ACID Transactions in MongoDB

Welcome to Week 4! 💳 This week is about Advanced Operations.

For years, critics said: “MongoDB is fast, but it doesn’t have Transactions. It’s not ACID compliant.” Since MongoDB 4.0 (2018), that is FALSE. MongoDB supports fully ACID multi-document transactions.

1. What is ACID?

• A - Atomicity: All or nothing. If one part fails, everything rolls back.
• C - Consistency: Database constraints are preserved.
• I - Isolation: Other users don’t see partial data while the transaction is running.
• D - Durability: Once committed, it’s saved forever.

2. When do you need them?

In MongoDB, single-document operations are ALWAYS atomic. If you update a User document, no one sees a “half-updated” user.

You ONLY need transactions when updating multiple documents across multiple collections where data integrity is critical.

Example: Money Transfer

1. Deduct $100 from Alice (Collection A)
2. Add $100 to Bob (Collection B)

If Step 1 succeeds but Step 2 fails, money disappears! 😱 You need a Transaction.

3. How to use Transactions (Node.js)

You need a Session.

async function transferMoney(fromId, toId, amount) {
  const session = await mongoose.startSession();
  session.startTransaction();

  try {
    const opts = { session };

    // Step 1: Deduct
    const sender = await Account.findOneAndUpdate(
      { _id: fromId },
      { $inc: { balance: -amount } },
      opts
    );

    if (sender.balance < 0) {
      throw new Error("Insufficient funds");
    }

    // Step 2: Add
    await Account.findOneAndUpdate(
      { _id: toId },
      { $inc: { balance: amount } },
      opts
    );

    // Commit!
    await session.commitTransaction();
    console.log("Success!");

  } catch (error) {
    // Rollback!
    await session.abortTransaction();
    console.error("Failed. Rolled back.", error);
  } finally {
    session.endSession();
  }
}

4. Performance Warning ⚠️

Transactions are expensive.

• They hold locks.
• They increase latency.
• They require a Replica Set (Transactions don’t work on a standalone server).

Best Practice: Design your schema to avoid transactions if possible (Embedding!). Use them only when absolutely necessary (like handling money).

🧠 Daily Challenge

1. Open your code.
2. Create a mock function buyProduct(userId, productId).
3. It should:
   • Decrement Product.stock.
   • Create an Order.
4. Wrap it in a Transaction so you don’t create an order if stock hits -1!

See you on Day 2 for Bulk Operations! 📦

Next Step

Next: Bulk Writes Operations →