Question

When implementing the Saga pattern for distributed transactions across microservices, what is the mandatory mechanism used to undo changes if a step in the transaction chain fails?

Accepted Answer

The mandatory mechanism used to undo changes in a Saga pattern is the compensating transaction. In a distributed system, a long-running transaction is broken into a sequence of smaller local transactions, where each microservice performs its own work and commits its data to its local database immediately. Because these local transactions cannot be rolled back using standard database atomic commit protocols, a failure requires a semantic undo operation. A compensating transaction is a specific function or service request designed to reverse the effects of a previously completed local transaction. For example, if a Saga processes a payment and then fails at the inventory reservation step, the system must invoke a compensating transaction to refund the payment. Each step in the Saga must have a corresponding compensating action defined beforehand to ensure the system returns to a consistent state, either by undoing the previous changes or by performing an alternative action that logically cancels the impact of the failed step.

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When implementing the Saga pattern for distributed transactions across microservices, what is the mandatory mechanism used to undo changes if a step in the transaction chain fails?