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November 02, 2018

Fine-Tune: Ensure Logical Data Consistency

Database_design_concepts
NewsWire Classic

The MPE/iX Transaction Manager for IMAGE does not guarantee logical consistency of your data. How do you ensure logical consistency? Use DBXBEGIN and DBXEND calls around all the DBPUT, DBUPDATE and DBDELETE calls that you make for your logical transaction. Yes, the definition of a logical transaction is up to the programmer.

There can be a lot of confusion about logical consistency, mostly because IMAGE kept adding logging and recovery features over its years of development. Gavin Scott gives a clear explanation of the state of affairs.

It’s amazing how much superstition exists surrounding this kind of stuff, and how many unnecessary rituals and sacrifices are performed daily to appease the mythical pantheon of data integrity gods. Real broken chains are supposed to be impossible to achieve with IMAGE on MPE/iX, no matter what application programs do, or how they are aborted, or how many times the system crashes!

The Transaction Manager provides absolute protection against internal database inconsistencies, as long as there are no bugs in the system and as long as the hardware is not corrupting data. No action or configuration is required on the part of the user.

Logical inconsistencies (order detail without an associated order header record, for example) can easily be created by aborting an application that’s in the middle of performing a database update that spans multiple records. Of course, IMAGE doesn’t care whether your data is logically correct or not, that’s the job of application programmers.

Using DBBEGIN/DBEND will have no effect whatsoever on logical integrity, unless you actually run DBRECOV to roll forward or roll back the database to a consistent point every time you abort a program or suffer any other failure.

By using DBXBEGIN/DBXEND XM style transactions, you can extend IMAGE’s guarantee of physical integrity to the logical integrity of your database. The system will ensure that no matter what happens, either all changes inside a DBX transaction will be applied, or none of them will be. Of course, it’s still possible to use this feature incorrectly (locking strategies are non-trivial as you need to lock the data that you read as well as that which you intend to write in many cases).

HP introduced a feature, far back in the MPE V days, called Intrinsic-Level Recovery (ILR). ILR can still can be enabled for a database. This was sort of a mini-XM that forced updates to disk each time an Intrinsic call completed in order to ensure structural integrity of the database in the face of system failures.

I believe that on MPE/iX, enabling ILR for a database does something really nasty like forcing an XM post after every update intrinsic call, which is a serious performance problem. ILR is no longer required on MPE/iX as XM will ensure integrity without it. With ILR you might be guaranteed that every committed transaction will survive a system abort, whereas without it XM might end up having to roll back the last fraction of a second’s worth of transactions. For almost any application this difference is negligible. Do not turn ILR on!

There are more complexities if your application performs transactions that affect multiple databases or databases and non-database files. It’s possible to do multi-database IMAGE transactions, but only if the databases reside on the same volume set, I believe.

01:44 PM in Hidden Value, Homesteading | Permalink

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