Example: Consistent View

Consistent views are only available for SAP liveCache database instances. You configure transaction consistency in OLTP (MaxDB) database instances through the isolation level.

liveCache database instances use undo log entries to enable consistent views of the data. The following example shows how two transactions simultaneously can have consistent views of the same data record.

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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 yet, 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 first 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 reads of data record 3600.

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

See also:

History Management

Logging Data Changes