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Function documentation Repeating Transactions Locate the document in its SAP Library structure

Use

After a system failure you have to restore your database instance or perform a restart. After the database system has analyzed log information, it can repeat the transaction since the last savepoint.

Activities

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       1.      The redo log manager restarts the user tasks of the transactions from 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:

For COMMIT transactions:

¡        The redo tasks search in the redo list for transactions that were completed with a COMMIT and are therefore ready to be imported again. The transactions are sorted in the redo list by the log sequence number of their COMMIT.

¡        The first redo task starts processing the transaction with the smallest COMMIT log sequence number. The redo log file belonging to this transaction is processed sequentially. A redo log entry in this redo log file with a higher log sequence number than the smallest COMMIT log 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. This mechanism ensures the consistency of the data.

¡        If a COMMIT transaction was completely processed, a redo task deletes the redo log file and, if it is an OLTP database instance, it also deletes the undo log file of the transaction. For liveCache database instances, it passes the undo log file to the history management. The transaction is then removed from the redo list.

For ROLLBACK transactions:

¡        The redo tasks search in the redo list for transactions that were completed with a ROLLBACK.

¡        The redo tasks delete the corresponding redo log files. This avoids transactions from being repeated unnecessarily.

¡        If the start of a ROLLBACK transaction was before the savepoint, then the redo task analyzes the undo log file of the transaction in order to transfer the transaction to the status it had before the savepoint.

After the ROLLBACK transaction was processed completely, the redo task deletes the undo log file of the transaction.

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

 

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