Compensation Log Records

• CLRs: redo only log records

• Used to record actions performed during transaction rollback

  • one CLR for each normal log record which is undone

• CLRs have a field UndoNxtLSN indicating which log record is to be undone next

  • avoids repeated undos by bypassing already undo records
    • needed in case of restarts during transaction rollback)
  • in contrast, IBM IMS may repeat undos, and AS400 may even undo undos, then redo the undos

Notes: