MongoDB Transactions in Node.js: Complete Guide with Real Examples

MongoDB transactions bring ACID guarantees to multi-document operations — essential when multiple collections must update atomically. Using Mongoose, you start a session, begin a transaction, perform operations within that session context, then commit or abort. The classic example: a wallet transfer debiting one account and crediting another — without a transaction, a crash between operations leaves money lost.

Key implementation details: always pass the session to every operation, wrap in try/catch with explicit abortTransaction() on failure, and implement retry logic for transient errors (write conflicts). Critical gotcha: transactions require a replica set — they won't work on standalone MongoDB instances (even in development, use --replSet).

Performance considerations: transactions hold locks, so keep them short. For read-heavy operations, consider read concerns and causal consistency instead of full transactions.