IBM 000-8697 Manual de usuario descargar pdf (Pagina 652)

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9-32 IBM Informix OnLine Database Server Administrator’s Guide

Heuristic Decisions: What and Why

Once a heuristic rollback or end-transaction occurs, you have started down

theroadtowardmanualrecovery,a complex and time-consumingprocess.It

isimportantforyoutofullyunderstandheuristicdecisions,asthatisthebest

means for avoiding them. You should always be wary of executing

tbmode -z or tbmode -Z within the context of two-phase commit.

Figure 9-8on page 9-33and Figure 9-9 onpage 9-35 illustratethe key charac-

teristicsofaheuristicrollbackandaheuristicend-transaction,respectively.A

general discussion of a heuristic rollback begins on page 9-32. A more

detailed discussion, including why the rollback might occur and the

messages that are generated, begins on page 9-36. A general discussion of a

heuristic end-transaction begins on page 9-32. A more detailed discussion,

including why you might take this action and its consequences, begins on

page 9-40.

Two points about the heuristic rollback bear emphasis.

The ﬁrst point is that a heuristic rollback does not end or close the trans-

action.Ifthepieceofworkiscreatingalong-transactioncondition,theaction

of rolling back the piece of work (whether done by tbinit or by you, the

administrator), does not solve the problem. This is a key difference between

thetwo-phasecommitprotocolandthestandardsituationinasingleOnLine

environment. The transaction remains open, and the logical log records

associatedwiththetransactionremainopen.Logicallogﬁlescannotbefreed.

The transaction ends only after the coordinator receives information about

the rollback from the participant, and, in response, directs the participant to

end the transaction. This additional instruction from the coordinator

removes the entry from the participant’s transaction table, ends the piece of

work,andenablesthelogicallogrecordsassociatedwiththe piece ofworkto

be closed.The complete protocol is illustrated in Figure 9-11 on page 9-49.

Thesecondpointisthataheuristicrollbackleavesthetransactionentryinthe

participant’sshared-memorytransactiontable. This is another way of saying

thatthetransactionremainsopenoractive.Eventually,whenthecoordinator

investigates why the participant OnLinedid not acknowledgethat the piece

of work committed, the entry in the transaction table is interpreted by the

coordinatorasevidenceofaheuristicrollback.Asmentionedintheprevious

paragraph,thecoordinatorrespondstotheheuristicrollbackbydirectingthe

participant to end the transaction.

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