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 combines synchronous, asynchronous, and page-level rollbacks.

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

  • The ROLLBACK keyword in a transaction block triggers a synchronous rollback.
  • If an error is reported during transaction running, the COMMIT keyword has the same function as ROLLBACK and triggers synchronous rollback.
  • If a fatal/panic error is reported during transaction running, the system attempts to synchronously roll back the transaction bound to the thread before the thread exits.
• 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.

• Page-level rollback

  If a transaction has not been rolled back to the current page, but other transactions need to reuse the TD occupied by this transaction, the page-level rollback operation is performed for all modifications on the current page. Page-level rollback only rolls back modifications on the current page. Other pages are not involved.

  The rollback of a Ustore subtransaction is controlled by the ROLLBACK TO SAVEPOINT statement. After a subtransaction is rolled back, the transaction can continue to run. The rollback of a subtransaction does not affect the status of the transaction. If a subtransaction is not released during transaction rollback, roll back the subtransaction first. The transaction is not rolled back until all subtransactions are rolled back.