What is Saga and Transactional Outbox? Why are distributed transactions needed?

Saga is a pattern for managing distributed transactions, breaking a large transaction into a sequence of local transactions, each with a compensating action in case of error. This allows ensuring data consistency in distributed systems without using global locks.

Transactional Outbox is a pattern where state changes and messages for other services are recorded in a single transaction in the database (in an "outbox" table). Then a separate process reads these messages and sends them to a queue or event bus. This guarantees that messages are not lost and will only be sent after a successful state change.

Distributed transactions are needed when an operation involves multiple services or databases, and data consistency between them must be ensured. However, classical distributed transactions (e.g., two-phase commit) are complex and poorly scalable, so patterns like Saga and Transactional Outbox are used to ensure reliability and consistency without locks.