Write concern and acknowledgement

How many replicas must confirm a write before the client is told it succeeded. Waiting for a majority survives a failover; waiting for the primary only is faster but can lose acknowledged writes.

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

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