What are transactions and how are they handled?

Short answer: Transactions allow multiple database operations to execute atomically — either all succeed or none do — ensuring data consistency. In MongoDB (using Mongoose): const session = await mongoose.startSession(); session.startTransaction(); try { await User.create([{ name: 'Bob' }], { session });

Example code

await Order.create([{ userId: user._id }], { session });
await session.commitTransaction(); } catch (error) { await session.abortTransaction(); } finally { session.endSession(); } In Sequelize (MySQL/PostgreSQL): const t = await sequelize.transaction(); try { await User.create({ name: 'Bob' }, { transaction: t });
await Order.create({ userId: user.id }, { transaction: t });
await t.commit(); } catch (error) { await t.rollback();
} Deployment & Production

Real-world example (ShopNest)

A ShopNest BFF in Node.js aggregates catalog and price APIs for the React storefront.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.