User's Manual

Chapter 10. SQL Remote Administration
The message tracking system
SQL Remote has a message tracking system to ensure that all replicated
operations are applied in the correct order, no operations are missed, and no
operation is applied twice.
Message system failures may lead to replication messages not reaching their
destination, or reaching it in a corrupt state. Also, messages may arrive at
their destination in a different order from that in which they were sent. This
section describes the SQL Remote system for detecting and correcting
message system errors, and for ensuring correct application of messages.
If you are using an e-mail message system, you should confirm that e-mail is
working properly between the two machines if SQL Remote messages are
not being sent and received properly.
The SQL Remote message tracking system is based on status information
maintained in the remoteuser SQL Remote system table. The table is
maintained by the Message Agent. The Message Agent at a subscriber
database sends confirmation to the publisher database to ensure that
remoteuser is maintained properly at each end of the subscription.
For Adaptive Server Anywhere, the remoteuser table is the
sys.sysremoteuser system table. For Adaptive Server Enterprise, this is the
sr_remoteuser table.
Status information in the remoteuser table
The remoteuser SQL Remote system table contains a row for each
subscriber, with status information for messages sent to and received by that
subscriber. At the consolidated database, remoteuser contains a row for
each remote user. At each remote database, remoteuser contains a single
row maintaining information for the consolidated database. (Recall that the
consolidated database subscribes to publications from the remote database.)
The remoteuser SQL Remote system table at each end of a subscription is
maintained by the Message Agent.
Tracking messages by transaction log offsets
The message-tracking status information takes the form of offsets in the
transaction logs of the publisher and subscriber databases. Each COMMIT is
marked in the transaction log by a well-defined offset. The order of
transactions can be determined by comparing their offset values.
Message ordering When messages are sent, they are ordered by the offset of the last COMMIT
of the preceding message. If a transaction spans several messages, there is a
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