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 History Management (liveCache) 

History management is responsible for the storing undo log files and later deleting them when they are no longer required. History management also ensures that transactions are able to read data consistently without locking.

Only SAP liveCache instances have a history management. Consistent reading for OLTP instances is achieved by selecting the appropriate isolation level.

Before a user task carries out a modifying SQL statement in a transaction, it writes undo log entries into an undo log file. With OLTP database instances, undo log files are deleted at the end of the transaction. With liveCache database instances, however, the user task transfers them to history management.

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       1.      At the end of a transaction, the user task writes a reference to its undo log file in a history file.

       2.      If no more applications need to access the data, for which details of the modifications made to it have been stored in the undo log file, the garbage collector reads the storage location of the undo log file from the history file and deletes the corresponding reference.

       3.      The garbage collector deletes the undo log file and any data records that the user task has marked for deletion.

See also:

Using Log Entries at Restart or Recovery

Example: Reading Data Consistently Without Locking

 

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