Backend & systems·Transactions and isolation
you break the cross-service operation into local transactions and write an undo step for each one, because there is no shared rollback.
Saga
A sequence of local transactions across services, with a compensating action for each step, so that a failure part-way is unwound rather than rolled back. It buys availability and loose coupling at the cost of never having true atomicity: intermediate states are visible, and compensation is not the same as undo. Every saga step must be idempotent, because retries are guaranteed.