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 serial number within the transaction to order the messages correctly. The default maximum message size is 50,000 bytes, but you can use the Message Agent -l option to change this setting.

Sending messages

The log_sent column holds the local transaction log offset for the latest message sent to the subscriber. When the Message Agent sends a message, it sets the log_sent value to the offset of the last COMMIT in the message. Once the message has been received and applied at the subscribed database, confirmation is sent back to the publisher. When the publisher Message Agent receives the confirmation, it sets the confirm_sent column for that subscriber with the local transaction log offset. Both log_sent and confirm_sent are offsets in the local database transaction log, and confirm_sent cannot be a later offset than log_sent.

Receiving messages

When the Message Agent at a subscriber database receives and applies a replication update, it updates the log_received column with the offset of the last COMMIT in the message. The log_received column at any subscriber database therefore contains a transaction log offset in the publisher database's transaction log. After the operations have been received and applied, the Message Agent sends confirmation back to the publisher database and also sets the confirm_received value in the local SYSREMOTEUSER table. The confirm_received column at any subscriber database contains a transaction log offset in the publisher database's transaction log.

Subscriptions are two-way

SQL Remote subscriptions are two-way operations: each remote database is a subscriber to publications of the consolidated database and the consolidated database subscribes to a matching publication from each remote database. Therefore, the remoteuser SQL Remote system tables at the consolidated and remote database hold complementary information.

The Message Agent applies transactions and updates the log_received value atomically. If a message contains several transactions, and a failure occurs while a message is being applied, the log_received value corresponds exactly to what has been applied and committed.

Resending messages

The remoteuser SQL Remote table contains two other columns that handle resending messages. The resend_count and rereceive_count columns are retry counts that are incremented when messages get lost or deleted for some reason.

In general, the log_send column has the same value as the log_sent column. However, if the log_send has a value that is greater than log_sent, the Message Agent sends messages to the subscriber immediately on its next run.