1 Chapter 27 Object-Oriented DBMSs - Standards and Systems Transparencies © Pearson Education Limited 1995, 2005.

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Chapter 27

Object-Oriented DBMSs - Standards and Systems

Transparencies

© Pearson Education Limited 1995, 2005

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Chapter 27 - Objectives

Object Management Group (OMG), CORBA, and other OMG standards.

Main features of ODMG Object Standard:– Object model– Object Definition Language (ODL)– Object Query Language (OQL)– Language bindings.

Main features of ObjectStore:– Architecture– Data Definition– Data Manipulation.


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Object Management Group (OMG)

International non profit-making consortium founded in 1989 to address object standards.

Several hundred member organizations including many platform and major software vendors.

Primary aims of OMG are:– Promotion of object-oriented approach.– Development of standards in which location,

environment, language, and other characteristics of objects are transparent.

Not recognized standards group but aims to develop de facto standards.


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Object Management Architecture

Four areas identified for reference model:

Object Model (OM) - Design-portable abstract model for communicating with OMG-compliant object-oriented systems.

Object Request Broker (ORB) - Handle distribution of messages between application objects in a highly interoperable manner.

Like distributed ‘software bus’ enabling objects to make/receive requests/responses from a provider.


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Object Management Architecture

Object Services - Provide main functions for realizing basic object functionality. Many of these services are database-oriented.

Common Facilities - Comprise a set of tasks that many applications must perform but are traditionally duplicated within each one.


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Object Reference Model


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Object Model


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Common Object Request Broker Architecture (CORBA)

Defines architecture of ORB-based environments. Basis of any OMG component, defining parts that

form ORB and associated structures. Some elements of CORBA are:

– Interface Definition Language (IDL).– Type model.– Interface Repository.– Methods for getting interfaces/specifications of objects.

Provides static and dynamic mechanism for clients to issue request to objects.


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CORBA ORB Architecture


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Other OMG Specifications

UML provides common language for describing software models.

Meta-Object Facility (MOF), defines common, abstract language for specification of metamodels.

XML Metadata Interchange (XMI) maps MOF to XML. XMI defines how XML tags are used to represent MOF-compliant models in XML.

Common Warehouse Metamodel (CWM) defines metamodel representing both business and technical metadata commonly found in data warehousing and business intelligence domains.


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CWM Sub-Metamodels


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CWM Relational Data Metamodel


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Model-Driven Architecture (MDA)

OMG hoped OMA would be common OO middleware standard. However:– Microsoft produced DCOM (Distributed Common

Object Model), – Sun developed Java, which came with its own ORB,

Remote Method Invocation (RMI), – another set of middleware standards emerged with

XML and SOAP (Simple Object Access Protocol). Also e-Business increased pressure on companies

to integrate their corporate databases. Enterprise Application Integration (EAI) is one of current key challenges for companies and, rather than helping, middleware may be part of problem.


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MDA is an approach to system specification and interoperability building upon 4 specifications discussed above.

Based on premise that systems should be specified independent of all hardware and software details.

Thus, while software and hardware may change over time, the specification will still be applicable.

MDA addresses complete system lifecycle from analysis and design to implementation, testing, component assembly, and deployment.


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To create an MDA-based application, a Platform Independent Model (PIM) is produced that represents only business functionality and behavior.

PIM can then be mapped to one or more Platform Specific Models (PSMs) to target platforms like CORBA Component Model (CCM), Enterprise JavaBeans (EJB), or Microsoft Transaction Server (MTS).

Both the PIM and the PSM are expressed using the UML.

MDA covers full range of pervasive services already specified by OMG, such as Persistence, Transactions, and Security.


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Object Data Management Group

Established by vendors of OODBMSs to define standards.

Have produced an Object Model that specifies a standard model for the semantics of database objects.

Design of class libraries and applications using these semantics should be portable across various OODBMSs.


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Between release 2.0 (1997) and 3.0 (late 1999), ODMG expanded its charter to cover the specification of universal object storage standards.

At same time, ODMG changed its name from Object Database Management Group to Object Data Management Group to reflect expansion of its efforts beyond merely setting storage standards for object databases.

The Java binding was submitted to JCP as basis for Java Data Objects (JDO).

In 2001, ODMG completed its work and disbanded.


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Under its extended charter, ODMG specification covers both OODBMSs that store objects directly and Object-to-Database Mappings (ODMs) that convert and store the objects in a relational or other database system representation.

Both types of products are referred to generically as Object Data Management Systems (ODMSs).

ODMSs make database objects appear as programming language objects in one or more existing OOPLs, and extend programming language with transparently persistent data, concurrency control, recovery, associative queries, and other database capabilities.


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Major components of ODMG architecture for an OODBMS are:

– Object Model (OM).– Object Definition Language (ODL).– Object Query Language (OQL).– C++, Smalltalk, and Java Language Binding.


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ODMG OM - Basic Modeling Primitives

Basic modeling primitives are object/literal. Only an object has a unique identifier. Objects/literals can be categorized into types. All objects of given type exhibit common

behavior and state. A type is itself an object. Behavior defined by set of operations that can be

performed on or by object. State defined by values objects carry for a set of

properties.


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ODMG OM - Basic Modeling Primitives

Property may be either an attribute of object or relationship between object and one or more other objects.

ODMS stores objects, enabling them to be shared by multiple users and applications.

ODMS based on a schema defined in ODL.


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ODMG Object Model - Objects

Object types decomposed as atomic, collections, or structured types.

Structured types as defined in ISO SQL standard.

Objects created using new() of corresponding factory interface provided by language binding.

Each object has a unique identity, the object identifier, which does not change and is not reused when the object is deleted.

May be given one or more names by user.


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Set of Built-in Types for ODMG Object Model


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ODL Interface for Objects


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ODMG OM - OIDs and Object Names

Each object is given a unique identity by ODMS, the object identifier, which does not change and is not reused when object is deleted.

Object may also be given one or more names that are meaningful to the user, provided each name identifies a single object within a database.

Object names act as “root” objects that provide entry points into the database.


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ODMG Object Model - Objects

Lifetime of an object is orthogonal to its type (persistence is independent of type).

Lifetime specified when object is created:– Transient: object’s memory allocated and

deallocated by programming language’s runtime system.

– Persistent: object’s storage managed by OODBMS.


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ODMG Object Model - Literals

A constant, with possibly complex structure. Literal types decomposed as atomic, collections,

structured, or null. Values of a literal’s properties may not change. Do not have their own identifiers and cannot

stand alone as objects. Embedded in objects and cannot be individually

referenced. Structured literals contain fixed number of

named heterogeneous elements.


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ODMG Object Model - Built-in Collections

Contains arbitrary number of unnamed homogeneous elements; each can be instance of atomic type, another collection, or a literal type.

Only collection objects have identity. Use iterator to iterate over collection. Ordered and unordered collections:

– ordered: traversed first to last, or vice versa; – unordered: no fixed order of iteration.


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ODMG Object Model – Built-in Collections

Set: unordered collections without duplicates.

Bag: unordered collections that do allow duplicates.

List: ordered collections that allow duplicates.

Array: 1D array of dynamically varying length.

Dictionary: unordered sequence of key-value pairs

with no duplicate keys.


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ODL Interface for Collections


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ODMG Object Model – Atomic Objects

Any user-defined object that is not a collection object is called an atomic object.

Atomic objects are represented as a class, which comprises state and behavior.

State represented by set of properties (attribute or relationship).

Attribute is not a “first class” object (i.e. not an object and so no OID).

Atomic objects can be related in a supertype/subtype lattice.


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ODMG Object Model - Relationships

Only binary relationships supported. Traversal paths are defined for each direction of

traversal. class Branch {

relationship set <Staff>Has inverse Staff::WorksAt}

class Staff {

relationship Branch WorksAt inverse Branch::Has}


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ODMG Object Model - Types, Classes, Interfaces, and Inheritance Two ways to specify types: interfaces and classes. Interface is a specification that defines only

abstract behavior of an object type, using operation signatures.

Behavior inheritance allows interfaces to be inherited by other interfaces/classes (but properties cannot be inherited from the interface).

Interface also noninstantiable – cannot create objects from an interface

Normally, interfaces used to specify abstract operations that can be inherited by classes or by other interfaces.


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ODMG Object Model - Types, Classes, Interfaces, and Inheritance

Class defines both the abstract state and behavior of an object type, and is instantiable.

Thus, interface is an abstract concept and class an implementation concept.

Can specify single inheritance between classes using extends keyword.

Multiple inheritance not allowed using extends but is allowed using behavior inheritance.


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ODMG Object Model - Types and Classes

Class definition specifies its extent and its keys:

Extents - set of all instances of given type. May request ODMS maintain index to members of this set.

Keys - uniquely identifies the instances of a type (similar to the concept of a candidate key).


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ODMG Object Model

Object model also specifies:

– Exceptions.– Metadata.– Transactions.– Databases.– Modules.


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Object Definition Language (ODL)

module DreamHomeClass Branch

(extent branchOffices key branchNo){

attribute string branchNo; ….relationship Manager ManagedBy inverse Manager::Manages;void takeOnPropertyForRent(in string propertyNo)

raises(propertyAlreadyForRent);}

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class Person {attribute struct Pname {string fName, string lName} name;

}Class Staff extends Person

(extent staff key staffNo){

attribute staffNo;attribute date DOB;….short getAge();

}


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class Manager extends Staff

(extent managers)

{

relationship Branch Manages

inverse Branch::ManagedBy;

}


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Object Query Language (OQL)

Provides declarative access to object database using SQL-like syntax.

Does not provide explicit update operators - leaves this to operations defined on object types.

Can be used as a standalone language and as a language embedded in another language, for which an ODMG binding is defined (Smalltalk, C++, and Java).

OQL can also invoke operations programmed in these languages.


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Can be used for associative and navigational access:– Associative query returns collection of objects.

How these objects are located is responsibility of ODMS.

– Navigational query accesses individual objects and object relationships used to navigate from one object to another. Responsibility of application program to specify procedure for accessing the objects.


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An OQL query is a function that delivers an object whose type may be inferred from operator contributing to query expression.

Query definition expressions is of form:

DEFINE Q as e

Defines query with name Q given query expression e.


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Expression can take several forms:

– Elementary - Construction

– Atomic type - Object

– Collection - Indexed collections

– Binary set - Conversion

Query consists of a (possibly empty) set of query definition expressions followed by an expression.

Result is object with or without identity.


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Example 27.2 OQL: Extents & Traversal Paths

Get set of all staff (with identity)

staff

Get set of all branch managers (with identity)

branchOffices.ManagedBy


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Find all branches in London

SELECT b.branchNo

FROM b IN branchOffices

WHERE b.address.city = “London”;

This returns a literal of type bag<string>.


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Assume londonBranches is named object (from last query). Find all staff who work at that branch.

londonBranches.Has

This returns set<SalesStaff>.


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Because of ambiguity over return result, cannot access sales staff salaries using:

londonBranches.Has.salary

Result may be set<float> or bag<float>.

Instead use:

SELECT [DISTINCT] s.salary

FROM s IN londonBranches.Has;


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Example 27.3 - OQL: Use of DEFINE

Get set of all staff who work in London (without identity).

DEFINE Londoners AS

SELECT s

FROM s IN salesStaff

WHERE s.WorksAt.address.city = “London”;

SELECT s.name.lName FROM s IN Londoners;

This returns a literal of type set<string>.


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Example 27.3 - OQL: Use of DEFINE

Can generalize this:

DEFINE CityWorker(cityname) AS

SELECT s

FROM s IN salesStaff

WHERE s.WorksAt.address.city = cityname;

CityWorker(“London”);

CityWorker(“Glasgow”);


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Example 27.4 OQL: Use of structures

Get structured set (without identity) containing name, sex, and age of all staff who live in London.

SELECT struct (lName:s.name.lName, sex:s.sex,

age:s.age)

FROM s IN Staff

WHERE s.WorksAt.address.city = “London”

This returns a literal of type set<struct>.


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Get structured set (with identity) with name, sex, and age of all deputy managers over 60:

class Deputy {attribute string lName; attribute sexType sex; attribute integer age;};

Typedef bag<Deputy>Deputies;Deputies (SELECT Deputy (lName:s.name.lName,

sex:s.sex, age:x.age)FROM s IN salesStaff WHERE position = “Deputy” AND s.getAge >

60)

This returns a mutable object of type deputies.© Pearson Education Limited 1995, 2005

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Get structured set (without identity) containing branch number and set of all Assistants at branches in London.

SELECT struct (branchNo:x.branchNo, assistants:

(SELECT y FROM y IN x.WorksAt

WHERE y.position = “Assistant”))

FROM x IN (SELECT b FROM b IN branchOffices

WHERE b.address.city = “London”)

This returns a literal of type set<struct>.


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Example 27.5 OQL: Use of aggregates

How many staff work in Glasgow.

COUNT (s IN CityWorker(“Glasgow”);

OQL aggregate can be applied within SELECT or to result of SELECT. Following equivalent:

SELECT COUNT(s) FROM s IN salesStaff

WHERE s.WorksAt.branchNo = “B003”;

COUNT(SELECT s FROM s IN salesStaff

WHERE s.WorksAt.branchNo = “B003”);


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Example 27.6 OQL: GROUP BY

Determine number of sales staff at each branch.SELECT struct(branchNumber,

numberOfStaff:COUNT(partition))FROM s IN salesStaffGROUP BY branchNumber: s.WorksAt.branchNo;

Result is of type: set<struct(branchNumber: string, partition:

bag<struct(s:SalesStaff)>)>

Note use of keyword partition to refer to each partition.


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OQL - Creating Objects

A type name constructor is used to create an object with identity.

Manager(staffNo: “SL21”,

fName: “John”, lName: “White”,

address: “19 Taylor St, London”,

position: “Manager”, sex: “M”,

DOB: date“1945-10-01”, salary: 30000)


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Language Bindings

Specify how ODL/OML constructs are mapped to programming language constructs.

Basic design principle is that programmer should think there is only one language being used.

C++ class library provided containing classes and functions that implement ODL constructs. Also, OML is used to specify how database objects are retrieved and manipulated within application program.


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Language Bindings - Creating a Working Application


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Language Bindings

Features that implement interface are prefixed

d_ (e.g. d_Float, d_String, d_List, d_Set, and

d_Bag).

Also class d_Iterator a class d_Extent.

Template class d_Ref(T) defined for each class

T in database schema that can refer to both

persistent and transient objects of class T.


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Language Bindings

Relationships handled by including either a reference (for 1:1) or a collection (for 1:*). For example, to represent 1:* Has relationship in Branch class:

d_Rel_Set<SalesStaff, _WorksAt> Has;const char _WorksAt[] = “WorksAt”;

and to represent same relationship in SalesStaff class:

d_Rel_Ref<Branch, _Has> WorksAt;const char _Has[] = “Has”;


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Language Bindings - OML

new operator overloaded so that it can create persistent or transient objects.

To create a persistent object, a database name and a name for the object must be provided. For example, to create a transient object:

d_Ref<SalesStaff> tempSalesStaff = new SalesStaff; and to create a persistent object:

d_Database *myDB;d_Ref<SalesStaff> s1 = new(myDb, “John White”)

SalesStaff;


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Language Bindings - OQL

OQL queries can be executed from within C++ in one of following ways:– using query member function of the d_Collection

class;– using d_OQL_Query interface.

d_Bag<d_Ref<SalesStaff>> wellPaidStaff; SaleStaff->query(wellPaidStaff, “salary > 30000”);

d_OQL_Query q(“SELECT s.WorksAt FROM s IN SalesStaff WHERE salary > $1”);

d_Bag<d_Ref<Branch>> branches;q << 30000;d_oql_execute(q, branches);


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Mapping Conceptual Design to Logical OO Design

Step 1 Mapping classes– Map each class or subclass to an ODL class, including

all appropriate attributes and methods. – Map composite attributes to a tuple constructor using a

struct declaration. – Map any multivalued attributes as follows:

» if values are ordered, map to a list constructor;» if values contain duplicates, map to a bag

constructor;» otherwise, map to a set constructor.

Create an extent for each class that will be iterated over. Specify EXTENDS for each ODL class that represents a subclass to inherit attributes and methods of superclass.


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Step 2 Mapping binary relationships– Add a relationship property (or reference attribute)

into each class that participates in relationship. – If supported, use inverse relationships where possible to

ensure system automatically maintains RI; otherwise program this functionality into class methods.

– If 1:1, each relationship property will be single-valued.– If 1:*, relationship property will be single-valued on

one side and collection type (list or set) on the other.– If *:*, each side will be a collection type.– Create tuple constructor (struct) for relationships

attributes of form <relationship reference, relationship attributes>.


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Step 3 Mapping n-ary relationships– For each relationship with degree greater than 2,

create separate class to represent relationship and include relationship property (based on a 1:* relationship) to each participating class.

Step 4 Mapping categories– For each category (union type), create class to represent

category and define a 1:1 relationship between category class and each of its superclasses.

– Alternatively, a union type can be used if OODBMS supports it.


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ObjectStore - Architecture

Based on multi-client/multi-server architecture, with each server responsible for controlling access to an object store and for managing concurrency control (locking-based), data recovery, and transaction log, among others.

A client can contact ObjectStore server on its host or any other ObjectStore server on any other host in network.


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ObjectStore - Architecture

For each host machine running one or more clients there is an associated cache manager process to facilitate concurrent access to data by handling callback messages from server to clients.

Also, each client has its own client cache, which acts as a holding area for data mapped (or waiting to be mapped) into physical memory.


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ObjectStore Architecture


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ObjectStore Server

Responsible for:– storage and retrieval of persistent data;– handling concurrent access by multiple client

applications;– database recovery.


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Client Application

Objectstore client library is linked into each client application, allowing it to:– map persistent objects to virtual addresses;

– allocate and deallocate storage for persistent objects;

– maintain a cache of recently used pages and the lock status of those pages;

– handle page faults on addresses that refer to persistent objects.


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Client Cache

Exists to improve access to persistent objects. When client application needs to access a

persistent object, then a page fault is generated when:– object is not in physical memory and not in client

cache;

– object is in client cache but has not yet been accessed;

– object is in client cache but has been previously accessed with different read/write permissions.


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Virtual Memory Mapping Architecture (VMMA)

C++ object is stored in database in its native format with all pointers intact (unswizzled).

Basic idea of VMMA is same as for virtual memory management in operating systems.

References to objects are realized by virtual memory addresses. If object has to be dereferenced and the page object resides on is in memory, there is no extra overhead in dereferencing this object (dereferencing is as fast as for any C/C++ program).


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Virtual Memory Mapping Architecture (VMMA)

If required page not in memory, page fault occurs and page is brought into same virtual memory address it originally occupied.

Thus, pointers to this object in other transferred objects are valid virtual memory pointers referring to their original target.

Unmapped range of virtual memory reserved for persistent objects, thereby ensuring that this range will be used for no other purpose than database pages.


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Building an ObjectStore Application


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ObjectStore Databases

ObjectStore supports two types of databases:– file database, a native operating system file that

contains an ObjectStore database;

– rawfs (raw file system) database, a private file system managed by the ObjectStore server, independent of file system managed by operating system.

Database divided into clusters and segments. A cluster is basic unit of storage allocation. When a persistent object is created storage is allocated from a cluster. Clusters are divided into segments.


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Data Definition in ObjectStore

Can handle persistence for objects created in C/C++ and Java through separate class libraries, and objects created in one language can be accessed in other.

For C++, ObjectStore uses C++ as a schema language so that everything in database must be defined by C++ class.

Persistence is orthogonal to type and persistent object support achieved through overloading new operator.

branchNo = new(DHomeDB, os_typespec::get_char(), 4) char[4];


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Data Definition in ObjectStore

Also a version of C++ delete operator to delete persistent objects and free persistent memory.

Once persistent memory has been allocated, pointers to this memory can be used in same way as pointers to virtual memory.


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Creating Relationships in ObjectStore

Relationship between Branch and SalesStaff handled by declaring two data members that are inverses of each other.

RI automatically maintained. Macros provided for defining relationships:

os_relationship_1_1; os_relationship_m_m;os_relationship_1_m; os_relationship_m_1.

os_relationship_1_m(SalesStaff, WorksAt, Branch, Has, Branch*) WorksAt;

os_relationship_m_1(Branch, Has, SalesStaff, WorksAt, os_Set<SalesStaff*>) Has;


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Data Manipulation in ObjectStore

Following operations must be performed before persistent memory can be accessed: – a database must be created or opened;

– a transaction must be started;

– a database root must be retrieved or created.


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Roots and Entry Point Objects

Database root provides way to give an object a persistent name, thereby allowing object to serve as entry point into the database.

From there, any object related to it can be retrieved using navigation (i.e., following data member pointers) or by a query (i.e., selecting all elements of a given collection that satisfy a specified predicate).


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Roots and Entry Point Objects


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Access Based on a Named Root

aBranch = (Branch*)(db1->find_root(“Branch3_Root”)

->get_value(WorksAtType);

cout << “Retrieval of branch B003 root: ” <<

aBranch->branchNo << “\n”;


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Iteration of Collections using Cursors

os_Cursor<SalesStaff*> c(aBranch->Has);

cout << “Staff associated with B003: \n”

for (p = c.first(); c.more(); p = c.next())

cout << p->staffNo << “\n”;


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Lookup of Single Object Based on Value of one or more Data Members

salesStaffExtent = (os_Set<SalesStaff*>*)

(db1->find_root(“salesStaffExtent_Root”)

->get_value(salesStaffExtentType);

aSalesPerson = salesStaffExtent

->query_pick(“SalesStaff*”,“!strcmp(staffNo,\“SG37\”)”,

db1);

cout << “Retrieval of specific member of sales staff: ” << aSalesPerson.staffNo << “\n”;


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Lookup of Single Object Based on Value of one or more Data Members

os_Set<SalesStaff*> &highlyPaidStaff =

salesStaffExtent->query(“SalesStaff*”,

“salary > 30000”, db1);

cout << “Retrieval of highly paid staff: \n”;

os_Cursor<SalesStaff*> c(highlyPaidStaff);

for (p = c.first(); c.more(); p = c.next())

cout << p->staffNo << “\n”;


1 Chapter 27 Object-Oriented DBMSs - Standards and Systems Transparencies © Pearson Education Limited 1995, 2005.

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