Distributed Objects: Enterprise JavaBean and Object Persistence

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Distributed ObjectsEnterprise JavaBean and
Object Persistence

• Jianguo Lu
• University of Windsor

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Outline

• Distributed object
• EJB concepts
• Components, JavaBean
• Backgrounds of EJB multi-tier application,
  distributed objects, middleware.
• EJB roles, EJB varieties.
• Object persistence
• Simple mapping
• Mapping aggregation
• Mapping inheritance

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Part 1 Distributed Objects and EJB
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Distributed objects

• Distributed computing part of the system located
  on separate computers
• Distributed objects allow objects running on one
  machine to be used by client applications on
  different computers
• Distributed object technologies
• Java RMI
• CORBA
• DCOM

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Implementing distributed objects
Distributed object
client
Remote interface
Remote interface
stub
Skeleton
network

• Stub client side proxy object
• Skeleton server side proxy object
• Stub and skeleton implements the same remote
  interface

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Write your own distributed object

• Client only knows the interface
• public interface Person
• public int getAge() throws Throwable
• public String getName() throws Throwable
• Client uses the object just as if it were local
• public class PersonClient
• public static void main(String args)
• try
• Person person new Person_Stub()
• int age person.getAge()
• String name person.getName()
• System.out.println(name age)
• catch (Throwable t)
• Networking is taken care of by Person_Stub

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From the client side

• public class Person_Stub implements Person
• Socket socket
• public Person_Stub() throws Throwable
• socketnew Socket(ip address here",
  8765)
• public int getAge()throws Throwable
• ObjectOutputStream outStream
• new
  ObjectOutputStream(socket.getOutputStream())
• outStream.writeObject("age")
• outStream.flush()
• ObjectInputStream inStream
• new
  ObjectInputStream(socket.getInputStream())
• return inStream.readInt()
• public String getName()throws Throwable

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From the server side

• public class Person_Skeleton extends Thread
• public void run()
• ServerSocket serverSocket new
  ServerSocket(8765)
• Socket socket serverSocket.accept()
• while (socket ! null)
• ObjectInputStream inStream new
  ObjectInputStream(socket.getInputStream())
• String method (String) inStream.readObject(
  )
• if (method.equals("age"))
• int age myServer.getAge()
• ObjectOutputStream outStream
• new ObjectOutputStream(sock
  et.getOutputStream())
• outStream.writeInt(age)
• outStream.flush()
• else if (method.equals("name"))
• public static void main(String args)
• PersonServer person new PersonServer("mike",
  24)
• Person_Skeleton skelnew Person_Skeleton(perso
  n)

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• public class PersonServer implements Person
• int age
• String name
• public PersonServer(String n, int a) namen
  agea
• public int getAge() return age
• public String getName() return name

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From hand-craft to RMI

• RMI technology
• Automatically generate appropriate stubs and
  skeletons
• Error and exception handling
• Parameter passing
• RMI is not good enough in
• Object persistence
• Transaction handling
• Security

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Explicit Middleware
Transfer(Account a1, Account a2, long amount)
Call middleware API to check security call
middleware API to start a transaction
subtract the balance of a1 and add the amount to
a2 call DB API to store the data call
middleware API to end the transaction

• Difficult to write, maintain, and support

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Implicit middleware
Transaction API
Transaction service
client
security API
Remote interface
Security service
Remote interface
stub
Skeleton
Database driver
DB API
implicit middleware
network
Transfer(Account a1, Account a2, long amount)
subtract the balance of a1 and add the amount to
a2
Declare the middle services needed in a text
file. Generate a RequestInterceptor from this
declaration

• Easy to write, maintain, and support

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EJBs intent

• EJB wants to provide basic services and
  environment to make enterprise systems easier to
  develop.
• EJB provides automatically-managed persistence
  logic
• EJB provides transactioning plumbing for
  components
• EJB provides an enforced-authorization framework
• EJB provides "best of breed" capabilities by
  providing all this in a vendor-neutral fashion

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Persistence Management

• Perform all state and persistence management
  under the control of the application server
• Access to relational data is done through SQL
• Most Java programmers aren't great database
  engineers
• Therefore let the application server worry about
  how to obtain the data

EmployeeBean
name
sal
findByName()
SELECT FROM EMP WHERE NAME?
SAL
NAME
Emp
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Opaque Transactional Plumbing

• Transactionally execute all actions to prevent
  inconsistency
• Despite our best programmatic efforts, sometimes
  things will go wrong.
• Asking programmers to deal with all the possible
  right/wrong combinations is a recipe for
  disaster.
• Data integrity still remains the highest priority
• Let the app server track the success or failure,
  and commit or rollback as necessary.

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Security Framework

• Ensure that only authorized clients can perform
  sensitive actions
• Certain operations (or resources) are sensitive
• Not all users should have access to those
  sensitive operations or resources
• As a result, the system needs to enforce
  authorization rights
• We can give the app server knowledge of what
  principals have what rights
• Therefore let the app server deal with the
  enforcement of authorization policy by checking
  roles of callers (either declaratively or
  programmatically)

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What is EJB (Enterprise JavaBean)

• Enterprise
• n. (In computer industry) an organization that
  uses computers.
• Could be corporations, businesses, government
  bodies.
• The organization is often large rather than
  small.
• adj. large, distributed (information system).
• JavaBean
• Independent reusable software component
• Extend the write once, run anywhere to reuse
  anywhere
• Enterprise JavaBean
• The architecture for a server-side platform that
  provides distributed components

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Components

• Components
• Reusable software building block.
• Objects with additional capabilities that enable
  them to function in large-scale information
  system.
• Easy to combine with other components.
• Components live in containers.
• Example GUI widgets such as a button.
• A container is an application program in which
  component is run
• Example Browser, MS word, IDE (Integrated
  Development Environment)

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JavaBeans components

• JavaBean is Sun's component architecture for the
  Java application environment.
• Visual programming tools can combine beans to
  create an application.
• A bean implements an additional set of interfaces
  from an ordinary Java object. The interfaces
  specific to a bean are
• Introspection - Allows a visual programming tool
  to analyze how a bean works, allowing developers
  to connect beans.
• Customization - Enables developers to customize
  the appearance and behavior of a bean, using a
  property sheet provided by a visual programming
  tool.
• Events - Enables beans to communicate by using a
  notification mechanism.
• Properties - Describes the bean's attributes and
  broadcasts a notification when an attribute
  changes.
• Persistence - Allows developers to customize
  beans and package them into a Java Archive (JAR)
  file. Also known as serialization.

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Enterprise JavaBean

• The architecture for a server-side platform that
  provides distributed components
• EJB is different from JavaBean
• Client-side? Server Side
• Standalone? distributed
• Need to take care of transaction etc.

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Multi-tier Enterprise Architecture
Database 1
Browser 1
Application server 1
Legacy application
Web server
Browser 2
Application server 2
Browser 3
Tier 3
Tier 4
Tier 1
Tier 2
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EJB architecture
Enterprise Information System (RDBMS)
EJB container/server
Client application
Enterprise bean
Enterprise Information System (ERP)
Web container
Browser
Enterprise bean
JSP

• EJB Servers examples
• Websphere Application Server
• WebLogic
• Web Servers examples
• Apache tomcat

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Roles in EJB system

• Enterprise Bean Provider
• Produce and sell EJBs such as CourseBean
• Domain expert (such as expert in course ware)
• Do not need to take care of system level
  programming such as transaction, security, and
  distribution.
• Application Assembler
• Uses EJB to build an application
• Business process expert (such as a domain expert
  in XYZ Training Co)
• E.g., a student should register a course after
  made a payment
• Deployer/System Administrator
• Expert in a specific operational environment
  (such as the system admin in XYZ Co)
• EJB Server Provider/EJB Container
  Provider/Persistence Manager Provider
• Take care of the transactions, security,
  persistency etc.
• System expert (such as IBM, Sun, BEA)

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Roles in EJB (cont.)
EJB Container server persistent Manager
Provider
EJB client application
EJB server container
Application assembler
Database
Enterprise JavaBean provider
System admin
EJB Deployer
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Types of EJBs
EmployeeBean
Entity Bean
name
sal
findByName()
SELECT FROM EMP WHERE NAME?
CMP
BMP
Stateless
stateful
SAL
NAME
BMP Bean Managed Persistency CMP Container
Managed Persistency Entity Beans were replaced by
the Java Persistence API in EJB 3.0.
Emp
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Session Bean and Entity Bean
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Part 2 Object Persistency

• Jianguo Lu

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Object Persistency

• Persistency characteristic of data that outlives
  the execution of the program that created it.
• A persistent object is one that can automatically
  store and retrieve itself in permanent storage.
• Objects can't be directly saved to and retrieved
  from relational databases.
• Approaches to object persistency
• Object serialization
• Convert object into a byte stream and store it as
  a whole in database/ file system.
• Object database
• Objects are the 1st class citizen in the database
• Object relational mapping (ORM)
• Supported by Hibernate, EJB, etc.

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Relational model vs. object model
from www.agiledata.org
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Objects and relations
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Mapping objects (Entity beans) to Tables

• Simple case
• Map entities to tables
• Map attributes (with primitive data types) in an
  entity bean to columns in the table
• Synchronizing the object with the table is taken
  care of by EJB container

EmployeeBean
ID
name
sal
findByName(n) SELECT FROM EMP WHERE NAMEn

• ejbLoad
• SELECT name, sal FROM employee WHERE id?
• ejbStore
• UPDATE employee SET name?, sal? WHERE id?
• ejbPostCreate
• INSERT INTO employee VALUES(?,?,?)
• ejbRemove
• DELETE FROM employee WHERE id?

SAL
NAME
ID
Employee Table
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Object attributes and table columns

• Not all attributes are mapped to table columns
• Describe the mapping using deployment descriptor
  using XML (or using annotation in EJB3.0)

ltenterprise-beansgt ltentitygt
ltejb-namegtEmployeelt/ejb-namegt
ltcmp-fieldgt name lt/cmp-fieldgt
ltcmp-fieldgt sal lt/cmp-fieldgt
lt/entitygt lt/enterprise-beansgt

• What if the attribute in an entity bean is an
  entity bean itself?
• E.g., Employee bean may have Address attribute
• It is a relations between objects

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Relationships between objects

• Aggregation
• Mapping strategies
• Single table aggregation
• Foreign key aggregation
• Manage cardinalities
• Manage many-to many relations
• Manage composition
• Inheritance
• One table for one inheritance tree
• One table for one class
• One table for one inheritance path

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Single Table Aggregation Pattern(1/4)

• Abstract map aggregation to a relational data
  model by integrating all aggregated objects
  attributes into a single table.
• Example CourseEdition has Instructor attribute
• Design pattern a general repeatable solution to
  a commonly-occurring problem in software design

CourseEdition
EndDate String
StartDate String
Code String
instructor Instructor
course Course
Instructor
Type Integer
SSN Integer
surName String
Age Integer
townOfBirth String
name String
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Single Table Aggregation(2/4)

• Solution Put the aggregated object's attributes
  into the same table as the aggregating objects.

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Single Table Aggregation(3/4)
CourseEdition
EndDate String
CourseEditionTable
StartDate String
Code String
instructor Instructor
EndDate VARCHAR(0)
course Course
StartDate VARCHAR(0)
Code VARCHAR(0)
course VARCHAR(0)
Type SMALLINT
SSN SMALLINT
Instructor
surName SMALLINT
Age SMALLINT
Type Integer
townOfBirth SMALLINT
SSN Integer
surName String
Age Integer
townOfBirth String
name String
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Single Table Aggregation(4/4)

• Consequences
• Performance
• Pro only one table needs to be accessed to
  retrieve an aggregating object with all its
  aggregated objects.
• Con the fields for aggregated objects
  attributes are likely to increase the number of
  pages retrieved with each database access,
  resulting in a possible waste of I/O bandwidth.
• Maintenance and flexibility If the aggregated
  object type is aggregated in more than one object
  type, the design results in poor maintainability
  as each change of the aggregated type causes an
  adaptation all of the aggregating object types
  database tables.
• Consistency of the database Aggregated objects
  are automatically deleted on deletion of the
  aggregating objects.
• Ad-hoc queries If you want to form a query that
  scans all Instructor objects in the database,
  this is very hard to formulate.

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Foreign Key Aggregation (1/3)

• Abstract The pattern shows how to map
  aggregation to a relational data model using
  foreign keys.
• Solution Use a separate table for the aggregated
  type. Insert an synthetic object identity into
  the table and use this object identity in the
  table of the aggregating object to make a foreign
  key link to the aggregated object.

AggregatingTable
aggregatedObjectID
attribute1

Foreign key
AggregatedObjectTable
ObjectID
attribute1

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Foreign Key Aggregation (2/3)

• Example Instructor and CourseEdition are mapped
  to two tables

CourseEdition
(from OM_Training)
CourseEditionTable
EndDate String
StartDate String
Code String
instructor Instructor
EndDate VARCHAR(8)
course Course
StartDate VARCHAR(8)
Code VARCHAR(8)
course VARCHAR(8)
instructorID VARCHAR(8)
Instructor
InstructorTable
(from OM_Training)
Foreign key
Type Integer
Type SMALLINT
SSN Integer
SSN SMALLINT
surName String
surName VARCHAR(8)
Age Integer
Age SMALLINT
townOfBirth String
townOfBirth VARCHAR(8)
name String
instructorID VARCHAR(8)
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Foreign Key Aggregation (3/3)

• Consequences
• Performance Foreign Key Aggregation needs a join
  operation or at least two database accesses where
  Single Table Aggregation needs a single database
  operation. If accessing aggregated objects is a
  statistical rare case this is acceptable. If the
  aggregated objects are always retrieved together
  with the aggregating object, you have to have a
  second look at performance here.
• Maintenance Factoring out objects like
  Instructor into tables of their own makes them
  easier to maintain and hence makes the mapping
  more flexible.
• Consistency of the database Aggregated objects
  are not automatically deleted on deletion of the
  aggregating objects.
• EJB container helps maintain the consistency of
  CMP entity bean. BMP entity bean will need
  additional code to maintain consistency.
• Ad-hoc queries Factoring out aggregated objects
  into separate tables allows easy querying these
  tables with ad-hoc queries.

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Specify relations using deployment descriptor
ltenterprise-beansgt ltentitygt
ltejb-namegtCourseEditionlt/ejb-namegt
lt/entitygt ltentitygt
ltejb-namegtInstructorlt/ejb-namegt
lt/entitygt lt/enterprise-beansgt
CourseEdition
EndDate String
StartDate String
Code String
instructor Instructor
course Course
1
1
ltejb-relationgt ltejb-relation-namegtInstructor-C
ourseEditionlt/ejb-relation-namegt
ltejb-relationship-rolegt
ltejb-relationship-role-namegt Instructor-For
lt/ejb-relationship-role-namegt
ltmultiplicitygtOnelt/multiplicitygt
ltrelationship-role-sourcegt ltejb-namegt Instructor
lt/ejb-namegt lt/relationship-role-sourcegt
lt/ejb-relationship-rolegt ltejb-relationship-ro
legt ltejb-relationship-role-namegt
Courses lt/ejb-relationship-role-namegt
ltmultiplicitygt One lt/multiplicitygt
ltrelationship-role-sourcegtltejb-namegt
CourseEdition lt/ejb-namegtlt/relationship-role-sourc
egt lt/ejb-relationship-rolegt lt/ejb-relationgt
Instructor
Type Integer
SSN Integer
surName String
Age Integer
townOfBirth String
name String
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Manage Cardinality 1 to many relationships
CourseEdition
ltejb-relationgt ltejb-relation-namegtInstructor
-CourseEditionlt/ejb-relation-namegt
ltejb-relationship-rolegt
ltejb-relationship-role-namegt Instructor-For
lt/ejb-relationship-role-namegt
ltmultiplicitygt Many lt/multiplicitygt
ltrelationship-role-sourcegt ltejb-namegt Instructor
lt/ejb-namegt lt/relationship-role-sourcegt
lt/ejb-relationship-rolegt ltejb-relationship-r
olegt ltejb-relationship-role-namegt
Courses lt/ejb-relationship-role-namegt
ltmultiplicitygt One lt/multiplicitygt
ltrelationship-role-sourcegt ltejb-namegt
CourseEdition lt/ejb-namegt
lt/relationship-role-sourcegt ltcmr-fieldgt
ltcmr-field-namegt courses lt/cmr-field-namegt
ltcmr-field-typegt
java.util.Collection lt/cmr-field-typegt
lt/cmr-fieldgt lt/ejb-relationship-rolegt
lt/ejb-relationgt
EndDate String
StartDate String
Code String
instructor Instructor
course Course
n
1
Instructor
Type Integer
SSN Integer
surName String
Age Integer
townOfBirth String
name String
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Manage cardinality many to many relationships

• Example A Trainee may register several
  CourseEditions, and one CourseEdition has a
  number of Trainees.
• Solution Create a separate table containing the
  object identifiers (or Foreign Keys) of the two
  object types participating in the association.
  Map the rest of the two object types to tables
  using any other suitable mapping pattern.

ltejb-relationgt ltejb-relation-namegt
Trainee-CourseEditionlt/ejb-relation-namegt
ltejb-relationship-rolegt
ltejb-relationship-role-namegt Trainee-EnrollIn-Cour
ses lt/ejb-relationship-role-namegt
ltmultiplicitygt Many lt/multiplicitygt
ltrelationship-role-sourcegt ltejb-namegt Instructor
lt/ejb-namegt lt/relationship-role-sourcegt
ltcmr-fieldgt ltcmr-field-namegt courses
lt/cmr-field-namegt
ltcmr-field-typegt java.util.Collection
lt/cmr-field-typegt lt/cmr-fieldgt
lt/ejb-relationship-rolegt
ltejb-relationship-rolegt
ltejb-relationship-role-namegt Courses-HaveEnrolle
d-Trainees lt/ejb-relationship-role-namegt
ltmultiplicitygt Many lt/multiplicitygt
ltrelationship-role-sourcegt ltejb-namegt
CourseEdition lt/ejb-namegt lt/relationship-role-sour
cegt ltcmr-fieldgt ltcmr-field-namegt
trainees lt/cmr-field-namegt
ltcmr-field-typegt java.util.Collection
lt/cmr-field-typegt lt/cmr-fieldgt
lt/ejb-relationship-rolegt lt/ejb-relationgt
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Object aggregation and composition

• Aggregation and composition
• Composition is a stronger form of aggregation.
• In composition, when deleting the owning objects,
  the contained object will also be deleted.
• In UML, composition relation is drawn as filled
  diamond, while aggregation as unfilled diamond.
• Examples
• When a car is destroyed, so is its carburetor.
  When a pond is destroyed, the ducks are still
  alive.
• When a university closes, the departments will be
  closed as well. However, data about professors
  should still be there.

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Manage composition in EJB 2.0

• Use cascaded delete in deployment descriptor, to
  indicate that deletion operation on Instructor is
  cascaded down to associated Telephone objects.
• ltejb-relationgt
• ltejb-relation-namegtInstructor-Telephonelt/ejb
  -relation-namegt
• ltejb-relationship-rolegt
• ltmultiplicitygtOnelt/multiplicitygt
• ltrelationship-role-sourcegt ltejb-namegt
  Instructor lt/ejb-namegt lt/relationship-role-source
  gt
• lt/ejb-relationship-rolegt
• ltejb-relationship-rolegt
• ltmultiplicitygtManylt/multiplicitygt
  ltcascade-delete/gt
• ltrelationship-role-sourcegt ltejb-namegt
  Telephone lt/ejb-namegt lt/relationship-role-sour
  cegt
• lt/ejb-relationship-rolegt
• lt/ejb-relationgt

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Mapping inheritance

• Strategies of mapping inheritance to tables
• One table for the inheritance tree
• One table for each class
• One table for each inheritance path

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One table for the inheritance tree

• Solution use the union of all attributes of all
  objects in the inheritance hierarchy as the
  columns of a single database table.

Base class instance DescendantA
instance DescendantB instance
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One table for the inheritance tree

• Consequences
• Write/update performance Reading/writing any
  objects in the hierarchy with one database
  operation
• Space consumption Requires more space to store
  the objects

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One table for each class

• Solution
• Map the attributes of each class to a separate
  table

DescendentA Table
DescendentA Attributes
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One table for each class

• Consequences
• Write and update performance More database
  operations involved
• Space consumption has near optimal consumption
• Maintenance cost As the mapping is
  straightforward and easy to understand, schema
  evolution is straightforward and easy.

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One table for each inheritance path

• Solution
• Map the attributes of each class to a separate
  table. To a classs table add the attributes of
  all classes the class inherits from.
• If BaseClass is abstract, BaseClassTable is not
  generated.

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One table for each inheritance path

• Consequences
• Write and update performance One database
  operation to read or write an object
• Space consumption No redundant attributes
• Maintenance cost Adding or deleting attributes
  of a superclass results in changes to the tables
  of all derived classes.