User Guide

SAP AG November 2002

Redo List

The redo list is an internal database list of transactions [Page 152], which is in the main

memory. The redo list is created by the redo log manager [Page

49].

If, during a restart or recovery [Page 49], the log reader [Page 50] finds a COMMIT [Page

129] in a redo log file [Page 50] for the whole transaction, this transaction is entered in the

redo list.

The redo tasks [Page

51] use the redo list to determine the transactions that need to be

repeated, and the sequence in which they are to be executed. The sequence for executing

the transactions, defined in the redo list, is important to ensure that the same data is not

changed simultaneously.

Redo Task

Redo tasks are active components during a restart or recovery [Page 49]. Redo tasks are

server tasks [Page

16].

Actions

COMMIT transactions

1. In the redo list [Page 51], the redo tasks look for transactions [Page 152] that were

completed with a COMMIT [Page 129], and are therefore ready to be re-imported. In the

redo list, the transactions are sorted by the log sequence numbers of their COMMIT.

2. The redo task starts by processing the transaction with the lowest COMMIT log sequence

number.

The redo log files [Page 50] that belong to this transaction are processed in sequence.

If a redo log entry [Page 44] of this redo log file has a higher log sequence number than

the lowest COMMIT log sequence number, it cannot be processed. In this case, the redo

task waits for the COMMIT of the other transaction. Only then can the redo task continue

processing the redo log file.

This mechanism ensures consistent data.

3. Once a COMMIT transaction has been processed in full, the redo task deletes the redo

log file and the undo log file [Page

48] of the transaction. The transaction is removed from

the redo list.

ROLLBACK transactions

1. In the redo list, the redo tasks look for transactions that were completed with a

ROLLBACK [Page 147].

2. The redo tasks delete the relevant redo log files. This prevents the transaction being

redone unnecessarily.

3. If the ROLLBACK transaction started before the savepoint, the redo task evaluates the

undo log file of the transaction, in order to set the transaction to the state before the

savepoint. Once the ROLLBACK transaction has been fully processed, the redo task

deletes the undo log file of the transaction.

If the ROLLBACK transaction started after the savepoint [Page 149], the redo task does

not process the transaction further.