What is the Saga rollback / compensating transaction pattern?

In a Saga (Q10), when a step fails, previously completed steps cannot be undone with a database ROLLBACK because each step has already committed its local transaction and those locks are released. Instead, the Saga executes compensating transactions — purpose-built operations that reverse the business effect of each completed step in reverse order.

A compensating transaction is a semantic undo, not a technical rollback. The key distinction:

• A technical rollback is performed by the database engine before a transaction commits — it undoes uncommitted SQL statements.
• A compensating transaction is a new, forward-moving operation that creates the business-level opposite of an already-committed action.

Example compensations:

• Forward step: reserve 5 units of stock → Compensation: release 5 units of stock reservation
• Forward step: charge customer 9.99 → Compensation: refund customer 9.99
• Forward step: create order in PENDING status → Compensation: update order status to CANCELLED

Important edge cases:

• Pivotal transactions — not all Saga steps can be compensated. A step that sends a physical shipment or charges a non-refundable fee is called a pivot transaction; if it succeeds, the Saga must complete rather than roll back.
• Retriable transactions — some steps after the pivot are guaranteed to succeed eventually (e.g., updating an order status). These steps are retried until success rather than being compensated.
• Idempotency — compensating transactions may be retried if the Saga coordination infrastructure fails, so each compensation must be idempotent.

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