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Oracle Distributed Systems by Charles Dye

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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 ...

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