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In either case, applications should not be allowed to update the
identity columns of a table to ensure that Oracle can identify rows and
preserve the integrity of replicated data.
Conflict Resolution
When a receiving site in an advanced replication system is using
asynchronous row-level replication and it detects a conflict in a transaction, the default behavior is to log the conflict and the entire transaction, and leave the local version of the data intact. In most cases, you should use Oracle’s
advanced replication facility to automate the resolution of replication
conflicts. You can also check each server’s DEFERROR data dictionary view
for transactions that caused conflicts, and resolve them manually, if necessary.
Column Groups
Oracle uses column groups to detect and resolve conflicts during asynchronous, row-level symmetric replication. A column group is a logical grouping of one
or more columns in a table. Every column in a replicated table is part of a
single column group. When configuring replicated tables, you can create
column groups and then assign columns and corresponding conflict
resolution methods to each group.
Each column group in a replicated table can have a list of one or more conflict resolution methods. Indicating multiple conflict resolution methods for a
group allows Oracle to resolve a conflict in different ways should others fail to resolve the conflict. When trying to resolve a conflict for a group, Oracle
executes the group's resolution methods in the order that you list for the
group.
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By default, every replicated table has a shadow column group. A table’s shadow column group contains all columns that are not within a specific column
group. You cannot assign conflict resolution methods to a table's shadow
group.
Conflict Resolution Methods
When designing column groups you can choose from among many built-in
conflict resolution methods. For example, to resolve update conflicts, you might choose to have Oracle overwrite the column values at the destination site with the column values from the originating site. Oracle offers many other update
conflict resolution methods.
However, Oracle offers no delete conflict resolution methods. Consequently,
applications that operate within an asynchronous, shared ownership data
model should avoid delete conflicts by not using DELETE statements to delete
rows. Instead, applications can mark rows for deletion and configure the
system to periodically purge deleted rows using procedural replication.
Unique Advanced Replication Options
Some applications have special requirements of an advanced replication
system. The following sections explain the Oracle unique advanced replication
options, including
•
Procedural Replication
•
Synchronous (Real-Time) Data Propagation
Procedural Replication
Batch processing applications can change large amounts of data within a
single transaction. In such cases, typical row-level replication could saturate a network with a huge quantity of data changes. To avoid such problems, a
batch processing application that operates in an advanced replication
environment can use Oracle's procedural replication to replicate simple stored procedure calls that will converge data replicas. Procedural replication
replicates only the call to a stored procedure that an application uses to update a table. Procedural replication does not replicate data modifications.
To use procedural replication, at all sites you must replicate the packages that modify data in the system. After replicating a package, you must generate a
wrapper for this package at each site. When an application calls a packaged procedure at the local site to modify data, the wrapper ensures that the call is ultimately made to the same packaged procedure at all other sites in the
Database Replication 30-25
replicated environment. Procedural replication can occur asynchronously or synchronously.
Conflict Detection and Procedural Replication
When an advanced replication system replicates data using procedural
replication, the procedures that replicate data are responsible for ensuring the integrity of the replicated data. That is, you must design such procedures
either to avoid or to detect replication conflicts and resolve them
appropriately. Consequently, procedural replication is most typically used
when databases are available only for the processing of large batch operations.
In such situations, replication conflicts are unlikely because numerous
transactions are not contending for the same data.
Additional Information: See Oracle8 Server Replication.
Synchronous (Real-Time) Data Propagation
Asynchronous data is the normal configuration for advanced replication
environments. However, Oracle also supports synchronous data propagation
for applications with special requirements. Synchronous data propagation occurs when an application updates a local replica of a table, and within the same

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