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by Burleson Consulting

The Data Warehouse Development Life Cycle

Distributed Oracle Data WarehousesMaster-Slave Oracle Replication

Using Oracle snapshots to create master-slave replication has become a very popular method for distributing warehouse data. In order to create a master-slave configuration with Oracle, the following three step are required:

1. Define and populate the slave database, using copies of the table descriptions from the master catalog tables. This is done with the CREATE SNAPSHOT command.

2. Create the propagation routines on the host database, and establish host gateways into the slave databases. This is done with the REFRESH clause of the CREATE SNAPSHOT statement.

3. Define snapshot log tables to hold updates that will be propagated from the original table to the replicated table.

The foremost concern when creating snapshots is the time interval in which the replicated tables will be refreshed. Data warehouse developers use database replication because they cannot afford the overhead of instant database updates (the two-phase commit issue). When using replication, developers must determine the length of the time lag between master database updates and slave table updates. Some data warehouse developers choose to allow updates to occur at predefined time intervals (say, each day), ensuring that updates occur when the time interval is reached. Another approach is to base slave table updates on the level of activity in the slave databases. A predefined threshold can be defined to start the slave updates when all of the slave databases fall below the activity threshold. This method allows slave tables to be updated when they are not busy, but, on the downside, end users can never be sure about the currency of their slave database. One interesting approach to this problem is to have each slave database automatically poll the master when it is not busy to see if there are any updates awaiting propagation.

This is an excerpt from "High Performance Data Warehousing".
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