggregation pipeline stages (`$match`, `$group`, `$lookup`), and secondary indexes. **Compound indexes** must match sort and filter patterns; use `explain()` to verify plans.

Replica sets provide high availability; sharded clusters horizontal scale for large datasets. Read concern and write concern tune consistency vs latency.

MongoDB is not schemaless magic. Treat it as a database with operational costs: backups, index builds, and capacity planning. Official docs: `mongodb.com/docs`.