Oracle Distributed Systems

Chapter 13. Updateable Snapshots

Updateable snapshots offer a means of deploying updateable copies of data at multiple sites. Unlike multi-master replication, which maintains copies of all records in a table at multiple sites, updateable snapshots may be partitioned horizontally. Another key difference between updateable snapshots and multi-master table replication is that updateable snapshot sites need not be in constant communication with the master site. Common usages of updateable snapshots include sales lead data on a salesperson’s laptop computer or grocery register receipt data. In both cases, the master data would reside at a headquarters site, and the updateable snapshot might push data back to the headquarters site at the end of the business day.

About Updateable Snapshots

As described in Chapter 11, updateable snapshots function by placing triggers on the master and snapshot tables. Thetrigger on the master table (TLOG$_table_name) populates the snapshot log table (MLOG$_table_name) at the master site. Similarly, the trigger on the snapshot base table (USTRG$_master_table_name ) populates the snapshot log table (USLOG$_table_name) at the snapshot site.

Like their read-only counterparts, updateable snapshots must be refreshed in order to reflect changes that have occurred to the master table. In addition, updateable snapshots must propagate changes from the snapshot site back to the master table. This propagation can happen either at the same time as the snapshot refresh ...