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System Level Consistency

System level consistency is a promise given by the transactional system that the execution of transactions that uphold certain requirements (as a minimum they should be correct, i.e., operation level consistent) leave an initially correct system state into another correct system state.

If the system guarantees that transactions are atomic, isolated, and durable then system level consistency is no more than operation level consistency--if transactions that produce correct result when executed alone, are executed as if they were alone, then they will produce correct results.

If, for instance, the system does not guarantee isolation, additional requirements^4.3may be put on the transactions for the system to make any promises. In ORACLE transactions are not isolated and additional locking is required [37]:

"SELECT FOR UPDATE [that acquires explicit row locks] is recommended when you need to lock a row without actually changing it. For example, if you intend to base an update on the existing values in a row, you need to make sure the row is not changed by someone else before your update."

Footnotes

... requirements^4.3

In addition to being operation level consistent.

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