Multi-document transactions

Since MongoDB 4.0, multi-document ACID transactions on replica sets mirror SQL BEGIN/COMMIT for critical flows.

Session transaction (mongosh)

session = db.getMongo().startSession()
session.startTransaction()
try {
  const orders = session.getDatabase('practice').orders
  const inventory = session.getDatabase('practice').inventory
  orders.insertOne({ sku: 'A1', qty: 1 }, { session })
  inventory.updateOne({ sku: 'A1' }, { $inc: { stock: -1 } }, { session })
  session.commitTransaction()
} catch (e) {
  session.abortTransaction()
  throw e
} finally {
  session.endSession()
}

Practice: Concepts apply to Atlas and self-hosted; try read-only commands in mongosh where safe.

When to use

Inventory + order creation, wallet transfers—prefer single-document atomicity when possible; transactions add latency and require replica set.

Important interview questions and answers

1. Q: Replica set requirement?
   A: Transactions need replication protocol—standalone dev converts to RS for testing.
2. Q: Retry on transient errors?
   A: Drivers retry commit on UnknownTransactionCommitResult—design idempotent ops.

Self-check

1. Why prefer single-doc atomicity?
2. What does abortTransaction do?

Tip: Prefer single-document atomic updates when they satisfy the business rule.