Oracle Distributed Systems

The Built-in Methods

Oracle supplies 11 built-in conflict resolution methods (see Table 15.3), which you can designate for column groups and priority groups. You will notice that data convergence for replicated environments of three or more sites is very challenging to obtain.

Table 15-3. Built-in Conflict Resolution Methods

Method	DML Supported	> 1 Master?	Convergence Requirements
Minimum Value	UPDATE	Yes	Always decreasing or < 3 masters
Maximum Value	UPDATE	Yes	Always increasing or < 3 masters
Earliest Timestamp	UPDATE	Yes	< 3 masters
Latest Timestamp	UPDATE	Yes	Always increasing or < 3 masters
Overwrite Update	UPDATE	No	< 2 masters
Discard Update	UPDATE	No	< 2 masters
Average	UPDATE	No	< 2 masters
Additive	UPDATE	Yes	Always converges
Append Site Name	INSERT	Yes	Never guaranteed to converge
Append Sequence	INSERT	Yes	Never guaranteed to converge
Ignore Insert/Discard Insert	INSERT	Yes	Never guaranteed to converge

For the most part, these techniques are self-explanatory, but certain peculiarities of their usage warrant further explanation.

Minimum Value/Maximum Value

The Minimum Value and Maximum Value methods are appropriate for sites with any number of masters and can be used with any replicatable datatype. Data is always guaranteed to converge with two masters and with three masters if values are always decreasing (for Minimum Value) or always increasing (for Maximum Value). If the data from the originating site and destination site are the same, these resolution techniques will fail. Therefore, you should always provide ...