Design Considerations

In another example, delete conflicts can be avoided by a simple strategy. Instead of deleting the rows, the rows can simply be marked for deletion. One specified site can then take charge and delete them. In this way, a potential delete conflict issue can be avoided.

Pre-built update conflict handlers

Before moving on to an examination of the conflict handlers, it is important to understand what a column list is and what the resolution columns are. The column list indicates the set of columns for which the update conflict handler is called when there is an update conflict. The resolution columns are the columns which will be used to identify an update handler. Also in some cases, resolution columns will be used to resolve the conflict.

Oracle provides four types of pre-built update conflict handlers. They are OVERWRITE, DISCARD, MAXIMUM and MINIMUM. The following example will illustrate the use of these handlers.

A row in the ALLINSURED table with polno 452366 is updated at the NY site and at the same time at London site. The update at the NY site changes the value of the SUM_ASSURED column to 45000 from 35000, but the London site changes the value to 62000 from 35000. When the row LCR from NY reaches the London site, it finds the old value of 62000 instead of expected 35000.

When used, the OVERWRITE handler, in the above situation, will overwrite the value at the London site value with 45000 and the conflict is resolved.