trayambakojha

 Profile Answers by trayambakojha 

• Sep 3rd, 2009

 

Triggers are stored procedures that are invoked when certain conditions or
events occur. The granularity and flexibility of the conditions and events that
cause a trigger to fire are database specific. This technique assumes that the
DBMS supports triggers. Therefore, file-based storage cannot use trigger-based
capture.

This approach is very similar to the application-assisted technique in that
triggers, once activated, can perform specialized tasks. In this case, the
invocation of the trigger can be used to save changed records of interest to a
persistent storage area for retrieval. One drawback to this approach is that it
should only be used in cases where the number of events captured is within the
system's performance limits.

For example, if all updates to a table are captured, this method creates
twice the workload for the database. An additional problem with this approach is
that it requires that the granularity of the information in the target database
match that of the source database.

For example, if we are interested in capturing bank transactions in the
target database, and the source database only stores account summary
information, then we may not be able to derive the transaction (event)
information. One benefit to this approach is that data capture occurs at the
source of the data and, therefore, both the before and after image are
available.