Transactions and ACID Compliance

While MongoDB's document model minimizes the need for multi-document transactions, some operations still require atomicity across multiple documents or collections. MongoDB supports full multi-document ACID transactions since version 4.0.

1. ACID Properties Recap

2. Single-Document Atomicity (Always On)

Every write to a single document is always atomic in MongoDB by default — even when updating deeply nested fields or arrays.

3. Multi-Document Transactions

const session = db.getMongo().startSession();
session.startTransaction();

try {
  const accounts = session.getDatabase("bankDB").accounts;

  accounts.updateOne(
    { accountId: "A100" },
    { $inc: { balance: -500 } },
    { session }
  );

  accounts.updateOne(
    { accountId: "B200" },
    { $inc: { balance: 500 } },
    { session }
  );

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

4. When to Use Transactions

• Bank transfers or financial ledger updates across accounts
• Inventory deduction that must stay in sync with order creation
• Any operation spanning multiple collections that must all succeed or all fail

5. When to Avoid Transactions

Transactions carry performance overhead. Whenever possible, model your schema (via embedding) so related data updates within a single document — avoiding the need for a transaction altogether.

6. Transactions Checklist

• ✅ Understand ACID properties in MongoDB's context
• ✅ Know single-document writes are always atomic
• ✅ Use startSession()/startTransaction()/commitTransaction() correctly
• ✅ Handle abortTransaction() on errors
• ✅ Recognize when embedding avoids the need for transactions