IBM 000-8697 Manual de usuario descargar pdf (Pagina 662)

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9-42 IBM Informix OnLine Database Server Administrator’s Guide

Heuristic End-Transaction

In the ﬁrst case, when you execute tbmode -Z to end a global transaction at

the coordinator OnLinedatabaseserver, you interferewith participant recovery

in the following way. If a participating OnLine database server was down at

the time that the coordinator issued its decision to commit, participant

recovery eventually queries the coordinator for information. Because the

transaction entry has been removed from shared memory at the coordinator

OnLine server, the coordinator is unable to provide information about the

transaction. The participant interprets the lack of information as a evidence

that the coordinator issued a decision to rollback the transaction. The partic-

ipant rolls back its piece of work even though all other participating OnLine

servers committed theirs. This is one way in which an inconsistent database

can develop.

In the second case, when you execute tbmode -Z to end a piece of work that

is in progress at a participant OnLine database server, you interfere with the

protocol in the following way. If the coordinator issues a decision to commit,

it waitsfor acknowledgment from allparticipants that the commit occurred.

WhenthecoordinatorOnLinedoesnot receiveanacknowledgmentfromthe

participant OnLine server where the tbmode -Z occurred, it investigates the

situation. The coordinator queries the participant OnLine server, which no

longer has information about the transaction. The lack of a transaction table

entry at the participant OnLine server is taken as evidence that the trans-

action committed. The coordinator OnLine assumes that the

acknowledgment message was sent from the participant, but somehow it

wasnotreceived.Becausethecoordinatordoesnotknowthatthisparticipant’s

piece of work did not commit, it does not generate messages indicating that

the global transaction was inconsistently implemented.

Only the administrator who executed the tbmode -Z command is aware of

the inconsistent implementation. (The complete protocol is illustrated in

Figure 9-12 on page 9-50.)

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