Transaction Rollback

Rollback is a process in which a transaction cannot be executed if a fault occurs during transaction running. In this case, the system needs to cancel the modification operations that have been completed in the transaction. Astore and UB-tree do not have rollback segments. Therefore, there is no dedicated rollback operation. To ensure performance, the Ustore rollback process supports synchronous, asynchronous, and in-page instant rollback.

1. Synchronous rollback.
   Transaction rollback is triggered in any of the following scenarios:

   1. The ROLLBACK keyword in a transaction block triggers a synchronous rollback.
   2. If an error is reported during transaction running, the COMMIT keyword has the same function as ROLLBACK and triggers synchronous rollback.
   3. If a fatal/panic error is reported during transaction running, the system attempts to roll back the transaction bound to the thread before exiting the thread.
2. Asynchronous rollback. When the synchronous rollback fails or the system is restarted after breakdown, the undo recycling thread initiates an asynchronous rollback task for the transaction that is not rolled back completely and provides services for external systems immediately. The task initiation thread Undo Launch of asynchronous rollback starts the working thread Undo Worker to execute the rollback task. The Undo Launch thread can start a maximum of five Undo Worker threads at the same time.
3. In-page rollback. If the rollback operation of a transaction page is not completed, but other transactions need to reuse the TD occupied by this transaction, the in-page rollback operation is performed for all modifications on the current page. In-page rollback only rolls back modifications on the current page. Other pages are not involved.

   The rollback of a Ustore sub-transaction is controlled by the ROLLBACK TO SAVEPOINT statement. After a sub-transaction is rolled back, the parent transaction can continue to run. The rollback of a sub-transaction does not affect the transaction status of the parent transaction. If sub-transactions that are not released during the parent transaction rollback, the sub-transactions are rolled back before the parent transaction is rolled back.