Multi-document Transactions

A transaction makes several reads and writes on a session commit as one or abort as one. You need a replica set (or mongos). Standalone mongod is the wrong demo for production semantics. Snapshot isolation: inside the transaction you see a consistent cut; outside, others see nothing until commit.

const session = client.startSession();
try {
  session.startTransaction();
  await db.collection("orders").insertOne(
    {
      userId: ObjectId("64a1b2c3d4e5f6a7b8c90001"),
      items: [{ name: "Mechanical Keyboard", quantity: 1, unitPrice: 4500 }],
      amount: 4500,
      status: "pending",
    },
    { session },
  );
  const stock = await db.collection("products").updateOne(
    { name: "Mechanical Keyboard", stock: { $gte: 1 } },
    { $inc: { stock: -1 } },
    { session },
  );
  if (stock.matchedCount === 0) {
    throw new Error("sold out");
  }
  await session.commitTransaction();
} catch (err) {
  await session.abortTransaction();
  throw err;
} finally {
  await session.endSession();
}

The interview classic is the same shape with two accounts: $inc: { balance: -500 } on Rahul, $inc: { balance: 500 } on Priya, abort if Rahul's filter balance: { $gte: 500 } misses. In Node, `withTransaction` retries transient errors (failover, TransientTransactionError). Do not retry a sold-out matchedCount as if it were transient.

• Pass `{ session }` on every operation that belongs in the txn. Forget one write and it commits outside.
• Lifetime. Default ~60 seconds. Long reports do not belong here.
• Write concern. Majority is the sane commit; unacknowledged writes cannot be transactional.
• Creates. Avoid createIndex / createCollection inside a txn. Have the collections first.