Picture this scenario: a customer clicks "Place Order" on your e-commerce site. Behind the scenes, three things need to happen. The Inventory Service must reserve the items, the Payment Service must charge the customer's card, and the Order Service must create a record of the purchase. All three must succeed together, or none should happen at all.

What happens if the payment goes through, but the inventory update fails? You've just charged someone for a product you can't deliver. What if the order gets created, but the payment fails? You've promised goods without getting paid.

When all your data lives in a single database, solving this is straightforward. You wrap everything in a transaction, and the database guarantees that either all operations commit or none do. But the moment your data spreads across multiple services or databases, you've entered the world of distributed transactions, one of the genuinely hard problems in distributed systems.

This chapter covers why distributed transactions are so challenging, the different approaches engineers have developed to handle them, and how to choose the right approach for your system.

1. What is a Distributed Transaction?