Document databases

Each record is a JSON-like document with its own structure, queried by fields and secondary indexes. Great when the data is naturally hierarchical; awkward when you need many-to-many relationships.

Where it shows up in real systems

  • Document database

    A replica set elects a new primary when the current one fails. With write concern majority, an acknowledged write survives that failover; with w:1, recent writes can be rolled back. Reading from secondaries scales reads but returns slightly stale data.

    In production: MongoDB Atlas, Amazon DocumentDB, Couchbase, Firestore

See it break, then fix it

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