BDR provides facilities for users to override the default last-update-wins data row conflict resolution strategy on row key conflicts.
A user defined conflict handler, if provided, is called before default row conflict resolution is performed. The user defined handler may choose to ignore the new row and keep the original local row, to apply the new row, or to generate a new row (possibly merging old and new) and apply that instead of the new incoming row. A conflict handler may also choose to ERROR out, which can be useful if it wishes to abort apply of a transaction and retry it later.
Conflict handlers cannot skip whole transactions.
Note: User-defined conflict handlers do not have access to both the old and new versions of the remote row, so they cannot tell which field(s) in the remote incoming tuple changed. It is thus not possible to do reliable row merging. Attempts to so for the general case will usually prove to be incorrect in an asynchronous replication envirionment. It's possible in some application-specific situations where the app "knows" more about the data.
See also: Conflict handler management functions