Presentation of the Relational Object Model

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Laila Alami Kasri et al. / International Journal of Engineering Science and Technology

Vol. 2(11), 2010, 6646-6656

attempted to store XML documents in RDBMS either as massive BLOB or CLOB objects or by the technique of

"shredding", which consists in breaking the document XML in fragments that can be stored into relational

tables, as DB2XML6, XML Extender dIBM7, XPERANTO[Carey et al (2000)], SilkRoute[Fernandez et al

(2002)] and others [Bohanon et al (2002), Chen et al (2003), Florescu and Kossmann (1999)].

These methods are unsatisfactory. The first requires to reassemble the XML data before making requests

and, depending on the volume, it can take time and fundamentally impact the performance, especially as these

databases, relatively new on the market are expensive and do not replace relational databases. They rather serve

as intermediate cache between Web applications and data sources to improve performance. The second breaksthe structure of the XML document and makes it lose its properties. Algorithms have been proposed to realize

the mapping between XML documents and databases, and vice versa [Lee et al (2001), Lee et al (2002), Wu et

al (2001)].

Our objective is to combine the advantages of relational databases and objects databases in a storage model

represented strictly in XML. This model is independent of any platform, it allows the storage of complex data of

any kind (text, image, sound, ...), its structure is composed of a set of collections. In our model of storage, we

adopt the global structure of the relational model by suggesting decomposing an XML document into several

collections. Thus, we eliminate one of the main problems facing the storage of large volumes of data into one

document: the repetition and the redundancy inherent to strictly hierarchical data.

We propose the creation and manipulation of XML data via SQL3 queries. Our choice of language is due to

the predomination of the relational model and to its evolution towards the objects and the possibilities it

provides precisely by the support of the complex objects.

We have developed a FRAMEWORK JAVA/XML, allowing the ultimate user write SQL3 queries in order

to specify the structure of the database, creation of types, and definition of XML documents. Throughout this

article, we present the different phases of the expression of SQL3 queries, the generation XML schema at the

storage of the valid documents in relation to generated schema.

This Solution helps define a visible correspondence between the relational-object Template and XML

Template of database. It helps automatically provide the XML storage Template which we are proposing, limit

the multiple imbrications of standard XML data, and remarkably reduce redundancy.

2. XML STORAGE TEMPLATE PROPOSES2.1. Abstract Syntax of TemplateOur solution consists of defining an XML storage Template that is based on the relational Template; its structure

is made up of a set of collections. A collection is a document which contains elements that are simple (atomic)

or compound, verifying constraints of keys and domains.

BD XML ::= {Collection }* + {Constraints }*

Collection ::= {Element }*

Element ::= {Components }*

Component ::= {Element | Attribute }

Constraints ::= {constraint key + constraint domains }

The global structure of our XML Template is similar to the relational Template, by considering relational

table as a collection XML except that the attribute of relation is atomic whereas the collection can have

components that are either attributes or elements of a simple or complex nature. We shall limit in our Template

imbrications in order to reduce redundancy of standard hierarchical XML Template and gain certain flexibility

in relation to the relational Template. We adopt such limitation in order to simplify the processing and

representation of complex (or compound) data.

2.2. Structure of the directory containing database

4 http://www.oracle.com/database/berkeley-db/xml/index.html5 http://www.dbxml.org6 http://www.informatik.fh-wiesbaden.de/~turau/DB2XML7

http://www-4.ibm.com/software/data/db2/extender/xmlext

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The database is organized in the form of a set of directories; each of which corresponds to a collection. The

directory contains a file XML collectionName.xml which represents the collection. Generally, it is a large

size and entirely operates in the server memory. Operations, such as adding, deleting, updating of objects of the

collection, are so complex and cannot be applied to the entire collection.

A sub-directory of xml-objects thus contains the different objects XML in distinct files. All updating

operations will be therefore applied separately to objects XML.

+ database+ .metadata+ .resources+ collection1

- collection.xml+ xml-objects

object1.xmlobject2.xml...

+ .resources+ collection2

- collection.xml+ xml-objects

object1.xmlobject2.xml...

+ .resources+ ... other collections

Fig.1. Structure of the database.

The database also contains a directory metadata defined in the root directory which contains meta-

information on the collections. Another directory, resources is created to enclose external resources relating to

the objects of the database. There are two kinds of resource; they are namely absolute resourcesand relative

resources.

The absolute resources are independent of any collection. They are defined in the root directory of the

database to be referenced by any collection.

The relative resources are specific to each collection. They are defined in a directory of resources linked to

the collection. The correspondent objects of the collection can alone make reference to these resources.

TheGeneralArchitectureoftheApplication

Fig. 2. The general architecture of the application.

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Fig.3. Diagram of main class

Figures 2 and 3 present the architecture of the application. The text SQL in entry is analyzed by SQL Parser.

The latter analyzes the syntax and produces abstract syntax trees. A NodeVisitor skims through all the absolute

syntax trees and creates objects which correspond to the queries. These objects are used to create either the

schema XML (ParseTree2Xsd) for creations of queries (DDL), or documents XML (ParseTree2XML) for

queries of insertion on the document XML (QueryTranslator).

The documents that are inserted in the database should conform to the generated schema. It is compulsory

therefore to create a validator of schema to check the coherence of the model and to allow the validation of the

documents XML of the database.

3. Mapping between Types SQL3 and Types XML SCHEMAOur goal is to present data SQL in a model XML in a way that preserves the maximum of information.

3.1. Mapping predefined types SQLTo insure the correspondence between the predefined SQL types, the strategy consists of look at a correspondent

XML Schema type that is close to a SQL type and to do a direct conversion or define a XML Schema type or

define one on the basis of retrieved information on a SQL type.

Table1. Table of correspondence between the predefined SQL types and XML SCHEMA.

SQL type XML SCHEMA type

CHARACTER,CHARACTER

VARYING,

CHARACTER LARGE OBJECT,

CLOB

VARCHAR,NCHAR

xsd:string ou restriction du type xsd:string contenant

xsd:length ou xsd :maxLength

BINARY LARGE OBJECT ou BLOB xsd:hexBinary ou xsd:base64Binary, avec la facet

xsd:maxLength

NUMERIC,DECIMAL Xsd:decimal, avec les facets xsd:precision et xsd:scale

INTEGER, BIGINT,SMALLINT Xsd:int

avec xsd:minInclusive et xsd:maxInclusive

FLAOT, REAL Xsd:float

DOUBLE PRECISION Xsd:double

BOOLEAN Xsd:Boolean

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Table1 (Continued)

DATE Xsd:date

TIME Xsd:time

TIMESTAMP Xsd:dateTime

INTERVAL xsd:duration

Table 2. Example of mapping between SQL type and XML SCHEMA.

SQL type XML SCHEMA typeINTEGER

SMALLINT

DECIMAL(12,2)

BLOB(12)

CHARACTER (33)

INTERVAL YEAR(4) TO

MONTH

3.2. Mapping of User predefined TypeA SQL object type defined by the user corresponds to a complex type in XML Schema.

1. If it has to do with a distinct type, it will be translated into an extension of the as it is shown in thefollowing case:

CREATE TYPE NAME_TYPE AS CHARACTER(100);

1: Create of complexeType defined by string type

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2: Define the complexType name_type with complexType stringType for constrain the restriction maxLenght

value = 100.

2. In the case of a type defined by the user which is not distinct, we create a complex type containing assequence the attributes of the defined type

CREATETYPE XOD_TYPE AS(

id CHARACTER(30),

nameCHARACTER(30),

equivalent_names REF(NAME_)ARRAY [100]

--others attributes

);

became

complexType defined

sequenceofSQL type attributes

< !element representing the SQL type attributes -->

3.3. Mapping of tablesA SQL table is transformed into a complex type element whose sequence contains the different columns of the

table. Nevertheless, one table contains also constraints relating to columns in the manner of the types defined bythe user.

A table can contain 0 with several tuples; thus the complex type will include a sequence with minOccurs

with value 0 as an attribute and an attribute maxOccurs with unbounded value.

The columns with constraint automatically become elements whereas the columns with constraints, unique

primary key become attributes as the following table shows:

Table 3. The correspondence between SQL concepts and XML SCHEMA

SQL XML SCHEMA

Table element of complex type

Column without constraint element of the sequence with complex type

Column of primary key Attribute of complex type with attribute use= "required"

Column with constraint not null element with attribute nullable="false"

Unique constraint Attribute | xsd:unique

Constraint foreign key and reference element xsd:keyRef

Table 4. The correspondance between SQL create table and XML SCHEMA

Create a table Schma XML corresponding

CREATE TABLE

colouring_compounds(

index INTUNIQUE,

description CLOB,

iupac_name

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Table 4 (Continued)

CHARACTER(100)

);

3.4. Express heritage through mappingThe types as well as tables that are created from a given type and the correspondent schema will be constructed

from the extension from base type.

Table 5. Mapping of the table defined with a type

SQL XML SCHEMA

CREATE TABLE dystuffs OF XOD_TYPE (

--defining the additional columns and

constraints

);

Table 6. Mapping of the heritage

SQL XML SCHEMA

CREATE TYPE RESOURCE_TYPE

ASCHARACTER(100);

CREATETYPE RESOURCE UNDER

RESOURCE_TYPE AS(

src CHARACTER(50),

type CHARACTER(50)

);

3.5. Mapping REFERENCE type columnsREF type is directly mapped into reference in XML SCHEMA

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Table 7. Mapping of the columns with reference type.

SQL XML SCHEMA

CREATE TYPE LINK

( src character(20) not null );

CREATE TYPE IMAGE_TYPE(

images ref(LINK),

-- others columns

);

...

...

3.6. Mapping types of named linesTo map the ROW type column, the column is transformed into a complex XML type whose name is and that of

the column and the sequence consists of elements representing ROW type columns. However ROW type is

recurrent because it can in its turn contain columns of this type. We therefore apply recurrent mapping:

Table 8. Mapping of column with ROW type

SQL XML Schma

CREATE TABLE dyestuffs OF

XOD_TYPE(

biological_sources ROW

(biological_source ROW

(key CHARACTER(100)

) ),

);


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The parser is based on a specification of language SQL [ANSI-ISO IEC 1999 SQL SPECIFICATION] as

a primary source of syntax SQL1999.

The syntax is described in Backus-Naur Form or BNF (Language of definition of formal languages) can be

found at the following URL:http://savage.net.au/SQL/sql-99.bnf

SQL3 grammar was not entirely implemented; the SQL instructions recognized by the parser are the queries

DDL (Data Definition Language) for the Description of the structure of the database, the creation of types and

tables, and insertion in queries DML (Data Manipulation Language).

Below, we will define a table and provide the representation of the obtained abstract syntax tree,

CREATETABLE art_objects OF XOD_TYPE(

type_of_object CHARACTER(100),

title CHARACTER(30),

collection CLOB,

owner CHARACTER(100),

historical_period CHARACTER(50),

provenance CHARACTER(100),

dimensions CHARACTER (100),

description CLOB,

creation CHARACTER(50),

images ROW(

img RESOURC ),

samples ROW(sample ROW( key CHARACTER(100) )

)

);

The syntactic tree corresponding to the precedent query

Fig.4. Sample of an abstract syntactic tree

Taking into account, the number of grammar rules SQL3 to process, we have espoused a generator of

syntactic analyser JavaCC. The tool reads the specifications of a grammar and converts it into program Java; it

provides other possibilities relative to the generation of parser such as construction of tree and debugging.

It includes module JavaCC to generate the parser and module JJTree to generate nodes. JJTree is a

preprocessor for JavaCC which inserts the syntactic trees of actions of rules of a given grammar in various parts

in the source JavaCC. It defines an interface Node which provides methods for operations such as placing the

parent of node and adding and searching children.

Fig.5. Functioning of JJTree

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5. GENERATION OF SCHEMA XML AND VALIDATIONThe generation of code XML Schema should be done from a schema which complies with model XML. This

part shows the different necessary steps starting from the extraction of the data of the table and types SQL to the

generation of a hierarchical structure of elements appropriate for the norm XML. We have defined the process

of transformation:

queries of creation in schema XML, queries of insertion in documents XML, Mapping consists in browsing all the created objects, retrieving their property and transforming them

into their equivalents; i.e. in element, or sequence of elements of a simple or complex type.

A schema which is compliant with model can be derived from a conceptual schema of data using theformalism Entities-Relations. To achieve this objective, we have devised a semi-automatic method of

transformation of schema. The main steps of this method are:

The transformation of complex structures such as the relations of heritage or types of ternaryassociations;

The construction of an arborescent structure deleting some cycles and the resolution of the conflicts ofthe father,

The modification of cardinalities that are not accepted in a model; The specification of the mode layout of elements son in relation to father; The transformation of attributes of schema in elements or attributes XML.

Fig.6. Validation of documents XML against schema XML.

6. Comparaison and PerspectivesGenerally, the native database XML save documents XML by preserving the arborescent logical structure. The

system DB2XML produce a document XML which directly represents the tables and their attributes in the

document XML without taking into account the relations which can exist between the tables. The system

XPERANTO and SilkRoute provide the user with a language of construction on database. It is evident that themanager of these systems is alone capable of using this language since it is only he who knows the schema of

the database. The use of the system SilkRoute requires the knowledge of the language of the definition of views

RXL. The system XPERANTO defines a global schema in XML Schema on the entire database and then the

manager, if necessary, formulates the queries XML-QL which define the sub-views.

Our solution allows for the production in model of storage for a database XML; the big advantages of this

achievement is allow for the creation and manipulation of a database XML by queries SQL3 just in the same

way as a relational-object database, which provides the users wich several possibilities to access and manipulate

data without having to necessarily master neither XML SCHEMA, XQuery, XPATH or XSLT. On the one

hand, the adopted relational structure considerably reduces the redundancy of the standard hierarchical model

and allows the management of the relations between the collections, and on the other hand, the use of SQL3,

allows us to introduce the complex types that are provided by XML Schema. Our perspective is to extend this

work to provide a complete model of conversion of SQL3 to XML on the one hand, and to efficiently complete

the management of the relations between the collections which applied in the relational model in the form of

constraints of referential integrities.

Conclusion

The relational-object model and the XML hierarchical model have similarities and differences. Our contribution

in this work is as follows:

The prolongation of the relational model in XML format by making use of the advantages of XMLsuch as simplicity, extensibility, portability, and integrability

The introduction of the notion of object (to express heritage, definition of type ROW, array, REF )expressed in SQL3 and defined in the complex type of XML Schema.

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The visible correspondence between SQL3 and XML Schema for the description and validation of thedata in the base.

Minimisation of imbrication and redundancy produced by the standard structure of hierarchical XML The possibility provided to the user to work with the relational data on the Web with the advantage of

having XML format; these data can easily transported and can be exploited and integrated by the

applications

The advantage of working with XML data without having to master the technological space linked to it.References

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Presentation of the Relational Object Model