The Saga Pattern: Choreography vs Orchestration

Originally formulated by Hector Garcia-Molina and Kenneth Salem in 1987, a Saga is a sequence of independent local transactions that spans multiple microservices to accomplish a long-lived distributed business workflow.

Unlike Two-Phase Commit (which holds locks across services until the entire distributed transaction completes), each local transaction in a Saga:

1. Executes locally against its own private database.
2. Commits its changes immediately in milliseconds.
3. Publishes a domain event or message to trigger the next step in the workflow.

The Compensating Transaction Mechanism:

If any step in the workflow fails (e.g., inventory is out of stock, or fraud detection flags the account), the Saga cannot execute a traditional database ROLLBACK because preceding steps have already committed.

Instead, the Saga executes a series of Compensating Transactions (C_1, C_2, \dots, C_k) in reverse order that semantically undo the side effects of committed steps (e.g., issuing a credit card refund, un-reserving warehouse stock, sending an apology email).