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This graphic is explained in the accompanying text Example: Reading Consistent Data Without Locking (liveCache) Locate the document in its SAP Library structure

Undo log entries are used by liveCache database instances so that several transactions can access the same data record at the same time and read it consistently, without the data record having to be locked for one of the transactions.

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       1.      Transaction T1 accesses data record 3600 for the first time in table CUSTOMER and reads the value Smiht as the name. Transaction T1 does not lock the data record, however.

       2.      Transaction T2 corrects the name from Smiht to Smith in data record 3600. Transaction T2 is ended with a COMMIT.

       3.      Transaction T1 accesses data record 3600 again and first checks in the history file whether a different transaction has changed data record 3600 since the last read.

As the data record was changed in the meantime by transaction T2, transaction T1 uses the undo log entries for transaction T2 to obtain the same result for the second read as for the first read. Without redo log entries, transaction T1 would obtain different results for the first and second read of data record 3600.

This graphic is explained in the accompanying text

       4.      The garbage collectors do not delete the undo log entries from transaction T2 until transaction T1 is ended with a COMMIT.

Note

Only liveCache database instances can read consistently without locking in this way. For OLTP instances data record 3600 would have to be locked until transaction T1 is ended. You can achieve this using an appropriate selection of the isolation level.

 

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