How Tags are used in Streams

In a Streams-based replication configuration, there is often a need to define bi-directional or multi-way replication of specific tables. Though the configuration of such multi-way replication is fairly simple, the management of data flows within this environment gets complicated due to conflicts. The following section provides information on multi-way replication issues and the conflict resolution methods.

Multi-Way Replication and Conflict Resolution

In a bi-directional configuration, a typical table is replicated in both ways. For example, table ORDERS at the NY site is replicated to London site. And again, any changes made on the ORDERS table at the London site are replicated to the ORDERS table at the NY site. When this is extended to additional sites, it is known as multi-way replication.

An important issue that must be dealt with in multi-way Streams replication is conflict resolution. Oracle Streams provides many useful built-in features that are intended to detect, avoid, and resolve the conflict.

Before examining the conflict resolution methods, it will be useful to detail what kinds of conflicts are possible in a typical bi-directional or multi-way replication.

• Update Conflict: This occurs when a specific row is updated with different values in two sites at almost the same time. When the row LCR arrives to the destination, it finds that the column’s old value does not match that of the source. This transaction typically gets into an error queue. This situation is called an update conflict.