EDB

Database technologies for engineering applications
since 25 June, 1998, last modified 25 June, 1998

In this section, we review database technologies that support facilities for engineering applications. These databases include object-oriented databases, active databases, and heterogeneous databases.

Basic Concept of Object-Oriented Database

object - all conceptual entities
attribute - some private memory that hold object's state
method - procedures encapsulate the behavior of objects
message - a medium through which an object can communicate with one another
class - a set of similar objects; all the objects described by the same set of attributes and methods
inheritance -
domain - the domain of attributes can be other classes

Schema Evolution in Object-Oriented Databases

Schema - the class definitions and the inheritance structure of the class lattice
Conventional databases (e.g. SQL/DS) provide dynamic creation and deletion of relation and addition of new
columns in a relation without requiring system shutdown
Program language oriented databases have deficiencies for schema change
Taxonomy of schema change and invariant

Taxonomy of Schema Change and Invariants

Changes to the contents of a node

Changes to an attribute

Changes to a method
Changes to an edge

Changes to a node

Invariance

Class lattice invariant
Distinct name invariant
Domain compatibility invariant

Transaction Management of Object-Oriented Databases

Serializable transaction
Session - a sequence of transactions
Long duration transactions as a set of transactions, rather than a sequence of transactions
Any active transaction of a session will compete for resource with those of all other session
Norman and Hypothetical transaction - a transaction which always aborts.
Experimenting with the effects of 'what-if' changes to the database

Event-Condition-Action Rule [Urban,1994]

The event specifies an operation or situation to be monitored
When event occurs, the condition is then evaluated
If the condition is true, the action is triggered automatically by database systems
Alter, Trigger, Integrity Maintenance Rule (IMR)
Alter - no action only send message to the user
Trigger - causes a specific database or application action
IMR - specifically developed to maintain constraints

An Active Database Model [Beeri1993]

In active databases operations can act as triggers that cause the execution of other operations.
Its advantages include, among others, code reuse, and centralization of code storage and management.
model of nested transactions in which transactions can have sub transactions, thus allowing for a hierarchical
organization of processing.
Object-oriented database supports the representation of data processing in the form of code.
Active Database, OODB, nested transactions

What an Active Database Model needs

How are triggers specified;
When are triggers enabled and executed;
How is the environment of trigger execution specified and implemented;
How do we specify and concurrently enabled triggers.

The Database Model

Object, Methods, Classes and Encapsulation
Encapsulation : An object encapsulates both data(its state) and behavior(its methods) and can only be
manipulated using the(external) methods associated with it.
Postulate that the methods are the triggering events.
The triggered operations are methods.
OODB's allow redefinition(overriding) of methods in subclass, even redefining methods for individual objects in a class.
In the nested transaction model a transaction may execute both operations and sub transactions.

Methods and Triggered Actions

TYPE method = [ m-name, m-code]
extension for triggers
TYPE method = [m-name, m-code, {triggered-action}]
triggered-action is name of an action.
An Example
- Database stores information about customers account and calculate total balance of a customer's account.

Triggered Actions Scheduling

When should the trigger be executed?
In HIPAC ; immediate, deferred, and decuple execution
execution interval: in which triggered action should be executed
example: update-account before get-balance
TYPE execution-interval = [start-event, end-event]
Two observations; execution in appropriate time, possibility of defining interval.

Failures

transaction management sub language: used in triggered action definition for specifying what should be done in the various failure case
Triggering Method Failure
Either case; roll back of triggered action, execution as planned (log system example)
HIPAC's decoupling mechanism
Independent from top transaction (ignore, abort)
Failure in execution interval specification

Triggered Action Failure

example of recalculation of balance
If recomputation(triggered-action) is fail,
1) roll back the balance update that triggered it
2) retry to execute and block retrievals of the total balance
" Try to run the action at lease 5 times and if they all fail try to run t instead. If t also fails, abort the transaction"

The Triggered Action Description

should include action and target object
TYPE triggered-action-description = [triggered-action, act-to-perform(location, parameters), execution-interval, scheduling-information, triggered-action-fail, trigger-fail]
- triggered-action :- name of the trigger
- act-to-perform :- name of the action to be taken
- scheduling-information :- priority and etc.

Active Objects

structure used for storing information about triggered actions
until executed.
for each action waiting for execution
TYPE active-action = [triggered-action(parameters), triggering-method-identity(location),...]
location describes the object where the triggering method executed

Methods for Handling The Active Part

inserting, retrieving and executing, and deleting active-actions
use for every triggered-action

An Execution Model

So far what information and how it is recorded
method processor
General Execution Plan
set of actions T1 : do not use any of parameter off M or do T2 : Scheduled after m terminates

Korean Engineering Databases �� copyright Namchul Do, 1998