History management is responsible for temporarily saving undo log files of liveCache database instances and deleting them later. History management enables consistent reading without locking.
Only SAP liveCache instances have history management. You can achieve consistent reading for OLTP instances by choosing the appropriate isolation level.
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1. At the end of the transaction, the user task writes a reference to its undo log file in a history file.
2. History management assigns a garbage collector to the history file. The garbage collector processes the history file sequentially and asks the history management which undo log files may be deleted from the history file that has been assigned to it. History management sends a request to the lock administration, which in turn checks whether the corresponding transactions still are used in a consistent view.
3. If no other applications have to access the data whose changes are saved in the undo log file, the garbage collector imports the log entries in the undo log file; if, for example, deletion requests are found, the corresponding objects or object files are deleted from the data area. After the complete undo log file was processed in this way, the reference to the undo log file in the history file is deleted and the undo log file itself is deleted.
Example: Reading Consistent Data Without Locking