IBM 000-8697 Bedienungsanleitung PDF herunterladen (Seite 644)

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9-24 IBM Informix OnLine Database Server Administrator’s Guide

Two-Phase Commit and Automatic Recovery

Coordinator recovery is not needed if the following occurs:

■ The coordinator has not recorded a decision.

■ The coordinator’s decision is to roll back the transaction.

Ifeitherone of these twoconditionsis true,the situation is handledbypartic-

ipant recovery.

Coordinator failure can occur as a result of either a system failure or the

isolatedfailureof the coordinatordatabaseserver process. If asystemfailure

occurred, the administrator reinitializes shared memory and restarts the

sqlexecd process. (Refer to page 9-23.) Otherwise, coordinator recovery

begins as part of OnLine processing.

As part of its regular activity, the tbinit process at the coordinator OnLine

detects ﬁrst that a two-phase commit protocol was underway; second, that

the coordinator had reached a decision to commit; and third, that the coordi-

nator was terminated prematurely. To complete the transaction, tbinit forks

a database server process. (The tbinit process reads the value of SQLEXEC to

fork a new database server process. If SQLEXEC is not set, tbinit uses the

pathname $INFORMIXDIR/lib/sqlturbo. The database server process is

owned by the user who started tbinit, either informix or root.)

Thisserverprocessactsasthenewcoordinator.Itforksanewdatabaseserver

process at each participant OnLine. (To do this, an sqlexecd processmust be

running at each participant OnLine.)

Thenew coordinatorsends a messagetoeachparticipant to obtainthestatus

of its piece of work. If the participant received a commit message from the

coordinator before it failed, the participant would have committed the piece

of work, removed the entry from its transaction table, and would have no

knowledge of the transaction. Thus, if the new coordinator receives a

messagethatthetransactionstatusisunknown,it knows the piece of work is

committed.

If the new coordinator receives a message that the transaction status is “can

commit,”amessagetocommitissenttotheparticipant.Afterallparticipants

sendanacknowledgmentindicatingthattheirpiecesofworkcommitted,the

coordinator ends the transaction.

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