Replication Guide and Reference

Data Consistency Requirements

IBM Replication always guarantees table consistency, that is, all data in the target table is actually committed in the base table. IBM Replication also provides consistency across sets of tables with the replication subscription, which can have multiple, related members. These members are each source and target pairs and the members are usually bound by transaction relationships.

The replication subscription helps maintain referential constraints that exist among a set of replication source and replica tables. The replication subscription also limits the boundary for cascade rejection due to RI violation or update collision. Finally, replication subscriptions keep the source-to-target definitions for all the components of a view target table together. The replication subscription should contain all target tables that are related, such as with RI constraints.

Replication Subscription Rules

The following section describes the rules and constraints you need to be aware of in planning and defining a replication subscription.

• If any member of the replication subscription requires full-refresh copying for any reason, the entire set is full refreshed. Full-refresh copying occurs only from the replication source to the replica, not the other way around.

• Each replica target table must be of the same generation as all the other replicas in the replication subscription and come from the same replication source table.

• A single synchpoint is maintained for the replication subscription to indicate the copy progress for the entire replication subscription.

• Collision detection levels must be specified while defining a replication source and affects the processing performed on a replica table in the replication subscription.

• Target tables with RI constraints must be full-refreshed by an outside means using the ASNLOAD exit to bypass RI checking.

• RI violations cannot be detected by application logic in an update-anywhere environment. Declarative RI constraints must be used.

• Referential constraints should not be defined for read-only target tables.

• All referential constraints that exist among the source tables should be included in the replication subscription replicas to prevent RI violations.

• The first occurrence of any RI violation terminates the current replication cycle. The subscription cycle will be retried automatically, after the transaction is rejected and compensated.

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