What is MongoDB's write concern?
Why Interviewers Ask This
This tests whether you can apply MongoDB knowledge to real-world scenarios. Interviewers are looking for clarity of thought and evidence that you've encountered this in production code.
Answer
Write concern specifies the level of acknowledgment requested from MongoDB for write operations — it controls the trade-off between write speed and data durability. Write concern options: w (required acknowledgment): 0 — fire and forget, no acknowledgment (fastest, data loss possible); 1 — primary acknowledges (default — primary has written to memory); "majority" — majority of replica set members have acknowledged (most durable — survives primary failure); N — N members must acknowledge. j (journal): false (default) — acknowledged when in memory; true — acknowledged only after written to the on-disk journal (WAL) — survives process crash. wtimeout: timeout in milliseconds for the write concern — throws WriteConcernError if not acknowledged within this time. Common configurations: w:1, j:false — default, in-memory acknowledgment — fast but minor data loss window (MongoDB crash before journal flush); w:1, j:true — on-disk durability on primary — survives crashes; w:"majority", j:true — strongest guarantee — survives primary failure with on-disk durability. Setting at operation level: db.orders.insertOne({ ... }, { writeConcern: { w: "majority", j: true, wtimeout: 5000 } }). Setting at connection level: in the connection string: mongodb://host/?w=majority&j=true. For financial or critical data, always use w:"majority", j:true.
Pro Tip
Demonstrate both theoretical understanding and practical experience. Say what it is, then give an example of how you actually used it in a MongoDB codebase.
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