IBM 000-8697 Manuel d'utilisateur télécharger pdf (Page 642)

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9-22 IBM Informix OnLine Database Server Administrator’s Guide

IBM Informix STAR and Two-Phase Commit Protocol

Example Transaction Rollback

Figure 9-5 is a simple representation of a two-phase commit protocol that

results in a transaction being rolled back. Notice that the participants do not

senda conﬁrmation tothe coordinatorwhen the pieceofworkisrolledback.

The coordinator does not keep a record that a transaction completed. The

beginning of phase two is considered to be the instant at which the coordi-

nator records its decision—in this case, when the roll back work logical log

record is written to disk.

Figure 9-5

Simple illustration

of a two-phase

commit protocol

that results in a

transaction being

rolled back

End Protocol

Start Protocol

P1 P2 P3

P1 Participant:

Determines that the commit cannot occur.

Returns message: “cannot commit.”

P2 and P3 Participants:

Determine that the commit can occur.

Record the fact that a commit can occur.

Return message: “can commit.”

Coordinator:

Records decision to begin precommit phase. Sends a “prepare”

message to participants.

All Participants:

Roll back piece of work.

Remove transaction entry from shared-memory transaction table.

Release shared-memory resources.

Coordinator:

Receives “cannot commit” response.

Records decision to roll back the transaction.

Phase 2 Begins:

Rolls back work on behalf of transaction.

Removes transaction entry from shared-memory transaction table.

Releases shared-memory resources.

Sends a “roll back” message to participants.

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