EXTENDING OBJECT ORIENTED DATABASES TO SUPPORT

THE VIEWPOINT MECHANISM

Fouzia Benchikha, Mahmoud Boufaida

LIRE Laboratory, Departement of Computer Science, Mentouri university of Constantine

25000 Constantine, Algeria

Keywords: Object-oriented databases, data model, Viewpoint approach, Distributed databases.

Abstract: An important dimension in the database technology evolution is the development of advanced/sophisticated

database models. In particular, the viewpoint concept receives a widespread attention. Its integration to a

data model gives a flexibility for the conventional object-oriented data model and allows one to improve the

modeling power of objects. On the other hand, the viewpoint concept can be used as a means to master the

complexity of the current systems permitting a distributed manner to develop them. In this paper we propose

a data model MVDB (Multi-Viewpoint DataBase model) that extends the object database model with the

viewpoint mechanism. The viewpoint notion is used as an approach for a distributed development of a

database schema, as a means for object multiple description and as a mechanism for dealing with the

integrity constraint problems commonly met in distributed environment.

1 INTRODUCTION

The expansion and the distribution of databases

towards the new co-operative applications of

increased complexity need a new way to schema

design that fulfills several requirements of data

modeling. However, in complex applications, like

computer aided design and engineering, it is difficult

to work out a single abstraction, which is appropriate

to all the participants of a same project. Each expert,

according to his field of knowledge and his

objectives, focuses on certain aspects of the

Universe of Discourse (UoD), which are not the

same ones for an another expert. Thus, it would be

necessary to specify data so as to take account

several points of view, while keeping to each one its

specificity and allowing the sharing and the

exchange of information. This perception mode of

data is called the “viewpoint approach".

The viewpoint paradigm takes various

significances according to its study from diverse

standpoints by different fields of computer science

including software engineering (Charrel, and al.

1993), knowledge representation (Dekker, 1994),

database systems (Rundenteiner, 1992) (Bertino,

1992), web applications (Gergatsoulis, and al. 2001),

requirements engineering (Menzies, and al. 1999)

and complex systems modeling (Carn, 1992).

Several terms have been assigned to this concept

such as roles, aspects, perspectives, dimensions,

domains and viewpoints.

In databases, we notice few works on the

integration of the viewpoint concept in the data

models. Most of these works consider the view

mechanism. Views are external schemas that provide

to the user part of the global schema, a kind of

viewpoint on the description of its entities. In the

context of our work, viewpoints accords to the same

UoD several descriptions. Each description is a

partial representation of data according to a given

“viewpoint”. The various partial descriptions are

supported by database schemas that together provide

the global schema of the same real word data. The

fact of achieving such an approach requires

necessarily a distributed environment that permits

the integration and the collaboration of a collection

of databases.

In this paper, we propose extending the

conventional object data model to support the

viewpoint mechanism. The choice of the object

oriented paradigm is primarily due to the basic

concepts that it offers, in particular a great power of

expression, a better reutilisability and evolution of

objects. These concepts correspond to the new

aspirations of the viewpoint notion. We propose a

new data model MVDB (Multi-View-Database

model) that uses viewpoints as an approach for a

distributed development of a database schemas.

273

Benchikha F. and Boufaida M. (2005).

EXTENDING OBJECT ORIENTED DATABASES TO SUPPORT THE VIEWPOINT MECHANISM.

In Proceedings of the Seventh International Conference on Enterprise Information Systems, pages 273-278

DOI: 10.5220/0002532602730278

 SciTePress

The paper is structured in the following way. In

section 2, some related works concerning the

integration of viewpoints in databases are briefly

presented. Section 3 defines the MVDB structures

and section 4 concludes our work.

2 RELATED WORKS

In the database field, the concept of viewpoint is

mainly studied within the framework of view

mechanism in object-oriented databases. Various

view models are proposed such as the view model of

(Abiteboul, 1991), the view model of Rundensteiner

(Rundenteiner, 1992) and the view model of Bertino

(Bertino, 1992). In these works views are exploited

to allow different applications to see the same

database according to different viewpoints. The

viewpoint concept here supports external schema,

which is the third level of the ANSI architecture

standard upon which the construction and the use of

relational database systems and later object-oriented

ones are centred. Many problems are arised like how

a view schema (view class) is inserted in the global

schema (class hierarchy) and does an instance of a

view own an identity.

Abiteboul (Abiteboul, 1991) provides a general

framework for view definition. A virtual class

mechanism is used for instantiating views in object-

oriented databases. Here, classes for views are

explicitly defined where the attributes of these

classes are really methods that retrieve the

information from where it is actually stored. A view

can be treated as a database but it does not preserve

object identity.

Rundensteiner (Rundenteiner, 1992) and Bertino

(Bertino, 1992) introduce the concepts of multiview

and schema view, respectively, that provide the

capacity to restructure a database schema so that it

meets the need of specific applications. They present

support for view design by automating some tasks of

the view specification process and by supporting

automatic tools for enforcing the consistency of a

view schema. Indeed, different views of the same

object is allowed, depending on the context in which

the object is considered. Here views preserve object

identity but there is any relation between the various

instances of the same object.

All these models consider the viewpoint as a view or

external schema defined with the aim of adapting an

existing structure to new needs. In our study we

argue that the view and the viewpoint concepts

concern respectively the exploitation step and the

design one (Benchikha and al. 2001). A viewpoint

must directly be related to the objects description

and confers new properties to them. Our MVDB

model deals with this requirement and allows the

construction of distributed object databases based on

viewpoints.

3 MVDB MODEL

In this section we present the basic notions and

concepts of our model.

3.1 Basic notions

MVDB model is based on the object oriented

paradigm intensively studied within the DBs

framework. We adopt this object model as the

common one for the various database schemas.

In the MVDB model a database is a multiple

description of the same UoD. The database schema,

shown in figure 1, is viewed as a set of viewpoint

schemas (VP schemas) managed by different

database systems. Each viewpoint schema represents

an aspect of the data description and is held by an

independent database. The viewpoint schemas

construction is based on the referential one that

holds basic data on the real word entities shared by

all viewpoint schemas. Objects in the referential

base are called global ones. Global objects have a

basic description in the referential base and one or

more descriptions according to viewpoints.

We point that object identity is a central notion in

our approach. It is the same object described in a

multiple way according to its membership in the

various viewpoint schemas. However, in order to

ensure the components autonomy, local objects can

be created and managed locally by viewpoint

databases. Local objects are objects with a single

description according to one viewpoint and can’t be

accessed at the global level.

VP Schma

Referential schema

VP Schema

VP Schema3 VP Scheman

Figure 1: An example of a global schema description according to multiple viewpoints

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274

Viewpoint databases are complementary and

provide a global distributed database called multi-

viewpoint database. A coherent exploitation of this

global database is then recommended. This

modeling approach confers a decentralized vision of

a database schema, facilitates the parallel work of

several designers and leads to a great power of data

structuring. Generally these features are particularly

needed in the great complex applications of the

industrial world. As a matter of fact, the companies

are at least logically distributed in offices,

departments, working groups, etc. Consequently we

can deduce that the data are also already distributed.

Each unit in the company must possibly manage the

relevant data for its operation and should be able, if

necessary, to reach remote data that exist in the other

units. The data in the various units are

complementary and operated by collaborating users.

We illustrate the viewpoint approach through a

simple modeling example. It concerns the

representation of a laboratory’s scientific staff (see

figure 2).

We are particularly interested in the teaching and

research activities concerning each member of the

laboratory. Let us consider the following viewpoints:

the Research viewpoint and the Teaching viewpoint.

A basic Referential consists of the common

information to all the viewpoints (family name, last

name and age of the members). On the other hand,

each viewpoint contains only information, that is

relevant to it. The Research viewpoint, for example,

contains, for each member of the laboratory, its

research topic, its research-institution and its

research-time.

3.2 Formal model

An object-oriented database (ODB), according to the

database technology, contains two fundamental sorts

of information: data, represented by the object’s

state (the database extent), and metadata,

implemented in the database schema.

An ODB schema is organized as a class hierarchy

according to the sub-classing relationship. This last

provides the conditions to establish if two classes are

in a generalisation/specialization. MVDB proposes

an extension of the conventional object model to

support the viewpoint paradigm.

Let be MVDB (Sr, VP, C, O) the specification of

the data model signature, where:

• Sr is a referential schema name.

• VP is a set of viewpoint schema names,

• C is a set of class names,

• O a set of objects.

In the following, we define formally the basic

concepts of the model.

Definition 1. Referential schema

The referential schema is defined as any common

object schema Sr = (Cr , ∝r ) where:

Cr (Cr ∈ C) is the finite set of class names in the

schema and ∝r is the sub-classing relationship,

which is a strict partial ordering among Cr.

Sr is well-formed iff:

∀ (c, c’) ∈ Cr x Cr , c ∝ c’ ⇒ (Ext (c) ⊆ Ext (c’))

where Ext is the extension of a class.

The referential schema is the basic schema that

describes all the entities independently from any

viewpoint. A viewpoint schema is the customization

of the whole or of a part of the referential schema

defined as follows.

Definition 2. Viewpoint schema

Let be S = (C, ∝ ) and S’ = (C’ , ∝’ ) two schemas.

S’ is a projection of S (conversely, S is an extension

of S’ on C), denoted S’ ∇ S (conversely S ∆ S’) iff:

(C’ ⊆ C) and (σ’

⎢C’

= σ)

A viewpoint schema Svp is an extension of a

projection S’ on the referential schema Sr (as

depicted in figure 3). A viewpoint schema is

obtained by two steps: at first, a projection operation

(∇) is carried out on the referential schema to select

Référential

family name

last rname

age

…

Teaching ViewPoint

nbr-teaching-hours

teaching-institution

Research

ViewPoint

topic

research time

Figure 2: A multi-viewpoint modeling example

EXTENDING OBJECT ORIENTED DATABASES TO SUPPORT THE VIEWPOINT MECHANISM

275

S’= Projection on Sr

Svp= Extension of S’

Figure 3: Viewpoint schema = Projection + Extension of the referential schema

the part (the whole) of it, which will be described

according to the considered viewpoint. Then, an

extension operation (∆) of the resulting schema

customizes the entities description according to the

viewpoint. However, in order to support

independence between the various viewpoint

schemas, and to keep the specificity of each one, we

choose a decentralized description. For that, we

benefit from the “slicing technique” used in certain

approaches as described in (Bertino, 1992)

(Rundensteiner, 1992). This technique consists of

the distribution of the referential projected schema in

the viewpoint schema.

In the following, we give an example showing

the definition of the multi-viewpoint database that

describes the laboratory’s scientific staff schema.

The laboratory-schema is the referential schema that

models information about members of a given

laboratory. Each member is an object stored in

laboratory-base. The research-viewpoint schema

refines the members description by adding new

attributes. All the members are concerned with this

description here. The whole of the referential

schema is thus imported. The schema definition is:

Referential schema Definition

Schema laboratory-schema ;

Base laboratory-base;

Class member

Public type tuple (

family-name : string,

last-name : string,

age : integer)

End;

Name members = set (member);

Export-schema class member;

Export-base name members;

Viewpoint schema Definition

Viewpoint research-viewpoint from laboratory;

Base researchers-base;

Import-schema laboratory-schema class member;

Import-schema laboratory-base name members;

Class research from member

Public type tuple (

research-time : integer,

research-Institution : string)

End.

The various schemas (referential and viewpoints)

hold all the persistent objects, instances of the

different schema classes. Before giving the formal

definition of them, we first present the extension that

is brought to the object concept.

In the context of our work, we address the two

following object properties.

Property 1: An object is an instance of the

referential schema and an instance of one or several

viewpoint schemas.

Thus, an object has a basic description and may be

described according to various viewpoints

simultaneously. This is of the most broadly accepted

properties of the viewpoint concept. Because an

object in a viewpoint schema is seen as an instance

of a viewpoint schema, it amounts to the multiple

description of the objects.

Property 2: The state of an object is viewpoint

oriented.

It means that the state of an object may vary

depending on the viewpoints in which it is being

described. This seems to suggest that each

description of an object according to a viewpoint

should be viewed as a separate instance of it. This

property that allows the object multiple instanciation

suits together with the first one.

According to the afore mentioned properties, we

integrate a new concept, that is the object referent.

We distinguish a local object referent from a global

one.

Definition 3. Local object referent

A local object referent, denoted by r

is the

identification of the object in a viewpoint (local)

database such that :

= (vp, o

) where :

− vp is the viewpoint schema name,

− o

is a viewpoint identification (OID).

An object in a viewpoint database becomes a pair (r

) where v

is its local state value.

Definition 4. Global object referent

A global object referent, denoted by r

represents

the identification of the object in the multi-viewpoint

database such that :

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276

= (o

, , L(r

)) where :

− o

is the referential OID of the object,

− L is the list of its viewpoint identifications that

is : L(r

) = U

vp∈ VP

An object becomes a pair (r

, v

) where v

is its

global state value.

The objects constitute the associated bases of the

referential and the viewpoint schemas that are

defined in the following.

Definition 5. Viewpoint base

Let O

be the finite set of the objects identifier of a

viewpoint schema S

A viewpoint base, denoted Bvp, is a schema state

specified commonly as a tuple (π

, O

, √

) where :

− π

: C

→ O

is the function that associates

to each class c∈C

its object identifiers,

− O

is the set of the objects in B

= U

c ∈ Cvp

⎨π

(c)⎬,

− √

is the function that associates a value to

each object of B

, such as :

∀ c ∈ C

vp ,

∀ o ∈ π

( c) , √

( o) ∈ σ

( c).

Definition 6. Referential base

Let R

be the finite set of the objects referent of

the referential schema S

and VP the finite set of the

viewpoints schema names.

A referential base, denoted Br, is a schema state

specified as a tuple (π

, O

, √

) where :

− π

: C

→ R

is the function that associates to

each class c ∈ C

the objects referent,

− O

is the set of the objects in Br : O

= U

c ∈ Cr

⎨(π

(c), U

vp ∈ VP

(rl))⎬,

− √

is the function that associates a global value

to each object of B

, such us :

∀ c ∈ C

r ,

∀ o ∈ π

( c) , √

( o) ∈ σ

( c).

where Br describes the UoD entities.

Referential base and viewpoint bases constitute

the multi-viewpoint base (extent) of the multi-

viewpoint schema. They give the complete

description of objects according to multiple

viewpoints. Lus us illustrate this by the following

example.

The associated bases of the referential and the

viewpoint schemas presented in the above example

are populated with objects. Each object has an

instance in the ‘laboratory-base’ (referential base)

and can possibly be instantiated in the ‘research-

base’. Instances are collected under the root of

persistence of each schema as presented in follows.

Bases

laboratory-base ((oid1, ⎨Benali, Mohamed, 40 ⎬) ,

(research-vp , oidvp1))

(oid2, ⎨Bencharif, Ali, 55 ⎬, nil)

researchers-base (oidvp1, ⎨ Benali, Mohamed, 40 , 12,

‘Constantine-Univer’ ⎬)

In the following we focus a bit more on an

essential and complementary functionality to every

data model: the coherence. This one is directly

related to the integrity constraints study.

3.3 The integrity constraints in

MVDB

Unlike the traditional approach (mono viewpoint)

where the integrity constraints are defined on the

global schema, we distinguish in the viewpoint

approach two types of constraints.

• Local constraints: they contribute to ensure the

local coherence of the entities in a viewpoint

database and independently of the other bases.

• Global constraints: they contribute to ensure the

global description coherence of the entities

according to several viewpoints.

If the local constraints are apprehended, it is

difficult to take into account the global constraints.

Classically, the principal conflicts met in federated

databases are the names, the semantic and the

structural conflicts. In our work, these latter can be

solved by the viewpoint mechanism.

• Naming conflicts: traditionally, the solving of

this type of conflicts is done by assertions

specifying the synonyms and the homonyms. In

our context, the existence of the referential

solves any conflict coming from a problem of

synonymy. Thus, all the common properties are

described by the referential schema. On the

other hand, a conflict coming from homonyms

is solved by the viewpoint mechanism itself. As

a matter of fact, two distinct homonymous

constructions can be differentiated by prefixing

them, for example, by the name of the partial

schema.

• Semantic and structural conflicts: they are of a

weak or a lack, in a database schema designed

according to various viewpoints. Nevertheless,

each partial schema describes an aspect of the

data semantically different from the other

descriptions. In addition, the referential permits

a representation, and by the same way an

unified structure of the real world entities that

will have different descriptions according to

different viewpoints .

EXTENDING OBJECT ORIENTED DATABASES TO SUPPORT THE VIEWPOINT MECHANISM

277

However, within the framework of MVDB, we

distinguish other types of conflicts. These ones

permit to guarantee the compatibility and the

coordination between the different object

descriptions in the system. Let us consider the

following cases.

• Mutual exclusion between partial DBs: when

the description of the entities by a partial

schema compromises their description by

another partial schema.

Example: any temporary teacher does not have the

right to acquire a teaching activity, and cannot have

a description according to this viewpoint.

• Interdependency between partial DBs: when the

partial schemas contain linked properties.

Example: any teacher whose the research time

exceeds twenty hours a week must reduce his

official teaching time by 40 percent.

• Referential integrity between partial DBs: when

the creation (possibly deleting) of a database

entity requires a preliminary creation (possibly a

deleting) of one (or many) entity (ies) of another

databases.

Example: any teacher must be a member of a

research group. This implies that the creation of any

object instance according to the teaching viewpoint

must generate the creation of the same object

instance in the research viewpoint.

A multi-viewpoint base is coherent (here we

speak about coherence with respect to the semantics

of the applications and not about the implicit

coherence induced by the model) if the following

conditions are satisfied:

1. The referential base is locally coherent with

respect to its schema, i.e. the object set

checks the local constraints of the

referential schema.

2. Each viewpoint base is coherent with the

corresponding viewpoint schema, i.e. each

viewpoint objects check the viewpoint

schema constraints.

3. The gathering of the various bases is

coherent, i.e. all the global constraints are

satisfied.

4 CONCLUSION

In this paper we have sketched how the object

oriented database model can be extended to support

the viewpoint mechanism. The ability to express

multiple viewpoints allows for database construction

versatility, richer expression of data semantics and

permits a distributed manner for design, which is not

captured by the current generation of object oriented

database systems. Indeed, it could be interesting to

study the behaviour aspects of the objects that can be

very useful for the constraint management that we

consider. In addition, it would be interesting to

develop an expression language for the specification

of the integrity constraints and extend a query

language like OQL, for dealing with the multi-

viewpoint aspect of the objects. These last can then

be a query according to one or several viewpoints.

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