Download IBM Informix Dynamic Server Enterprise Replication Guide

Log-Based Data Capture
The update-anywhere model provides the greater challenge in asynchronous
replication. For example, if a replication system contains three replication
sites that all have read and write capabilities, conflicts occur when the sites
try to update the same data at the same time. Conflicts must be detected and
resolved so that the data elements eventually have the same value at every
site. For more information, see “Conflict Resolution” on page 3-10.
Log-Based Data Capture
Enterprise Replication uses log-based data capture to gather data for replication. Enterprise Replication reads the logical log to obtain the row images
for tables that participate in replication and then evaluates the row images.
Log-based data capture takes changes from the logical log and does not
compete with transactions for access to production tables. Log-based datacapture systems operate as part of the normal database-logging process and
thus add minimal overhead to the system.
Two additional methods of data capture, which Enterprise Replication does
not support, include:
■
Trigger-based data capture
A trigger is code in the database that is associated with a piece of
data. When the data changes, the trigger activates the replication
process.
■
Trigger-based transaction capture
A trigger is associated with a table. Data changes are grouped into
transactions and a single transaction might trigger several replications if it modifies several tables. The trigger receives the whole
transaction, but the procedure that captures the data runs as a part of
the original transaction, thus slowing down the original transaction.
About IBM Informix Enterprise Replication 1-5