About Rule-Based Transformations for Nonidentical Copies

A rule-based transformation is an additional configuration option that provides flexibility when a database object is not identical at the different databases. Rule-based transformations modify changes to a database object so that the changes can be applied successfully at each database. Specifically, a rule-based transformation is any modification to a message when a rule in a positive rule set evaluates to TRUE.

For example, suppose a table has five columns at the database where changes originated, but the shared table at a different database only has four of the five columns. When a data manipulation language (DML) operation is performed on the table at the source database, the row changes are captured and formatted as row LCRs. A rule-based transformation can delete the extra column in these row LCRs so that they can be applied successfully at the other database. If the row LCRs are not transformed, then the apply process at the other database will raise errors because the row LCRs have an extra column.

There are two types of rule-based transformations: declarative and custom. Declarative rule-based transformations include a set of common transformation scenarios for row changes resulting from DML changes (row LCRs). Custom rule-based transformations require a user-defined PL/SQL function to perform the transformation. This guide discusses only declarative rule-based transformations.

The following declarative rule-based transformations are available:

• An add column transformation adds a column to a row LCR.

• A delete column transformation deletes a column from a row LCR.

• A keep columns transformation keeps a list of columns in a row LCR, while deleting the columns that are not in the list.

• A rename column transformation renames a column in a row LCR.

• A rename schema transformation renames the schema in a row LCR.

• A rename table transformation renames the table in a row LCR.

When you add one of these declarative rule-based transformations, you specify the rule to associate with the transformation. When the specified rule evaluates to TRUE for a row LCR, Oracle Streams performs the declarative transformation internally on the row LCR. Typically, rules and rule sets are created automatically when you configure your Oracle Streams replication environment.

You can configure multiple declarative rule-based transformations for a single rule, but you can only configure one custom rule-based transformation for a single rule. In addition, you can configure declarative rule-based transformations and a custom rule-based transformation for a single rule.

A transformation can occur at any stage in the Oracle Streams information flow: during capture, propagation, or apply. When a transformation occurs depends on the rule with which the transformation is associated. For example, to perform a transformation during propagation, associate the transformation with a rule in the positive rule set for a propagation.