mark j. tseytlin 11/08/2002

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THE OBSERVER DESIGN PATTERN AND THE MAINTENANCE OF CONSISTENCY

CONSTRAINTS IN AN OBJECT-ORIENTED DATABASE

Mark J. Tseytlin

11/08/2002

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Initial Thoughts

• One of the current trends in database theory is towards total object orientation [WK95]

• Problem: Current totally OODB’s are not as robust as RDB’s

• Reason: The need to maintain consistency between related objects in an OODB [GHJVB95]

• Solution: Consistency or Integrity constraints are used to govern the procedural actions needed to maintain consistency in a database

• The observer pattern [GHJVB95] is a software design pattern that creates a new outlook on the implementation of actions to maintain consistency constraints in object-oriented systems.

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Objectives of the Study

• Implement a concrete database system that illustrates the concept of observer pattern

• Keep this database system as totally Object-Oriented as possible to prove usability and feasibility of the observer pattern for OODB applications

• Investigate maintenance of consistency rules that are used for consistency checks during database transactions.

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History and Introduction

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Data Models and Their Evolution

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A Typical Database SystemDatabase systems are computerized systems in which the interpretation and storage of information are of primary importance [MD92].

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A Typical Database System (cont.)

• Conceptual Schema - database designer’s view of the system

• DML - Data Modeling Language that is used to implement the logical databasedescription

• DBMS - Database Management System is a software package that supports the implementation of a database and performs operations on the data that is stored in thedatabase. These operations include:- storage of the data in a database- search and manipulation of stored data- display/receipt of old/new data to/from the end user

• The characteristics of these operations are directly dependent on the data model used as the conceptual guideline for the implementation of the DBMS

• HMI - Human Machine Interface is the end user’s view of the database system

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Evolution of Data Models

Variable length records and files

Flat files Relational data model

Object-relational data model

Semantic data models

Object-oriented data model

Hierarchical data model

Network data model

Data model refers to a model used for the design of database schemas

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Flat File

• Contain fixed length records• Can’t have repeating groups of information, i.e. the data has to be continuous [DK97]• Are widely used in relational databases

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Relational Data Model

• It was the first to exemplify a model and a query language in which the layout of the data on a disk drive was not determined by the data model [EN00]

• It was implemented by employment of one level of abstraction with a mapping from the database schema to the physical layout of the data

• Flat file structure was used, but data was organized into normalized relations to prevent data-anomalies caused by manipulation of the data

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Relational Data Model (cont.)

• Normalized relations - the table• Relation - a cell in the table that contains a singular value• Attribute - a column in the table that contains data from one domain• Tuple - a row in the table that must contain unique information- Each tuple contains a logical key or index that makes it unique

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Relational Data Model (cont.)

data of type 1

data of type 2

………

data of type k data of type k

data of type k+1

………

data of type n-1

No physical link

primary key

foreign key

• Foreign key – a primary key of a data structure that’s included in another data structure in order to connect the two with a data relation instead of a pointer [JO98]

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OO Data Model

• OO Data Model was fist to allow more complete representation of real-world objects in computing [JO98]

• Encapsulation – incorporation of behavior of real-world object into the class that defines such object

• Specialization – extension of a general definition of an object with a specializeddefinition

• Database constraints defined on the highest level – abstract class level

• Insertion - creation of a new object

• Insertion constraints – rules that govern the creation process in order to keep thedatabase consistent [MD92]

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OO Data Model (cont.)

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OO Data Model (cont.)

• Most current OO Databases still don’t include full non-procedural query language and views [WK95]

• Many OO systems bear the cost of being implemented as a set of concrete classes[PMD95]

• Designs should be specific enough to the problem at hand, but general enough to address future problems and requirements [GHJVB95]

• Most OO systems employ similar classes and communicating objects arranged in reoccurring patterns

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Object Oriented Design Patterns

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OO Design Patterns - Elements

Essential elements [GHJVB95]:

• Pattern name - a handle describing a design pattern in context of the concept that this pattern represents

• Problem - suggests when the pattern should be applied

• Solution - a schema of the design with relationships, responsibilities and collaborations

• Consequences - represent the pros and cons of applying the pattern to the design

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Main questions [GHJVB95]:

• What does the design pattern do? - determine the purpose of each design pattern in search of best pattern for use • What are the situations in which this design pattern is applicable for use? - determine if the pattern selected is consistent with the purpose of the target software • Which classes are in the design and how do they collaborate to keep the design consistent? - determined whether the number and type of classes necessary for implementation of the pattern are reasonable for the implementation at hand • How does the pattern support its objectives and what are the trade-offs and results of using the pattern? - analyze the scope of the required operations, as well as the system and software requirements necessary to support these operations

OO Design Patterns - Questions

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Observer Design Pattern

Observer design pattern is intended to define a one-to-many relationship among objects so that they exhibit “active” behavior [GHJVB95]

Active behavior - when one object changes state, all its dependents are notified and updated automatically

- Observer query the subject - observer changes its state and requests commitment notification, from the subject. The subject accepts change and propagates it to all its dependent observers (also known as publish-subscribe method) [GHJVB95]

- Observers register with the subject - when specific event occurs and only in this case, an observer gets a change of state notification from the subject. This is the most efficient method. The modifications of interest are specified explicitly [GHJVB95].

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Observer Design Pattern

Subject – the data object Observers – the user interface objects Note: all objects are completely independent of each other

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Observer Design Pattern - Observer Query the Subject

Observer changes state

Observer queries the Subject

Subject checks validity of change

Subject accepts the change

Change valid?YesNo

Subject rejects the change

Subject notifies other observers

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JAVA Programming Language

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JAVA language

• JAVA is a totally object oriented language. This is very useful for the purposesof creating a totally object-oriented database.

• JAVA is free of pointers. This property makes JAVA highly portable.

• In JAVA, strings are objects. The advantage of this representation is that JAVA’sstrings cannot be accidentally overwritten. Also, JAVA’s string representation savesstorage space.

• JAVA language lacks multiple inheritance. This is very useful in a complex object oriented database environment where records are implemented as instancesof compound objects. • JAVA supports multithreading. This feature allows running concurrent operations. Concurrency is a key feature of modern database systems.

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Family Tree Database System and Observer Design Pattern

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High Level Overview of FTDB SystemObservers

Person Data Entry Form(PDEF)

Family Tree Graphical Interface(FTGI)

People Data Storage and Manipulation

(PDSAM)

Relationship Data Storage and Manipulation

(RDSAM)

Subjects

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High Level Overview of FTDB System (cont.)• FTDB System – an OO database system that stores and manipulates people’s records and family type relationships

• FTDB - a concrete database system implementation that uses concepts of the observer pattern. It uses “publish-subscribe” observer design pattern strategy with explicit specification of modifications of interest.

• Observers and subjects are implemented as completely autonomous sets of classes that correctly implement observer pattern characteristics.

• Subject object groups employ “change managers” to map subjects to their dependent observers, define update strategy, and notify all dependent observers in the event of a change

• The end user is able to enter information through one of the two HMIs

• The system checks consistency constraints when an attempt is made to enter a new record, modify existing record, or new family relationship is proposed

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The Observers - PDEF

• Data can be entered via keyboard or by accessing an input file

• Data is checked for consistency

• This HMI accepts complete new records and modifications to existing records

• Consistency checks are performed upon any insertion/modification attempts

• The message window shows OK/ERROR result for each transaction. These messages are also automatically saved in a log file

• All modifications are automatically saved in an output file after the application is terminated

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The Observers - FTGI• Used to graphically represent relationships among Person objects contained by the DB

• Each relationship type is color-coded

• Each new relationship is subject to satisfying existence rules

• The message window shows OK/ERROR result for eachTransaction. These messages are also automatically saved in a log file

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The Subjects - PDSAM

universalColor

hairColor eyeColor

Gender

Person

People Data Storage and Manipulation

People List

myUtilities

• Used to facilitate the storage of people data entered via PDEF interface • Checks consistency between observers

• Propagates changes made via PDEF to FTGI

• PDSAM object is a “change manager” for the group that facilitates all consistency checks when a record is entered or modified and when a new family type relationship is created

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The Subjects - RDSAM

• RDSAM object is a “change manager” that coordinates consistency checks when new relationship is entered

• Consistency checking is accomplished based on consistency rules

• Consistency rules are applied using existing relationships

• System supports marriage, ancestry, and sibling relationshipgroups

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FTDB – Example of Operation

OODB, Observer Pattern, and Consistency Constraints

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PDEF HMI – Records Imported from File

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FTGI HMI – Initial Set of Records

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FTGI HMI – Observer queries the subject

Mouse click

End User

PDSAM object group

New Info?

Yes(true) + New Info No(false)

Display New Info

FTGI HMI

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FTGI HMI - Creation of New H-W Relationship

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FTGI - Drawing Menu

relationshiplabels

buttons

Father –Son relationship

Father-Daughter relationship

Mother-Son relationship

Mother-Daughter relationship

Sister-Sister relationship

Brother-Brother relationship

Sister-Brother relationship

Husband-Wife relationships

boxes

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FTGI Observer

RDSAM subject object group

PDSAM subject object group

FTGI observer object group

MW class

PDSAM class

RDSAM class

End User

CirclesDrawing

Menu

FTGI class

Lines

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RDSAM Subject

FTGI observer object group

PDSAM subjectobject group

RDSAM subjectobject group

PDSAM class

FTGI class

<rel_name_list> class

RDSAM class

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HWL Class – Possible Consistency Search Results

Search type Finding Message returned

D duplicate ERROR: DUPLICATE Husband(P1)-Wife(P2)

N Husband(P1)-Wife(P3) ERROR: Husband(P1)-Wife(P2) RELATIONSHIP NOT (Husband-Wife (P1, P3))


D no duplicates OK Husband(P1)-Wife(P2) DUPLICATE and list of rules from Table 3.2-2

N no inconsistencies OK Husband(P1)-Wife(P2) NOT (Husband-Wife (P1, P2))

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Husband-Wife Relationship Existence Rules

Rule#

Rule Consistency Checked by class

1 Male (P1) PL class2 Female (P2) PL class3 Not (Husband-Wife (P1, P3)) HWL class4 Not (Husband-Wife (P3, P2)) HWL class5 Not (Sister-Brother (P2, P1)) SBL class6 Not (Father-Daughter (P1, P2)) FDL class7 Not (Mother-Son (P2, P1)) MSL class

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H-W relationship consistency checking roadmap

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The PDSAM Subject – PL Class

PL class

People_List

PDSAM class

Person class

Rules 1 and 2:1. Male (P1)

2. Female (P2)

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The RDSAM Subject – HWL Class

Rules 3 and 4:3. Not (Husband-Wife (P1, P3))

4. Not (Husband-Wife (P3, P2))

HWL class

Husband_Wife_List

RDSAM class

HW class

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HWL Class – Possible Consistency Search Results

Search type Finding Message returned

D duplicate ERROR: DUPLICATE Husband(P1)-Wife(P2)



D no duplicates OK Husband(P1)-Wife(P2) DUPLICATE and list of rules from Table 3.2-2

N no inconsistencies OK Husband(P1)-Wife(P2) NOT (Husband-Wife (P1, P2))

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FTGI HMI – ERROR new H-W Relationship

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Future Work

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Further Abstraction of the RDSAM Object Group

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RDSAM Abstraction (cont.)• This is a better design -reduces the number of interfaces and couplings among objects, but still exhibits modularity needed for different functionality.-RDSAM object is a true “change manager” rather an integral part of the RDSAMsubject object group

• Inter-group coordination- consistency checks are done on the relationship group object level- objects of the RDSAM subject object group remain a cooperating collection ofobjects responsible for maintenance of relationship consistency constraints

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Selecting the New Relationship Type

Sister-Brother relationships

New type relationship X

X

P1

P2

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Automatic Generation of Relationship Types and Existence Rules

New relationship data

New relationship data Valid new relationship/ new relationship type


Check for validity/ store

<rel_name_objects>


Generated/ violated

Baselined relationship group objects

New Relationship group

object


Check for validity/ create new relationship type

object

Violate/ don’t violate integrity

DM object MW object

Report action results Create new menu choice on the DM

FTGI object

Valid new relationship/ new relationship type

Relationship object

RDSAM object

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Results of Pattern Matching Exercise

XP1P6 P2

P4 P5P3

Rule #

Pattern Matching Result

Rule

1 Male (P1) Male (X)

2 Female (P2) Female (Y)

3 Husband-Wife (P1, P6) Not (Husband-Wife (X,Y))

4 Father-Son (P3,P1) --

5 Father-Daughter (P5,P2) --

6 Mother-Son (P4,P1) Mother-Son (Z,X)

7 Mother-Daughter (P4,P2) Mother-Daughter (Z,Y)

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New Half-Sister--Half-Brother Relationship

Sister-Brother relationships

Half_Sister-Half_Brother relationship

New Type relationship X

P2

P1

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Summary

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Accomplishment of Objectives

• FTDB System is a concrete database system that illustrates the concept of observer pattern. It uses “publish-subscribe” strategy with explicit specification of modifications of interest.

• FTDB is implemented in JAVA which is a totally OO programming language. The design of the database is specific enough for the problem at hand, yet it’s flexible enough to address future requirements.

• Complete investigation of maintenance of a subset of relationship consistency rules was conducted during this study. Also, a possible solution was proposed for automatic generation of existence rules.

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The end

mark j. tseytlin 11/08/2002

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