Nested Top Actions

• Main topic of discussion : Some of the updates of a transaction to be committed, even if the corresponding transaction ( which is making these updates) is committed or not.
• If the extension related updates to the system data in the DB were themselves interrupted by a failure before completion, it is necessary to undo them. These actions are performed by starting independent transactions, called TOP ACTIONS.
• In short a nested top action (NTA) (subsequence of actions of transactions) should not be undone once the sequence is complete and dependent transaction logs to stable storage.
• Aries supports top actions efficiently, without initiating independent transaction.
• The enclosing transaction which performs sequence of actions (NTA) consists of :

  a. ascertaining the position of the current transaction's log record.

  b. logging the redo and undo information associated with the actions of the nested top action.

  c. on completion of the NTA, writing a dummy CLR whose UndoNxtLSN points to the log in (a).

• Assumptions : Effects of any actions like creating a file and their associated updates to system data normally resident outside the databases are externalised.
• Now if the enclosing transaction is to roll back after the completion of NTA, the dummy CLR ensures that the actions of NTA are not undone. If the failure occurs because its log record was written as undo-redo log records, thus providing the atomicity of NTA. Dummy CLR serves the purpose of commit NTA record and there is nothing to redo.
• Advantages: Enclosing transaction need not wait for this record to be forced to stable storage before proceeding. The cost of initiating a new transaction is saved.

   
  Figure 7: Nested Top Action example
  

• Log record 6' acting as dummy CLR. Even though the enclosing transaction's activity is interrupted by a failure and hence it needs to be rolled back, 6' ensures that the nested top action is not undone.