IBM 000-8697 Uživatelský manuál stáhnout pdf (Strana 640)

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9-20 IBM Informix OnLine Database Server Administrator’s Guide

IBM Informix STAR and Two-Phase Commit Protocol

During the postdecision phase the coordinator directs each database server to

either commit the changes or roll back the transaction. If the coordinator

directsthe participantstocommitthetransaction,itwaitstoreceiveacknowl-

edgments from each before ending the global transaction. If the coordinator

directsthe participants to roll backthe transaction, noacknowledgments are

expected from the participants.

Informix has implemented the two-phase commit protocol with the

presumed-abort optimization, which allows faster processing because some of

thelogicallogrecordswrittenduringtheprotocolcanbebuffered.Theimpli-

cation of the optimization for automatic participant recovery is described as

part of the discussion of automatic recovery that begins on page 9-23. The

speciﬁc discussion of presumed-abort optimization begins on page 9-29.

Protocol Components

The two-phase commit protocol relies on two kinds of communication

messages and logical log records:

■ Messages

Messagesmustpassbetweenthecoordinatorandeachparticipantto

ensure transaction atomicity (that is, all-or-nothing changes). Mes-

sages from the coordinator include a transaction identiﬁcation

numberandinstructions(suchas“preparetocommit,”“commit,”or

“rollback”). Messagesfrom each participant include the transaction

status and reports of action taken (such as “can commit,” “cannot

commit,” “committed,” or “rolled back”).

■ Logical log records

Logicallog recordsofthetransactionarekeptonstable storage (disk

or tape) to ensure data integrity and consistency, even if a failure

occursataparticipatingOnLinedatabaseserver(participantorcoor-

dinator). For more details about automatic two-phase commit

recovery in the event of failure, refer to page 9-23. For more details

about the logical log records that are written during the two-phase

commit protocol, refer to page 9-44.

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