Why should you care?

Without transactions, debit succeeds but credit fails — money disappears.

See it live — copy this example

Paste into your MeanVerse project (Angular + Express + MongoDB), then run with ng serve / node / mongosh as noted.

const session = await mongoose.startSession();
try {
  await session.withTransaction(async () => {
    await Account.updateOne({ _id: fromId }, { $inc: { balanceCents: -amount } }, { session });
    await Account.updateOne({ _id: toId }, { $inc: { balanceCents: amount } }, { session });
    await Transfer.create([{ fromId, toId, amount, status: 'posted' }], { session });
  });
} finally {
  await session.endSession();
}

What happened?

• withTransaction retries on transient errors.
• All ops share session.
• Replica set required — standalone dev uses single-node replica set.

Practice next

1. Configure local Mongo as replica set for transactions.
2. Wrap transfer logic in withTransaction.
3. Simulate failure mid-way — verify rollback.
4. Add inventory decrement in same transaction as order create.
5. Log abort reason in catch block.

Remember

Transactions = ACID across multiple docs. Required for financial MeanVerse flows. Need replica set deployment.

Double-entry ledger

E-commerce order decrements stock and records payment together.

Outcome: Rollback prevents oversell when payment gateway times out.