TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML DTD A Conceptual and Generic Approach Philippe Thiran Computer Science Department Technische Universiteit Eindhoven The Netherlands

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Current Situation – XML as the standard for publishing and exchanging data over the Web – Data recorded and maintained in existing Databases Heterogeneous databases: different data models Limitation of database models – Database schema incompleteness (implicit/hidden structures) – Explicit and implicit interconnections among entities (no primary and foreign keys) Oracle V5 Model

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Migrating existing databases to XML – Principle XML description in DTD Bottom-up Approach Exploiting as much as possible the meaning of source data – Method and Tool Method – Not limited to any specific database model – Capturing the explicit and implicit structures and interconnections of the database schema Tool for supporting the method

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Schema Representation Database models and DTD Schema Manipulation Database schemas and DTD

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Schema Representation – Expressing database schemas and XML in terms of GER Extended object-entity relationship data model One rich and expressive model able to express data schemas whatever their operational data models – Operational database models like IMS, Relational, OO – XML-family models: XML DTD or XML Schema

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Schema Representation – Expressing XML in terms of GER DTD expressed in terms of GER – DTD concepts – Hierarchical organization – Sequence organization DTD ConceptsGER Interpretation Element typesEntity types Hierarchy of element types(root) entity types, relationship types, father roles Content type ELEMENTRelationship types Sequence organization (order of elements in the sequence) Seq groups Occurrence operators on sub-elements ?, *, + Role Cardinalities IDREF, GID attributesIDREF, GID groups Attribute modifiersAttribute cardinalities

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Schema Representation – Expressing XML in terms of GER <!ATTLIST Product Reference ID #REQUIRED Label CDATA #IMPLIED UnitPrice CDATA #REQUIRED>

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Schema Manipulation – Transforming XML DTD within GER Schema transformations defined on GER – Reverse transformations, semantics-preserving transformations – Transformation operators Standard transformations – For manipulating schemas expressed in operational database models – Example: transforming an entity type into an attribute DTD-specific transformations

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Schema Manipulation – Transforming XML DTD within GER Standard transformations – For manipulating schemas expressed in classical structured models – Example of a semantics-preserving transformation: transforming an relationship type into a entity type RT-ET: Transforming a relationship type into an entity type. Inverse: ET-RT

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Schema Manipulation – Transforming XML DTD within GER DTD-specific transformations (example) – Suited to derive a DTD from a structured data schema DTD-RT-to-HIER: Transforms a one-to-many (or one-to-one) binary relationship type into a hierarchical relation. The 1-1 role becomes the child role. Inverse: DTD-HIER-to-RT Create-SEQ-GROUP: Adds a seq group to an entity type. That group contains the child roles played by its children (in an aleatory order). Inverse: Del-SEQ-GROUP

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Converting (legacy) databases into DTD Exploiting as much as possible the meaning of source data Capturing the explicit and implicit structures and interconnections

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Exporting Databases – Bottom-up approach (from the source to the target) – Semi-automated 4-step method Extraction of the database schema (automated) – Extraction of the explicit structures and constraints Semantics recovering (semi-automated) – Recovery of the implicit structures and constraints Model translation (semi-automated) – Translation of a schema expressed in the GER into a schema expressed in the GER DTD – Use of the relations among entities DTD exportation (automated) – Generation of the DTD document

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Exporting XML – Reverse Engineering Recovering of the conceptual schema of an existing database – Augmentation of the knowledge about the data semantics – Database reverse engineering process (DB-MAIN) – Elicitation of hidden structures and constraints Database Schema Schema transformations Conceptual Schema

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Exporting XML – Model Translation DTD-specific transformation Non-deterministic process – It requires some design choices – The user-inputs might have consequences on the properties and the semantics of the resulting schema 5-step transformation process – Schema preparation – Hierarchy structure creation – Constraint relaxation – Attribute representation – Ordering definition

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Exporting XML – Model Translation Schema preparation – Removing invalid constructs » Multivalued/compound attributes » Complex relationship types Conceptual Schema 1.Schema preparation 2.Hierarchy structure creation 3.Constraint relaxation 4.Attribute representation 5.Ordering definition

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Exporting XML – Model Translation Hierarchical structure creation Entity types, relationship types are transformed into a tree by electing natural roots (significant concepts) by resolving father conflicts by breaking cycles by (eventually) adding a unique root 1.Schema preparation 2.Hierarchy structure creation 3.Constraint relaxation 4.Attribute representation 5.Ordering definition

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Exporting XML – Model Translation Constraint relaxation – Role cardinalities extension – Gid and idref groups creation 1.Schema preparation 2.Hierarchy structure creation 3.Constraint relaxation 4.Attribute representation 5.Ordering definition

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Exporting XML – Model Translation 1.Schema preparation 2.Hierarchy structure creation 3.Constraint relaxation 4.Attribute representation 5.Ordering definition Attribute representation Ordering definition

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML CASE Support – DB-MAIN Model Expression Database models and DTD Model Translation DTD-specific transformation

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML CASE Support – DB-MAIN –Basic Features Dedicated to database application engineering Based on the GER Includes transformation operators, reverse engineering processors and schema analysis tools Extraction facilities (SQL, Codasyl, RPG, IMS, etc.)

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML CASE Support –*-to-DTD Transformation DTD-Specific transformations Assistant

TU/e eindhoven university of technology / faculty of mathematics and informatics Exporting Databases in XML Conclusions –Rich and expressive data model Translating semantics of both database and XML models –Non-deterministic aspect of the model translation The same database schema can lead to a large set of equivalent XML structures –CASE Support (application) Automatic production of XML documents –that comply with the DTD that has been computed –based on the schema transformations used to convert the database schema in XML DTD