IBM 000-8697 Manual do Utilizador descarregar pdf (Página 673)

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Product Environment 9-53

Determining Database Consistency

Step 2: Determine If the Networked Database Contains Inconsistent

Data

Suppose that you have determined that a transaction was inconsistently

implemented and that one or more participants committed the transaction

while one or more participants rolled it back. What does this mean to your

networked database system? When does this cause problems with data

integrity?

A transaction that is inconsistentlyimplemented causesproblemswhenever

the piece of work rolled back by one participant is dependent on a piece of

workthatwasupdatedbyanotherparticipant.Itisimpossibletodeﬁnethese

dependencies with SQL because distributed transactions do not support

constraints that reference data at multiple database servers. The pieces of

work are independent (no dependencies exist) only if the data could have

been updated in two independent transactions. Otherwise, the pieces of

work are considered to be dependent.

Beforeproceedingany further, considerthetransactionthatcausedthe error.

Are the pieces of data that were updated and rolled back dependent on one

another? Multiple updates might be included in a single transaction for

reasons other than maintaining data integrity. For example, here are three

possible reasons:

■ Reduced transaction overhead

■ Simpliﬁed coding

■ Programmer preference

Verifyalso thatevery participant OnLinethat isassumed to havecommitted

thetransaction actually modiﬁeddata. It might bethataread-onlyOnLineis

listed as a participant that committed a transaction.

Ifaninconsistenttransactiondoesnotleadtoaviolationofdataintegrity,you

can quit the procedure at this point.

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