Object-Oriented Database Systems - IfI

Object-Oriented Database Systems
Standardization Efforts
18.09.2002 INF 312, Ifi, UiO 1
Object-Oriented DBS vs. Object-Relational DBS
two ways to integrate object-orientation into DBS
-> both directions are also reflected in standards developments
commercial OODBS (examples): commercial ORDBS (examples):
- GemStone (Brokat Infosys AG) - ORACLE
- O2 (Ardent -> Informix) - Sybase
- ObjectivityDB - Informix (Illustra)
- ObjectStore (ODI -> eXcelon) - IBM
- POET - CinCom (UNISQL, Total)
- Versant - Microsoft
- Jasmine (Computer Associates) - ...
- ...
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Object Data System Vendors
• Object-Oriented
• Object-Relational
Overview
Object-Oriented DBS (OODBS) standards
• OMG: Object Model, OMA -> CORBA, POA, PSS
• ODMG 3.0 version
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Object-Oriented DBS vs Object-Relational DBS
OODBS ORDBS
• simpler way for programmer to use
DBS (familiar with OOPLs)
• “seamlessness”, no “impedance
mismatch”
• OO functionality + DBS functionality
=> higher performance for specific
applications
• “revolutionary” approach, no legacy
problems
• substantial investment in
SQL-based relational DBSs
=> evolutionary approach
• systems are more robust due
to many years of usage and
experience
• application development tools
• data security
• transaction processing
performance
• day-to-day business
requirements, e.g., online
backup
predictions:
- both kinds of systems will exist, used for different kinds of applications
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Object-Oriented DBS vs. Object-Relational DBS
Michael Stonebraker’s Application Matrix:
• Classify your problem into one of the four quadrants
• Use a DBMS optimized for the quadrant
Query
No Query
Relational DBMS
File System
Object-Relational DBMS
Object-Oriented DBMS
Simple Data Complex Data
How large is the upper-right corner?
• ”Many applications will migrate from the upper-left”
• ”It is growing and it will engulf upper-left and lower-right”
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Object Management Group (OMG)
International software industry consortium (started 1989)
today: 800+ companies
Original members:
• 3COM
• American Airlines
• Hewlett-Packard Co. (HP, Palo Alto, CA)
• Sun Microsystems Inc. (Mountain View, CA)
Examples of other members (vendors and end-users):
Informix Software Inc., Apple Computer, Philips, Olivetti, IBM,
AT&T, Groupe Bull, ICL, Enfin Systems, Canon, Unisys,
Architecture Project Management, British Telecom, Citicorp,
Royal Bank of Canada, John Deere, Microsoft, etc.
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Object Data System Vendors
• Object-Oriented
• Object-Relational
Overview
Object-Oriented DBS (OODBS) standards
• OMG: Object Model, OMA -> CORBA, POA, PSS
• ODMG 3.0 version
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OMG Focus
Common Goal: Create a component-based software marketplace
by hastening the introduction of standardized object software.
Establish industry guidelines and detailed object management
specifications to provide a common framework for application
development.
• IDL (Interface Definition Language)
• OMA (Object Management Architecture)
• CORBA (Common Object Request Broker Architecture)
– CORBA 1: 1991
– CORBA 3: 2000
• CORBA Facilities and CORBA Services
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OMG Technology
Platform Technology
• Analysis and Design
– UML (Unified Modeling Language)
– XMI (XML Metadata Interchange)
– MOF (Meta-Object Facility)
• Real-Time CORBA
• ORB (Object Request Broker)
Domain Technology
• 23 Domain Task Forces
– Business Objects, C4I, e-Commerce, Finance, Healthcare,
Life Science, Manufacturing, Telecommunications, etc.
Architecture Board
• OMA (Object Management Architecture and Reference Model)
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OMA (Object Management Architecture)
• OMA specifies the overall object model that is used in
distributed object computing environments, including
– how objects are defined and created,
– how client applications invoke objects,
– how objects can be shared.
• OMA sets the component software environment
– how components provide services
– how facilities provide services
– how CORBA services provide services
• Facilitated by the ORB (Object Request Broker)
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OMG Current Efforts
• Business Object Initiative
• CWM (Common Warehouse Model)
• CCM (CORBA Component Model)
• Java and Internet integration
• MOF (Meta Object Facility)
• PSS (Persistent State Service)
• Quality of Service Control
• Real Time CORBA
• UML (Unified Modeling Language)
• Workflow Management
• XMI (XML Metadata Interchange)
• etc.
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Application objects:
• Spreadsheet
• CAD tool
• Accounting
•Editors
•….
Application
Objects
OMA Reference Model
Horizontal facilities:
• Internationalization
• MOF
•….
Horizontal
Facilities
Vertical
Facilities
Vertical facilities:
• Healthcare
• e-Commerce
•….
Object Request Broker (ORB)
EJB
Component
EJB
Container
Components:
• Account Receivable
• Account Payable
• Ledger
•….
CCM
Component
CCM
Container
ORB:
• Name service
• Request dispatch
• Parameter encoding
•…
CORBA Object Services
Object Services:
•Naming
• Transaction
• Persistent State
• Security
• Query
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•…
12

OMG: CORBA Object Model
CORBA includes an object model of types, operations, and subtyping.
In the object model, the user can manipulate instances but not types or
operations as first-class objects.
- basic concepts
- object identity and object identifiers
- object types
- value types
- operations
- subtyping and inheritance
- argument passing and results
- interfaces of a type
- implementation
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OMG: Object Identiy & Object Identifiers
Each object has a unique identity (object identifier, OID) that is distinct
from and independent of any of its characteristics.
OID provides a means to denote or refer to the object.
Characteristics can vary over time, whereas identity (OID) is constant.
Implementation of OIDs is not specified.
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OMG Object Model - Basic Concepts
Client: An object system provides services to clients. A client of a service
is any entity capable of requesting the service..
Object: An object is an identifiable, encapsulated entity that provides one
or more services that can be requested by a client.
Type: A type is an identifiable entity with an associated predicate (a
single-argument mathematical function with a boolean result) defined over
entities. An entity that satisfies a type is called a member of the type.
Interface: An interface is a description of a set of possible operations that
a client may request of an object, through that interface.
Operation: An operation is an identifiable entity that denotes the
indivisible primitive of service provision that can be requested..
Attribute: An interface may have attributes. An attribute is logically equivalent to
a pair of accessor functions: one to retrieve the value and one to set the value.
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OMG: Object Types
Objects support only certain operations.
Operations applicable to an object characterize its behavior.
Types describe these operations and thus characterize the behavior of
objects.
Objects are created as instances of their types. Objects cannot change
their types.
Each operation has a signature, which consists of a name, set of
parameters, and set of results.
The set of operation signatures defined on a type is called the type´s
interface.
Every instance of a type satisfies the interface of that type.
Types are arranged into a type hierarchy that forms a directed acyclic
graph.
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OMG: Value Types
A value type is an entity which shares many of the
characteristics of interfaces and structs. It is a description of
both a set of operations that a client may request and of state
that is accessible to a client. Instances of a value type are
always local concrete implementations in some programming
language.
Objects + Value Types = Set of Denotable Values
Value Types provide ”pass by value” semantics
Basic values include Short, Long, LongLong, Ushort, Ulong,
UlongLong, Float, Double, LongDouble, Fixed, Char, Wchar,
String, Wstring, Boolean, Octet, Enum, Any
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OMG: Request in OMA
Client Object implementation
ORB
Request
A request consists of:
• target object
• operation
• parameters
• optional request context
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OMG: Operations
Operation (definition of signature) describes an action that can be
applied to parameters.
Operation Invocation = Request: is an event that indicates an operation
and possibly parameter lists some on behalf of a requester (client),
possibly causing results to be returned.
Request and operations are not objects.
Each operation has a signature consisting of:
-name
- set of parameters
- the result
- exceptions that may be raised by an invocation
Set of operation signatures defined on a type is called the type´s interface.
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OMG: Subtyping and Inheritance
Subtyping (specialization) is a relationship between types based on their
interfaces.
Subtyping defines the rules which objects of one type are determined to be
acceptable in contexts expecting another type.
Relationships between types define the type hierarchy (directed acyclic graph)
Inheritance is a mechanism for reuse.
Inheritance allows a type to be defined in terms of another type.
Operation dispatching: when an operation request is issued, a specific
operation implementation (method) is selected for execution.
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OMG: Argument Passing and Results
Execution semantics consist of arguments and return values to
formal parameters.
-> check: is request legal?
Expressions result in Value Types or OIDs (denotable values).
Operationally, the denotable values are copied into the formal
parameters. The objects the OIDs refer to are not copied.
If an operation request returns successfully, it was performed
exactly once; if it returns an exception indication, it was
performed at-most-once.
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OMG: Implementation
The object model formally specifies only the semantics of objects,
nothing about implementation.
The combination of a type specification and one of the
implementations defined for that type is called class.
An individual object, at any given point in time, is an instance of
one class.
Implementations may vary among different instances of a type
and even for the same instance over time.
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OMG: Interfaces of a Type
A type exports all the operations that are defined on it.
An interface is a description of a set of possible operations
that a client may request of an object, through that interface.
It provides a syntactic description of how a service provided
by an object supporting this interface, is accessed via the set
of operations of the object’s type.
Interface inheritance provides the composition mechanism
for permitting an object to support multiple interfaces.
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OMG: CORBA
Standard that defines specific ways for objects and clients
within a distributed environment to interact.
The basis distributed object computing components
defined in CORBA include:
- Static and Dynamic Invocation Interfaces (SII, DII)
- Interface Definition Language (IDL)
- C Language Mapping
- Interface Repository (IR)
- ORB Interface and Server Implementation
- Basic and Portable Object Adapter Interfaces
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OMG: CORBA – Remote Procedure Calls
Caller
Callee
before after
Interface
description
Compiler
Caller
Stub
Skeleton
Callee
Network
interface
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Client
OMG: Inter-ORB Interoperability
ORB1
Request
Object implementation
ORB2
Basic Object Adapter may be used in same ORB
Portable Object Adapter used across ORBs
Object implementation
IIOP (Internet Inter-ORB Protocol) hides differences in
platform and software (programming language, OS, ORB)
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Dynamic
invoke
OMG: CORBA Internal Architecture
Client
Client
stubs
1 interface
1 interface/adaptor
1 interface/operation
ORB
interface
ORB core
Object
implementation
Implement.
Skeletons
Object
adapter
Proprietary interface
Normal call interface
Upcall interface
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Interface
repository
Interface
definitions
OMG: CORBA - Interfaces
Client
stubs
Implementation
installation
Implement.
skeletons
Accesses Includes Includes
Describes
Implement.
repository
Client Object implementation
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OMG: CORBA - Types and Object Interfaces
Type in general refers to things that may be specified as
arguments or results in the signatures of operations.
Object Interfaces (IDL)
IDL Data Types: - basic types: -- integer types
-- floating point types
-- char, boolean, octet
- constructed types (struct, union, enum)
- template types (sequence, string)
- array
Pseudo-Objects: requests, named-value lists, contexts, interface
definition objects, implementation definition objects
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OMG: CORBA Object Services
• Naming Service
• Trader Service
• Event Service
• Notification Service
• Transaction Service
• Concurrency Service
• Security Service
• icensing Service
• Lifecycle Service
• Relationship Service
• Persistent State Service
• Externalization Service
• Property Service
• Query Service
• Time Service
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OMG: CORBA - Operations and Requests
A client performs a request by having access to an object
reference for an object and knowing the immediate type of the
object and the desired operation to be performed.
A client initiates a request by calling a stub routine specific to the
object type or by constructing the request dynamically via the DII
(Dynamic Invocation Interface).
Features of operations that can be specified in IDL:
- invocation semantics
- parameter directions
- exceptions raised
- context
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OMG: Horizontal (Common) Facilities
• User Interface Common Facilities
– Internationalization
– Printing
• Information Management Common Facilities
– Meta Object Facility
• System Management Common Facilities
– XCMF (superceded by CCM)
• Task Management Common Facilities
– Workflow Facility (with Workflow Management Coalition)
Services have now much higher priority than common facilities.
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OMG: Vertical (Domain) Facilities
• Business Object Facilities
– Task and Session
– Document Repository Integration
• Finance Facilities
– Currency
– Party
– General Ledger
• Electronic Commerce Facilities
– Negotiation
– Public Key Infrastructure
• Manufacturing Facilities
– Product Data Management
– High-Level Architecture (HLA Simulation)
• Telecommunications Facilities
– A / V Stream Control
– Notification
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OMG: CORBA Used to Create an OODBS
Object Services
interface repository
lifecycle services
transaction management
…
Object Request Broker
method
method
method
method
method
method
method
method
method
method
method
Object method
Object
Object
method Object
method
method
method
network-accessible objects
method
method
method
Object method
Object
Object
method Object
method
method
method
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OMG: Vertical (Domain) Facilities
• Telecommunications cont.
– CORBA/TMN and CORBA/IN Interworking Facilities
– Log Facility
• Transportation
– Air Traffic Control HCI Facility
• Healthcare
– Person Identifier Service
– Lexicon Query Service
• Utilities
– Utility Management System (in process)
• Life Science Research
– Biomolecular Sequence Analysis (in process)
– Genome Mapping (in process)
• Sigs: C4I, GIS, Analytical Data Mgmt, Distributed Simulation, …
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OMG: Application Access to an OODBS
Application
Program
method
method
method
method
application
structures
or objects
method
method
method
method
method
method
method
method
ODBMS
Database (persistent objects)
method
method
method
Object method
Object
Object
method Object
method
method
method
method
method
method
Object method
Object
Object
method Object
method
method
method
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OMG Persistent State Service
Interfaces
ORB domain Datastore domain
Servants
external interface internal interface
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PSS: Sub-Storagetyping
• If type_B is a subtype of type_A, and
if store_B is a storagehome for type_B, and
if store_A is a storagehome for type_A,
then store_B must be a subtype of store_A.
type_A
type_B
store_A
store_B
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PSS: Basic Concepts - 1
• Basic Concepts
– Datastore
• Provides persistent storage for object data, may be a RDBS,
OODBS, ORDBS or file system
– Abstract Storagetype
• The declared type of stored data to persistently encode an
object type (struct)
– Storagetype
• An implementation of an Abstract Storagetype
– Abstract Storagehome
• The declared type of a container to store instances of an
Abstract Storagetype
– Storagehome
• An implementation of an Abstract Storagehome, stores
instances of its corresponding Storagetype
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PSS: Basic Concepts - 2
• Basic Concepts, cont.
– Catalog
• Provides access to a storagehome
– PSDL
• Superset of IDL, used to declare PSS entities
• IDL + abstract storagetype, abstract storagehome, storagetype,
storagehome, catalog
– Session
• Kind of catalog, provides transactional semantics
– Storage Object
• A persistent state stored for an object. Is not a CORBA object,
cannot be invoked, has a pid, not a CORBA Ref
– Storage Object Incarnation
• A CORBA object (in a process) bound to a Storage Object
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In file People.psdl
abstract storagetype Person {
readonly state long social_security_number;
state string full_name;
state string phone_number;
};
abstract storagehome PersonHome of Person {
Person create(in long ssn, in string full_name, in string
phone);
};
catalog People {
provides PersonHome person_home;
};
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Using PSS
CosPersistentState.ConnectorRegistry connectorRegistry
= CosPersistentState.ConnectorRegistryHelper.narrow(
myOrb.resolve_initial_references(“PSS”) );
CosPersistentState.Connector connector
= connectorRegistry.find_connector(“”);
// create session
CosPersistentState.Session mySession
= connector.create_basic_session(
org.omg.CosPersistentState.READ_WRITE,“”,parameters);
// find person home (personHome is a storage home instance)
PersonHome personHome = (PersonHome)
mySession.find_storage_home(“PSDL:PersonHomeImpl:1.0”);
// create person Knut
Person p = personHome.create(00010154321, “Knut”, “2200 9000”);
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// In file PeopleImpl.psdl
#include
storagetype PersonImpl implements Person {};
storagehome PersonStoreImpl of PersonImpl implements
PersonStore {};
Notes: Any change to a storage object incarnation is reflected in
the persistent state of the corresponding storage object.
(after commit, flush, osv.)
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Transparent Persistence
// Java
public interface JPerson {
public long socialSecurityNumber();
public String fullName();
public void fullName(String newName);
public String phoneNumber()
public void phoneNumber(String newNumber);
}
public class JPersonImpl implements JPerson {
private long _ssn;
private String _name;
private String _phoneNumber;
public long socialSecurityNumber() { return _ssn }
// etc.
}
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Object-Relational Data Mapping
• When using “Objects” in an application,
Where are the objects?
– In the database only (ORDBMS?)
– In both application and database (OODBMS)
– In the application only (RDBMS)
• Must create an Object-Relational mapping to overcome
the impedance mismatch
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Changes for a DBMS
• Types from Application Object Model
• Schema
• Access Paths (Indexes)
• Clustering Strategies
• DBMS
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Naive Architecture for a Database
get
set
get
set
get
set
What Changes?
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Encapsulated Database Architecture
18.09.2002 INF 312, Ifi, UiO 48
<
>

Object-Relational Mapping References
• Relational Database Access Layers (from PLoP 96)
http://www.sdm.de/en/tec/pub/art/art/arcus/relz_abstract.htm
• Connecting Business Objects to Relational Databases (from
PLoP 98) http://www.joeyoder.com//papers/patterns/
• Crossing Chasms (from PLoP 95)
http://members.aol.com/kgb1001001/Chasms.htm
• There are links to several whitepapers on the ODMG site
http://www.odmg.org/library/whitepapers.htm#Database
• http://www.sun.com/software/javablend/index.html
• http://www.rational.com/products/rose/whitepapers.jsp
• There are many links to info on both OODBMSs and ORDBMSs
at http://www.cetus-links.org/oo_db_systems_2.html
18.09.2002 INF 312, Ifi, UiO 49
Object-Oriented Databases (1. Generation)
• Developed from object-oriented programming languages (PL)
• Intermediate stadium: persistent object-oriented PL
(persistence means data survives program run-time)
• Incorporate most object-oriented concepts, but lack good
support of several DBS concepts
• Most important application domains: GIS (Geographic IS),
CAD (Computer Aided Design) and Genome DBS
18.09.2002 INF 312, Ifi, UiO 51
Overview
Object Data System Vendors
• Object-Oriented
• Object-Relational
Object-Oriented DBS (OODBS) standards
• OMG: Object Model, OMA -> CORBA, POA, PSS
• ODMG 3.0 version
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ODMG (Object Database Management Group)
started in 1991
voting members: other members:
- ObjectDesign - American Management Systems
- Objectivity - Anderson Consulting
- Ontos - EDS
- O2 Technology (Ardent) - Fujitsu
- POET Software - Hewlett-Packard
- Servio (Gemstone) - Intellitic
(- SunSoft) - MITRE
- Versant - Persistence Software
- Sybase
-UniData
- Texas Instruments
18.09.2002 INF 312, Ifi, UiO 52

ODMG (Object Data Management Group) Today
Release 3.0 in 2000
voting members: other members:
- eXcelon (was ObjectDesign) - CERN
- Objectivity - Micro Data Base Systems
-POETSoftware -NEC
- Sun Microsystems - Versant
Plus 8 academic reviewers
(Alagic, Dittrich, King, Liskov, Maier, Moss, Soloman, Zdonik)
Object Data: OO and OR
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ODMG: Architecture
- common architecture for ODS products
- declarations for application schema (data + operations)
- source program
- schema declarations
- applications access DBs, types must conform to declarations
- DBs can be shared among applications on a network
- DBMS: shared service for transaction and lock management
18.09.2002 INF 312, Ifi, UiO 55
The Object Data Standard (Version 3.0)
Designed for ODS
Main components:
-(Architecture)
- Object Model
- Object Definition Language (ODL)
- Object Query Language (OQL)
- Language Bindings (OML - Object Manipulation Language)
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ODMG: Object Model
- based in OMG specifications, ODMG object model is
superset of OMG object model)
- common data model to be supported by OODBS
-- objects with OIDs
-- object behavior, state of objects, named objects
-- operation signatures
-- properties (attributes or relationships)
-- attribute / relationship signatures
-- collection types
-- inheritance
-- ...
- subset of the object model provides interoperability across PLs
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ODMG: Object Definition Language (ODL)
ODL is intended to define object types that can be implemented
in a variety of PLs:
- syntax for object model based on IDL (CORBA) +
referential integrity and collections
- used to define application schema; schema can subsequently
be translated into declarations of desired PL
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ODMG: Object Manipulation Language (OML)
- Language Bindings (C++, Smalltalk, Java) -
- based on extending a PL syntax and semantics in order to
provide DBS capabilities rather than embedding statements in
SQL or another language
-> goal: seamlessness
- one single unified type system across PL and DBS
- PL-specific binding respects the syntax and semantics of the
base PL -> small set of extensions to the base PL
- 3 components of PL binding: ODL, OQL, OML
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ODMG: Object Query Language (OQL)
- declarative query language for querying DB objects
- based on SQL syntax whereever possible
-> simple OQL queries
-> complex OQL queries
-> OQL as embedded language
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ODMG C++ Binding
class d_Object {
public:
mark_modified(); //mark Object as modified
void* operator new(size_t size);
void* operator new(size_t size, const d_ Ref_Any &cluster,
const char *typename);
void* operator new(size_t size, d_Database *database,
const char *typename);
virtual void d_activate();
virtual void d_deactivate();
}
An object which is an instance of a class that
inherits from d_Object is said to be
“persistence capable”.
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ODMG C++ Binding (cont.)
template class d_Ref {
public:
d_Ref();
d_Ref(T *fromPtr); //constructor
d_Ref(const d_Ref&); //constructor
...
T* operator->(); //dereference the Ref
operator T*(); //applies to non-const
T& operator T*() const;
void delete_object(); //delete object - all copies
...
}
d_Ref is an example of the Proxy Design Pattern
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Typical Ref-> Implementation
T* operator->() { //dereference the Ref
if (cachePtr != 0)
return cachePtr;
else {
// validate myDB exists, permissions, etc, then
...
DatabasePage* temp = new DatabasePage;
// read database page containing object oid
// from database myDB into temp
...
// locate object oid in temp
...
cachePtr = ... // Ptr to local object copy
}
}
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Typical Proxy Implementation
aRef
Type* 0
DatabaseName myDB
db_Object_Id
boolean dirty
...
oid
0
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}
Typical Ref-> Result
aRef
Type*
DatabaseName
db_Object_Id
boolean dirty
...
myDB
oid
0
Memory
Database
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}
Memory
Database

Object Query Language (OQL)
• OQL is the query language of the ODMG standard
• OQL is a declarative (not procedural) language allowing
to define what you want to know, instead of how you get
to the result
• OQL can be used as a stand-alone query language to
write ad hoc queries
• OQL can also be used from PLs like C++, Smalltalk, and
Java
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Conventions (like Eaglestone & Ridley)
• OQL reserved words are written in bold font
• Names of classes (of mutable objects) are written
beginning with a capital letter (stor forbokstav)
• Names of attributes, relationships, operations and
other variable names are written in lower case (små
bokstaver)
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Sets or Navigation? Both ways!
• OQL supports declarative queries that return a set of objects
(like SQL does for relational databases).
– The result to such queries is an collection object containing
a collection of objects, an object, a collection of literals, or a
literal (or undefined)
• OQL also supports navigational queries that use paths based
on pointers and relationships to navigate in the database to
locate the desired data (like the network data model).
– The result of a navigational query is the desired object in
the database, from which one can use a PL for further
computation.
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select distinct E.title
from Employees E
where department = 4
Result in SQL:
Title
Engineer
Consultant
Salesman
Secretary
Example: set query
Result in OQL:
:Set
:String “Engineer”
:String “Consultant”
:String “Salesman”
:String “Secretary”
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OQL: a Typed Expression Language
• A query is a query expression with no bound variables
• An expression returns an object or a literal
• With unary and binary operators , expressions can be built
recursively from sub-expressions:
or
• The type of sub-expressions are subject to restrictions
depending on the type checking rules for the operator used.
• ::= |
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OQL Grammar primaryExpr
primaryExpr ::= conversionExpr
| collectionExpr
| aggregateExpr
| undefinedExpr
| objectConstruction
| structConstruction
| collectionConstruction
| identifier [ argList ]
| queryParam
| literal
| ( query )
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Recursing on OQL expr
query In the OQL grammar, there is a
selectExpr | expr production rule from the undercastExpr
lined non-terminal to the next line
orExpr | ( type ) castExpr
orelseExpr { or orelseExpr }
andExpr { orelse andExpr }
quantifierExpr { and quantifierExpr }
andthenExpr | for all inClause : andthenExpr | …
equalityExpr { andthen equalityExpr }
relationalExpr { ( = | != ) relationalExpr } | …
additiveExpr { ( < | | >= ) additiveExpr } …
multiplicativeExpr { + multiplicativeExpr } | …
inExpr { * inExpr }
unaryExpr { in unaryExpr }
+ unaryExpr | … | postfixExpr
primaryExpr { ( . | -> ) identifier [ argList ] } | …
( query )
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Example OQL Queries
17 literal with value 17 of type integer
Pindex.lookup(“Earl”) set of Person objects with name=“Earl”
or the unique Person object with name = “Earl”
assuming type(Pindex) is dictionary
Cities set of City objects in the database
assuming City.extent is named Cities and there are no duplicates
select c.name bag of String (names of cities) where the city
from c in Cities has more than 5 museums in the database
where c.name in (
select m.name
from m in Museums
group by m->city
having count > 5)
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OQL Collections
collectionConstruction ::= array ( [ valueList ] )
| set ( [ valueList ] )
| bag ( [ valueList ] )
| list ( [ valueList ] )
| list ( listRange )
Class City {array(museum) museums }
oslo.museums[2]
norskMuseums = oslo.museums+bergen.museums
Type of e1+e2: if type(e1) = list(t1) and type(e2) = list(t2)
(e1||e2) and t1 is compatible with t2,
array, too then type(e1+e2) = list(lub(t1,t2))
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Set Operators
Set operators: union, intersect, and except (minus or
difference)
Operators work with multiplicity on bags.
bag(1,2) union bag(1,2,3)) = a bag of 1,2,1,2,3
bag(1,2,1,3) minus bag(2) = a bag of 1,1,3
distinct(bag(1,2,1,2,3)) = a set of 1,2,3
element({person1}) = Person: person1
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Conversion Operators for Collections
collectionExpr ::= first ( query )
| last ( query )
| unique ( query )
| exists ( query )
First(select students from course where course.name=“INF 312”)
conversionExpr ::= listtoset ( query )
| element ( query )
| distinct ( query )
| flatten ( query )
flatten(list(set(1,2,3),set(3,4,5,6),set(7)) = a set of 1,2,3,4,5,6,7
flatten(list(list(1,2),list(1,2,3)) = a list of 1,2,1,2,3
flatten(set(list(1,2),list(1,2,3)) = a set of 1,2,3
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OQL selectExpr
selectExpr ::= select [ distinct ]
projectionAttributes
fromClause [ whereClause ] [ groupClause ]
[orderClause ]
select f ( y [, partition] )
from x in X
where p(x) scope of x
group by y: g(x)
having h(y, partition)
order by o(y, partition)
scope of y
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Complex Example
For each museum, find all tours with a capacity of more than 20 that
have the same theme as some tour that already visits the museum.
select struct( M: m,
Tours: flatten ( select ( select s
from s in Tour
where s.theme = t.theme
and s.capacity > 20 )
from t in Tour
where m in ( select distinct w.what
from w in t.description ) ) )
from m in Museum
Type of result = struct(Museum, set(Tour))
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OQL is not Computationally Complete!
LEAST_FIXED_POINT operator cannot be expressed in
OQL
OQL is computationally supplemented by an OO
programming language through the ODMG programming
language bindings (DML).
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