IBM 000-8697 Bedienungsanleitung PDF herunterladen (Seite 667)

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Product Environment 9-47

Two-Phase Commit and Logical Log Records

The coordinator’s commit work record (COMMIT record) contains all infor-

mation needed toinitiate the two-phasecommit protocoland itserves as the

starting point for automatic recovery in the event of a failure on the coordi-

nator’s host machine. Because this record is critical to recovery, it is not

allowed to remain in the logical log buffer. The coordinator must immedi-

ately ﬂush the COMMIT logical log record.

The participants in Figure 9-10 must immediately ﬂush both the PREPARE

and the COMMIT logical log records. Flushing the PREPARE record ensures

that, ifthe participant’s host machine fails, fast recovery is able todetermine

that this participant is part of a global transaction. As part of recovery, the

participant might query the coordinator to learn the ﬁnal disposition of this

transaction.

Flushing the participant’s COMMIT record ensures that, if the participant’s

host machine fails, the participant has a record of what action it took

regarding the transaction. To understand why this is crucial, consider the

situation in which a participant crashes after the PREPARE record is written

but before the COMMIT record ﬂushes. After fast recovery, the PREPARE

recordisrestoredbuttheCOMMITrecordislost(sinceitwasinthelogicallog

buffer at the time of the failure). The existence of the PREPARE record would

initiateaquerytothecoordinatoraboutthetransaction.However,thecoordi-

nator would know nothing of the transaction since it ended the transaction

afteritreceivedtheparticipant’s acknowledgment that the commit occurred.

In this situation, the participant would interpret the lack of information as a

ﬁnal direction to roll back the transaction. The two-phase commit protocol

requirestheparticipantCOMMITrecordtobeﬂushedimmediatelytoprevent

this kind of misunderstanding.

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