9-42 IBM Informix OnLine Database Server Administrator’s Guide
Heuristic End-Transaction
In the first case, when you execute tbmode -Z to end a global transaction at
thecoordinator OnLine database server, you interfere with participant recovery
inthe following way. If a participating OnLine database server was down at
the time that the coordinator issued its decision to commit, participant
recovery eventually queries the coordinator for information. Because the
transactionentry has been removed from shared memory at the coordinator
OnLine server, the coordinator is unable to provide information about the
transaction. The participant interprets the lack of information as a evidence
that the coordinator issued a decision to rollback the transaction. The partic-
ipantrolls back its piece of work even though all other participating OnLine
serverscommitted theirs. This is one way in which an inconsistent database
can develop.
Inthe second case, when you execute tbmode -Z to end a piece of work that
is in progress at aparticipant OnLine database server, you interfere with the
protocol in the following way. If the coordinator issues a decision to commit,
itwaits for acknowledgment from all participants that the commit occurred.
Whenthe coordinator OnLine does not receive an acknowledgment from the
participantOnLine server where the tbmode -Z occurred, it investigates the
situation. The coordinator queries the participant OnLine server, which no
longerhas information about the transaction. The lack of a transaction table
entry at the participant OnLine server is taken as evidence that the trans-
action committed. The coordinator OnLine assumes that the
acknowledgment message was sent from the participant, but somehow it
wasnot received. Because the coordinator does not know that this participant’s
pieceof work did not commit, it does not generate messages indicating that
the global transaction was inconsistently implemented.
Only the administrator who executed thetbmode -Z command is aware of
the inconsistent implementation. (The complete protocol is illustrated in
Figure9-12 on page 9-50.)