IBM 000-8697 Manual de usuario descargar pdf (Pagina 641)

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Product Environment 9-21

IBM Informix STAR and Two-Phase Commit Protocol

Example Transaction Commit

Figure 9-4 is a simple representation of a two-phase commit protocol that

results in a transaction being committed. Notice that the coordinator’s

decision to commit or roll back the transaction is stored on stable storage

before the decision is propagated to the participants. This is done to facilitate

recovery. Ifafailureoccursduringthe operation, therecordofthedecision is

needed during recovery to indicate the status of the transaction. The

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

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

record is written to disk.

Figure 9-4

Simple illustration

of a two-phase

commit protocol

that results in a

committed

transaction

All Participants:

Determine that the commit can occur. Record the fact that a commit

can occur. Return message: “can commit.”

Coordinator:

Waits to receive “committed” responses.

Records that the transaction is committed.

Removes transaction entry from shared-memory transaction table.

Releases shared-memory resources.

Coordinator:

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

message to participants.

All Participants:

Commit the piece of work.

Remove transaction entry from shared-memory transaction table.

Release shared-memory resources.

Return conﬁrmation message: “committed.”

Coordinator:

Waits to receive “can commit” responses.

Records decision to commit transaction.

Phase 2 Begins:

Sends a “commit” message to participants.

Start Protocol

End Protocol

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