Download Common Layers in an Information System

Persistent Objects
Persistent objects are objects that continue to exist after the
application program terminates
Persistence is provided by a DBMS
• ACID properties … D: durable or persistent
Two main categories
• OODBMS
• RDBMS
Chapter 38:
Intro, 1- 2, 4 - 9, 16 -19
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OODBMS
An ODBMS is a DBMS which stores objects
Depending on how an ODBMS implementation chooses to
persist objects, there are two types of ODBMS:
non-native and native
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OODBMS
Non-native ODBMS
In a non-native object database, there are two separate object
models, one of the application and the other of the database
itself.
ODMG-compliant ODBMSs are examples of non-native
ODBMSs since they require a separate data schema to be
defined, regardless of the existence of the application object
model. In order to query or persist objects from and to nonnative object databases, a mapping between these two distinct
models is required.
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OODBMS
Native ODBMS
In native object databases, objects are stored exactly as they are,
without the need to map them into a different object model
supported by the databases and vice versa.
In the world of native object databases, there is just one single
object model: the application object model
databases are not easily portable across applications, languages,
platforms
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OODBMS
Native ODBMS
e.g. db4o
www.db4o.com
Consider the model for the Persistence example on the web site
section
*
student
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OODBMS
import …
Makes the db4o classes available to us.
ObjectContainer db =
Db4o.openFile(“…”);
If the database does not exist, db4o creates it.
The database is then opened for use.
db.commit();
This statement ends the current transaction. It
indicates the end of a unit of work that must
be committed in its entirety to the database.
db.close();
This statement closes the database for us and
makes it available to another application.
db.store(section);
This one statement is all that is necessary to
have the section object saved to the database.
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OODBMS
Consider code in folder Persistence example on web site:
SectionDriver, Section, and Student comprise a 1903 example
using arrays of objects
CreateStudentDatabase: creates a database of sections where a
section contains student objects
ListDatabase: lists section objects and student objects in database
ClearDatabase: removes all sections and students from the
database
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RDBMS
Because RDBMSs are used so heavily in practice, objects must
be stored and retrieved from an RDBMS
The mismatch between the object representation of data and the
relational representation leads to difficulties. A mapping between
the two models is required to be implemented.
• Relations have Primary Keys, Foreign keys
• Data types
• Complex objects vs tuples
• Inheritance
• Collections of objects
Larman examines how a framework for an O-R mapping service
should be constructed
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Architectural layers
User Interface
Application
Technical
Services
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NextgenPOS
Relational Database
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O/R Mapping
Service
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Patterns
Representing Objects as Tables 38.6
Object Identifier 38.8
Façade 38.9
Representing Object Associations as Tables 38.19
one-to-one
one-to-many
many-to-many
many-to-many with an Association Class
Aggregation and Composition
Superclass/Subclass
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Persistent Objects in an RDBMS
To use an RDBMS for NextGenPOS, we require an object to
relational mapping service
• persistence service is a subsystem within the Technical
Services Layer
• mismatch between record-oriented and object-oriented
representations of data
• recommendation : buy, not build
Hibernate, Toplink, Glassfish
• provides functions such as store, retrieve, modify, commit,
rollback
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Key Ideas
Mapping
There are two schemas: object and relational. A mapping is
required that translates from one to the other.
Object Identity
Records and objects have a unique identifier
Materialization/Dematerialization
need to be able to translate the non-object representation to an
object representation, and vice versa
Lazy Materialization
Some objects are not materialized all-at-once; part of the object
may be done first, and other parts later on
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Each class becomes a table
Representing Objects as Tables pattern
define a table for each persistent object class
Some mappings are straight forward
Class
An object
The table
Many of these patterns mentioned come from a paper “Crossing Chasms: A Pattern
Language for Object-RDBMS” published in 1996 in Pattern Languages of Program
Design
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<<Table>> stereotype
We can indicate tables in UML using the stereotype:<<Table>>
Note you can use UML for
designing relational databases
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OIDs
Object Identifier Pattern
assign an OID to each record and object
i.e. We augment the attributes
with a new attribute that we
refer to as the OID
We can use the OID to prevent from instantiating the same object
twice and to help find an instance
Objects have an OID that is used as the PK in a relational table.
Each object is uniquely mapped to a row in the table
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Façade
Persistence subsystem will have a Façade that provides access to
its services
façade
The façade is
implemented using
Singleton
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Requesting a
ProductSpecification with
a specific OID
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Representing Relationships in Tables
Representing Object Associations as Tables pattern
one-to-one
place an OID FK in one or both tables representing the objects in
the association
or, create an associative table that records the OIDs of each
object in the association
or, could merge the two classes into one table
Consider
Store
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1
1
houses
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Representing Relationships in Tables
Representing Object Associations as Tables pattern
one-to-many
create an associative table that records the OIDs of each object in
relationship
or, could use a Foreign Key on the Many side
Consider
ProductCatalog
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1
1..*
contains ProductSpecification
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Representing Relationships in Tables
Representing Object Associations as Tables pattern
many-to-many
create an associative table that records the OIDs of each object in
relationship
Company
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*
*
employs
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Representing Relationships in Tables
Representing Object Associations as Tables pattern
many-to-many AND with an Association Class
create an associative table that records the OIDs of each object in
relationship and attributes of the association class
Company
*
*
employs
Person
Employment
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Representing Relationships in Tables
Representing Object Associations as Tables pattern
many-to-many AND with an Association Class
create an associative table that records the OIDs of each object in
relationship ...
*
Person
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Marriage
*
marries
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Representing Relationships in Tables
Representing Object Associations as Tables pattern
Aggregation and Composition
1..*
Sale
contains
*
Programme
1..*
contains
SalesLineItem
Course
Treat as 1-to-many and many-to-many associations
(Aside: Note that relational integrity rules/triggers may need to be
defined.)
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Representing Relationships in Tables
Representing Object Associations as Tables pattern
Superclass/Subclass
Payment
CashPayment
CreditPayment
ChequePayment
3 basic choices:
•all classes represented as separate tables,
•only the ‘leaf’ classes, or
•one table for all
Others are possible (see ACS-3902)
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OR Mapping - Example
Suppose we have the following class model:
Program
name
type
Course
* name
*
contains
description
credit
Offering
* slot
offer
room
description
When we map this structure to a Relational Database, we are creating an objectto-relational mapping. We need to define the relational database schema and
specify which class(es) map to which relation(s), and whether the associations
are mapped to relations or to foreign keys.
When specifying a relations include all attributes, the PK, FKs and what the FK
references.
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Example - recall patterns for O-R mappings
O-R Mapping
•The Representing Objects as Tables pattern says that each class
will be represented in the RDb by a separate table (relation).
•The Object Identifier pattern tells us that each table
representing a class will have the OID as the PK.
•The Representing Object Associations as Tables pattern gives
us several options:
•use an association table to represent the association
•(for 1-1 and 1-m) we can choose to use a Foreign Key
•for a 1-1 we can choose to merge the two classes into one
table
•etc
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Example - mapping
O-R Mapping, for the given class model we could decide:
Class
Program
Course
Maps to Relation
Program
Course
Offering
Offering
Association Maps to Relation/FKey
contains
offer
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Table: Contains
FK in Offering (refers to Course)
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Example - mapping
Relational Schema
Program
pOID name
type
Offering
oOID slot
room cOID
Course
cOID name
description
•Attributes
•PKs underlined.
credit description
Contains
cOID pOID
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Illustrates the
structure of the
relations
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•FKs indicated via
dashed lines.
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Example - mapping
Table structure with some sample records
Program
pOID name
123
3-year
334
4-year
Course
cOID
256
333
456
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type
R
R
description
A 3-year programme of studies in Business Computing
...
name
credit description
C Programming
3
This course introduces …
Java Programming 3
...
Systems Analysis
3
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