How do write concern and read concern work in MongoDB?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Set the write concern

   Pass writeConcern: { w, j, wtimeout } on the operation or configure it at the connection level; w can be a count or "majority".

2. Step 2

   Understand journaling

   j: true forces the write to be flushed to the on-disk journal before acknowledgment, guarding against a crash before the next checkpoint.

3. Step 3

   Choose a read concern

   Select "local", "available", "majority", "linearizable", or "snapshot" based on how tolerant the read is of rollbacks.

4. Step 4

   Match the pair for a use case

   For strong guarantees combine w: "majority" writes with readConcern: "majority" reads so acknowledged data is never rolled back.

5. Step 5

   Bound the wait

   Use wtimeout so a write does not block indefinitely if not enough members are reachable; a timeout does not undo the write.

Code Example

// Insert acknowledged by a majority and flushed to the journal
await db.collection('orders').insertOne(
  { orderId: 'A-1001', total: 250 },
  { writeConcern: { w: 'majority', j: true, wtimeout: 5000 } }
);

// Read only data acknowledged by a majority (never rolled back)
const order = await db.collection('orders').findOne(
  { orderId: 'A-1001' },
  { readConcern: { level: 'majority' } }
);
console.log(order);

Follow-up Questions

• What is a rollback on a replica set and how does w: majority prevent reading rolled-back data?
• What is the difference between read concern and read preference?
• When would you use readConcern: "linearizable" and what is its cost?
• What happens to a write when wtimeout is exceeded?
• How do write concern and read concern interact inside a multi-document transaction?