IBM 000-8697 Manuel d'utilisateur télécharger pdf (Page 650)

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9-30 IBM Informix OnLine Database Server Administrator’s Guide

Heuristic Decisions: What and Why

Independent action in and of itself does not create the need for manual

recovery. For example, if a piece of work at a participant database server is

rolled back because it developed into a long transaction and the coordinator

issues a decision to roll back the global transaction, the complete database

remains consistent and there is no problem.

However, within the context of two-phase commit protocol, some

independent actionscan developinto heuristicdecisions. (Refer to page 9-30.)

Independent actions create a problem only when both of two conditions are

true:

■ The participant OnLine already sent a “can commit” message to the

coordinator.

■ The coordinator’s decision is to commit the transaction.

When both conditions are true, the net result is a global transaction that is

inconsistently implemented (committed by one or more OnLine database

servers and rolled back by another). The database system becomes incon-

sistent. An inconsistent database system requires manual recovery, if you

decide that the best course of action is to return the database system to a

consistent state. This might notnecessarily bethe case.A description of how

to make this determination, and how to manually recover from an incon-

sistent global transaction, is provided on page 9-51.

Heuristic Decisions: What and Why

Within the two-phase commit protocol, two independent actions take on

special signiﬁcance:

■ A heuristic rollback is an independent action taken by either tbinit or

the administrator to rollback a pieceof work that has already sent a

“can commit” message.

■ A heuristic end-transaction is an independent action taken by the

administrator to roll back a piece of work and remove all information

about the transaction from the OnLine shared-memory transaction

table.

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IBM 000-8697 Manuel d'utilisateur Page 650

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