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Unformatted text preview: ser or application program. However, some situations cannot be
effectively modeled by this pattern. For example, consider an inventory database
of a department store, which stores the current quantity of each item along with its
price and registered vendor's details. The database is directly connected to the
point-of-sale terminals of the store and as the items are sold, the inventory in the
database keeps getting updated. In order to ensure that an item never runs out of
stock in the store, it is required that an order for the item is placed as soon as its
quantity in the inventory falls below a threshold value. To do this in a passive
database system, a polling mechanism can be implemented that periodically checks the quantity of every item in the inventory and prints a list of those whose
quantity is less than its threshold value. New orders can then be placed for the
items in this list. The main difficulty in implementing this mechanism is how to
decide the most appropriate polling frequency. If it is too high, there is a cost
penalty. On the other hand, if it is too low, the reaction may be too late to meet the
requirement of no item running out of stock. The best way to handle such an
application would be to make the system automatically place an order with the
registered vendor as soon as the current quantity of an item goes below its
threshold value. This is exactly what an active database system does. For instance,
in case of our example, the system can be designed to automatically invoke an
application when the quantity of an item goes below its threshold value. This
application prepares a purchase order for the item and sends this order to the
registered vendor on e-mail.
An active database system supports applications of the type mentioned above by
moving the reactive behavior from the application (or polling mechanism) into the
database management system. Active databases are thus able to monitor and react
to specific circumstances of relevance to an application.
An active database system must provide a knowledge model (that is, a description
mechanism) and an execution mode...
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This document was uploaded on 04/07/2014.
- Spring '14