How do transactions work in MongoDB? | MongoDB Interview Questions

What You'll Learn

When to use transactions
Multi-document transactions
Best practices

When to Use Transactions

Transfer money between accounts
Order processing with inventory update
Any operation requiring atomicity across documents

Basic Transaction

code.jsJavaScript

const session = client.startSession();

try {
  session.startTransaction();

  // Deduct from sender
  await accounts.updateOne(
    { _id: senderId },
    { $inc: { balance: -amount } },
    { session }
  );

  // Add to receiver
  await accounts.updateOne(
    { _id: receiverId },
    { $inc: { balance: amount } },
    { session }
  );

  await session.commitTransaction();
} catch (error) {
  await session.abortTransaction();
  throw error;
} finally {
  session.endSession();
}

With Mongoose

code.jsJavaScript

const session = await mongoose.startSession();

await session.withTransaction(async () => {
  await Order.create([{ userId, items, total }], { session });

  await Product.updateMany(
    { _id: { $in: itemIds } },
    { $inc: { stock: -1 } },
    { session }
  );

  await User.updateOne(
    { _id: userId },
    { $inc: { orderCount: 1 } },
    { session }
  );
});

session.endSession();

Requirements

MongoDB 4.0+ for replica sets
MongoDB 4.2+ for sharded clusters
WiredTiger storage engine

Best Practices

Keep transactions short
Limit operations per transaction
Handle retries for transient errors
Consider single-document atomicity first