persistency (4)

8/3/2019 Persistency (4)

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Development Methodologies

Mapping Design Models to the Data Layer


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Data Layer

The general goal of an object-oriented design is tomodel a business process or set of processes

The goal of a data layer design is to model the

business data Typically a relational database

Mapping from system objects to data entities can bedifficult if the system contains

Complex class structure Large unstructured objects

Object inheritance


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Using Databases

3 approaches have been developed for the

management of the object storage:

Object relational mapping

ORDBMS

OODBMS


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Object Relational mapping

Most business database application use

relational databases

Need to map object in the database to tables

in the database

The mapping needs to be carefully considered

to allow data to be represented and accessed

as efficiently as possible


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RDBMS terminology

A relational database can contain one or manytables.

Tables make up the basic storage structure of arelational database management system.

A table holds the data about something in thereal world, such as employees, departments, orsalaries

A single row, or record, represents all data for a

particular employee. The order of rows stored ina table is insignificant. You can specify an orderwhen you retrieve data from a table.


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The class model

Artefact produced to represent the logical

structure of a system.

It captures both the data requirement and the

behaviour of the objects.

Behaviour is captured using the operations

that are defined for the class.


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Object-oriented Principles

Abstraction: indicates the process of identifying theessential aspects of an entity. Typically it relates to

Identifying an object

And what it does

Then deciding if it should be implemented

Encapsulation: An object contains both the datastructure and the set of operations which can beused to manipulate it

Information Hiding : External aspects of an objectare separated from its Internal details (these arehidden from the outside world)


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Relationship

Association : relationship between 2 classes.

The relationship may be functional or

structural

Aggregation: form of association that implies

Inheritance: a means of factoring out

common behaviour into generalised classes


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UML data profile

Table

column


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Stereotyped operations

Behaviour is represented as stereotypedoperations:

A primary key constraint (PK);

A Foreign key constraint (FK);

An index constraint (Index); A trigger (Trigger);

A uniqueness constraint (Unique);

A stored procedure (Proc) - not formally part of the dataprofile, but an example of a possible modellingtechnique; and a

Validity check (Check).


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Class model to Relational model

Step 1: Model classes

Step 2:

Identify persistent objects Step3:

Assume each persistent class maps to onerelational table

Step4: Map attributes to columns

Not all attributes are persistent


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Step 5:

Select an inheritance strategy

Filtered mapping

Each class hierarchy has a single corresponding table thatcontains all the inherited attributes for all elements - this table is

therefore the union of every class in the hierarchy

Horizontal mapping

Each class in the hierarchy has a corresponding table of only the

attributes accessible by that class (including inherited attributes). Vertical mapping

Each generation in the class hierarchy has a table containing only

that generation's actual attributes.



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Step 6:

For each class add a unique object identifier

It is often more convenient to implement absolute

identity using OID's rather than business related

primary keys



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Step 7a:

Map associations to foreign keys

Each association in the class model create a foreign key

from the child to the parent table



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Step 7b:

Map Aggregation and Composition

A weak aggregation could be implemented using

either an intermediate table (for the many-to-many case)

a foreign key in the aggregated class/table (one-to-many case). In the case of the many-to-many relationship, if the parent is

deleted, the entries in the intermediate table for that entity mustalso be deleted also. In the case of the one-to-many relationship, ifthe parent is deleted, the foreign key entry (ie. 'owner') must becleared.

In the case of composition, the use of a foreign key is mandatory,with the added constraint that on deletion of the parent the partmust be deleted also



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Step 8:

Model behaviour

Issue: Should one map some or all class behaviour to

the functional capabilities provided by databasevendors in the form of triggers, stored procedures,

uniqueness and data constraints, and relational

integrity.



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Relational Database Issues

Because the relational model is simple, relational

databases ...

have a "simple" formal model and semantics

support data independence (physical & logical)

have good ad-hoc query facilities

have good storage management facilities: recovery;backup

etc.

good concurrency

are fast and efficient


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Relational Database Issues contd.

But when building application

data in the application does not map well to table

heterogeneous collections

only fixed number of built-in types is available

It is not possible to represent/manipulate complex entities

as a single unit

The business rules and logical rules are written as

independent procedures, functions or triggers.


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Further Considerations

Manual ORM process outlined to date

Existing libraries/techniques for ORM

Hibernate / Nhibernate

QuickDB Orm

Service Data Objects

persistency (4)

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