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Process documentation Repeating Transactions Locate the document in the library structure

Purpose

After a system breakdown, you have to restore your database instance or perform a restart. After the database system has analyzed the log information, it can repeat the required transactions.

Process Flow

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       1.      The redo log manager restarts the user tasks for the transactions listed in the transaction list.

       2.      The redo log manager starts the redo tasks.

       3.      The redo tasks analyze the redo list and read the redo and undo log entries for the transactions listed in the redo list.

       4.      The redo tasks repeat the transactions since the last savepoint. They proceed as follows:

Transactions That Were Closed with a COMMIT

                            a.      The redo tasks search in the redo list for transactions that were closed with a COMMIT and need to be repeated. The transactions are sorted in the redo list by the sequence number of their COMMIT.

                            b.      The first redo task begins by processing the transaction with the smallest COMMIT sequence number. The redo log entries belonging to this transaction are processed sequentially. A redo log entry in this redo log file with a higher log sequence number than the smallest COMMIT sequence number must not be processed. In this case, the redo task waits for the COMMIT of the corresponding other transaction. Only then can the redo task continue processing the redo log file. In this way, the database system ensures data consistency.

                            c.      Once a COMMIT transaction has been completely repeated, the redo task deletes the redo log file and transfers the undo log file to the history management. Then the redo log manager removes the transaction from the redo list.

Transactions That Were Canceled with a ROLLBACK

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                            a.      The redo tasks search in the redo list for transactions that were canceled with a ROLLBACK and delete the corresponding redo log files. This prevents transactions from being repeated unnecessarily.

                            b.      If the start of a ROLLBACK transaction was before the savepoint, then a redo task analyzes the undo log file of the transaction to return the transaction to the status before the savepoint. Once the ROLLBACK transaction has been fully processed, the undo log file of the transaction is deleted.

If the start of a ROLLBACK transaction was after the savepoint, then the redo task does not process the transaction further.

See also:

Using Log Entries for a Restart or Restore

 

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