1 Object Persistence

8/8/2019 1 Object Persistence

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Object Persistence


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What is Persistence

One of the most critical tasks that applicationshave to perform is to save and restore data

In object-oriented systems, there are two kind ofobjects:

Transient objects: exist in memory and are discarded whenan application terminates

Persistent objects: exist from one execution of anapplication to another or be shared among different

instances of applications. Persistence is the storage of data from working

memory so that it can be restored when theapplication is run again


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Persistence Mechanisms

There are several object persistence mechanisms:

Files hold data typically on magnetic media: disks

and tapes

Objects can also be serialized directly to files

Database management systems (DBMS) hold

tables of data (relational DBMS) or objects (object

DBMS)

DBMS use files to store data or objects, but they hide the

physical processes for storing data beneath a layer of

abstraction


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1234567890123456789012345678901234567890123456789

Simon Bennett Leicester GB 21322012002

Simon,Bennett,Leicester,GB,

213,22-01-2002

1,Simon,Bennett

2,1,0077098641,20022,2,0077096738,2001

Simon

Bennett

4

File Systems

Files and record structures

Fixed length (padded)

Variable length (delimited)

Header and detail Tagged data (XML)


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Object Serialization


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Object Serialization

Simple persistence method whichprovides a

program the ability to read or write a whole object

to and from a stream of bytes

Allows Java objects to be encoded into a bytestream suitable for streaming to a file on disk or

over a network

Persistence by reachability handles complex

objects


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Java Object Serialisation

A serialisable class must do the following

implement the java.io.Serializable interface

identify the fields that should be serialisable

non-transient and non-static fields are serialised by default

use the serialPersistentField member or the transient keyword

Optionally, the class can define the following

methods

writeObject controls saved data or appendsinformation

readObject reads data corresponding to

writeObject


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Java Object Serialization Example

pu blic class Address implements Serializable {

// Class Definition

}

// Serialise an object

FileOutputStream f = new FileOutputStream("tmp");ObjectOutput out = new ObjectOutputStream(f);

out.writeObject(new Address());

out.flush();

out.close();

// Deserialise an object

FileInputStream f = new FileInputStream("tmp");

ObjectInput in = new ObjectInputStream(f);

Address address = (Address) in.readObject();

in.close();


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Database Management Systems (DBMS)

Problems with files and serialization:

Redundancy: number of files grows with

applications, and data is duplicated

Inconsistency: data is updated in one applicationsfiles, but not in anothers

Maintenance problems : changes to data

structures mean changes to many programs

Difficulty combining data: business needs may

mean users want data from different applications


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DBMS

Corporate database consolidates data for different

applications

Eachapplication then has its own view ofa subset

of the data

DatabaseApplication 1 Application 2


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DBMS Schema

Ultimately data in databases is stored in files, but

their structure is hidden from developers

Conceptual Schema

External Schema

Internal Schema

The view on data used by

application programs.

The logical model of data that isseparate from how it is used.

The physical storage of data in

files and indexes.


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Advantages & Disadvantages of DBMS

Advantages

Eliminate unnecessary duplication of data

Enforce data integrity through constraints

Changes to conceptual schema need not affect external schema

Changes to internal schema need not affect the conceptual schema Many tools are available to manage the database

Disadvantages

Cost of investing in the DBMS

Running cost, including staff (Database Administrators) to manage the

DBMS

Processing overhead in converting data to format required by programs


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Types of DBMS

Relational Database Management System

(RDBMS)

Object-Oriented Database Management System

(OODBMS) Object-Relational Database Management System

(ORDBMS)


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Relational Databases

Data organised as tuples in relations (tables)

Link between data tuples

primary and foreign keys

Relationalalgebra

project, select, join

Relational normal forms

Declarative language

data definition, consistency, manipulation and querying

Examples

Oracle 11g, Microsoft SQLServer, IBMDB2

PostgreSQL, MySQL, Derby


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Object-Relational Impedance Mismatch

Object-oriented application development and relational

data management results in clash of two incompatible

models

Code to map between models is considerable overhead,

costly and hard to maintain


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Object-Oriented Databases


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Object-Oriented Databases

Object store objects as objects

Seamless Transition no overhead

Designed to handle complex nested objects for graphical

and multimedia applications, e.g Jusmine, Ontos,

ObjectStore

Object databases are closely linked to

programminglanguages with ways of navigating

through the database

Some will transparently 'materialize' objects from the

database when they are referred to

Operations are not stored with classes


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Object Identifiers

RDBMS entities are uniquely identified by primary keys

relationships are represented by matching primary key-foreign key data

Identification depends on the values in the key fields

Object-oriented database

stores object identifiers (OIDs) within an object to indicateother objects to which it is related.

The object identifier is not visible to the user or databaseprogrammer

An object remains the same object even when its statetakes on completely new values


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Object identity and equality

every object has unique and immutable objectidentifier (OID)

sharing of objects through references

two objects are identical if they have the same

OID two objects are equal if they have the same state

Equality can exist at different levels: Shallow equality: same state values (e.g. two

CustomerOrder objects have same values) Deep equality: same state values and related objects also

contain same state values (e.g. two CustomerOrder objectshave same values and all their related OrderItem objectshave same values)


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Objects and Literals

Objects can change their state values, and are

described as being mutable

Another component of the object data model is the

litera

l, which represents avalue or set ofvalueswhich cannot be changed

A literal is described as being immutable. Literals

do not have OIDs.


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Advantages of OODBMS

Complex objects and relationships

Class hierarchy

No impedance mismatch

No need for primary keys

One data model

One programminglanguage

No need for query language

Highperformance for certain tasks


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Disadvantages of OODBMS

Schema changes

Lack ofagreed standards

Lack ofad-hoc querying

In general,

RDBMSs are probably more suitable for databases with a

variety of query and user interface requirements (i.e. most

mainstream business applications),

while OODBMSs are appropriate for applications withcomplex, irregular data, where data access will follow

predictable patterns (e.g CAD/CAM systems,

manufacturing databases)


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OODBMS Products

Versant

ObjectStore and PSE Pro from eXcelon

Objectivity/DB

Intersystems Cache

POET fastObjects

DB4O

Computer Associates Jasmine

GemStone


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The Object-Relational Database


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The Object Relational Model

The object relational model is an extension of therelational model, with the following features:

a field may contain an object with attributes andoperations.

complex objects can be stored in relational tables the object relational model offers some of the advantages

of both the relational and object data models

An object relational DBMS is sometimes referred

to as ahybrid

DBMS has the commercialadvantage of being supported

by some of the major RDBMS vendors

e.g. PostgreSQL, Oracle-X


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Many companies already committed to some

RDBMS (huge investment)

RDBMS is robust and in use for along time

ODBMS is yet to include some features

Why RDBMS is still used for objects?


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Using Relational DBMS for OBJECTS

Most modern business application development projects

use object technology and relational databases to store the

data.

To store objects in a relational database, the objects have

to be flattened into tables Complex objects have to be taken apart and the parts stored in different

tables

When retrieved from the database, the object has to be reassembled

from the parts in different tables

O-R Mapping (Object Relation Mapping)

"Mapping" will be used to refer to how objects and their relationships

are mapped to the tables in a relational database.


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Object-Relational Mappings


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OR Mapping Frameworks

Much effort has been put in recently to making ORmapping more convenient

Transparent persistence

Key features:

the programmer can workonly with objects no SQL statements inthe code

selected objects are initially marked as being persistent thereafter,changes in those objects are transparently changed in the database, andthe programmer does not have to write code specifically to update thedatabase

the frameworkhandles the mapping of the objects to relationaldatabase tables where they are actually stored

mapping of objects to database tables is usually defined in XMLdescriptor files


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OR Mapping Frameworks

Hibernate

maps Java types to SQL types

transparent persistence for classes meeting certain requirements

generates SQL for more than 25 dialects behind the scenes

provides data query and retrieval using either HQL orSQL can be used stand-alone with Java SE or in Java EE applications

Java Persistence API (JPA)

Enterprise Java Beans Standard 3.0

introduced annotations to define mapping javax.persistence package

Toplink

JDO (Java Data Object)

OBJ

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