CSC 480 Software Engineering Lecture 18 Nov 6, 2002.
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Transcript of CSC 480 Software Engineering Lecture 18 Nov 6, 2002.
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CSC 480 Software Engineering
Lecture 18Nov 6, 2002
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Application Layers
Presentation layer
Application processinglayer
Data managementlayer
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Three-tier architectures
In a three-tier architecture, each of the application architecture layers may execute on a separate processor
Allows for better performance than a thin-client approach and is simpler to manage than a fat-client approach
A more scalable architecture - as demands increase, extra servers can be added
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A Three-Tier C/S architecture
Client
Server
Datamanagement
PresentationServer
Applicationprocessing
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An Internet Banking System
Database server
Customeraccountdatabase
Web server
Client
Client
Client
Client
Account serviceprovision
SQLSQL query
HTTP interaction
Account serviceprovision
Application Server
HTML filetransfer
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Use of C/S ArchitecturesArchitecture ApplicationsTwo-tier C/Sarchitecture withthin clients
Legacy system applications where separating applicationprocessing and data management is impracticalComputationally-intensive applications such as compilers withlittle or no data managementData-intensive applications (browsing and querying) with littleor no application processing.
Two-tier C/Sarchitecture withfat clients
Applications where application processing is provided byCOTS (e.g. Microsoft Excel) on the clientApplications where computationally-intensive processing ofdata (e.g. data visualisation) is required.Applications with relatively stable end-user functionality usedin an environment with well-established system management
Three-tier ormulti-tier C/Sarchitecture
Large scale applications with hundreds or thousands of clientsApplications where both the data and the application arevolatile.Applications where data from multiple sources are integrated
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Distributed Object Architectures
There is no distinction in a distributed object architectures between clients and servers
Each distributable entity is an object that provides services to other objects and receives services from other objects
Object communication is through a middleware system called an object request broker (software bus)
However, more complex to design than C/S systems
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Distributed Object Architecture
Software bus
o1 o2 o3 o4
o5 o6
S (o1) S (o2) S (o3) S (o4)
S (o5) S (o6)
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Advantages
It allows the system designer to delay decisions on where and how services should be provided
It is a very open system architecture that allows new resources to be added to it as required
The system is flexible and scaleable It is possible to reconfigure the system
dynamically with objects migrating across the network as required
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Uses – distributed object architecture
As a logical model that allows you to structure and organise the system. In this case, you think about how to provide application functionality solely in terms of services and combinations of services
As a flexible approach to the implementation of client-server systems. The logical model of the system is a client-server model but both clients and servers are realised as distributed objects communicating through a software bus
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A Data Mining SystemDatabase 1
Database 2
Database 3
Integrator 1
Integrator 2
Visualiser
Display
Report gen.
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Data Mining System
The logical model of the system is not one of service provision where there are distinguished data management services
It allows the number of databases that are accessed to be increased without disrupting the system
It allows new types of relationship to be mined by adding new integrator objects
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CORBA
CORBA is an international standard for an Object Request Broker - middleware to manage communications between distributed objects
Several implementation of CORBA are available DCOM is an alternative approach by Microsoft to
object request brokers CORBA has been defined by the Object
Management Group
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Application structure
Application objects Standard objects, defined by the OMG, for a
specific domain e.g. insurance (Vertical) Fundamental CORBA services such as
directories and security management Horizontal (i.e. cutting across applications)
facilities such as user interface facilities
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CORBA Application Structure
CORBA services
Object request broker
Domainfacilities
HorizontalCORBA facilities
Applicationobjects
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CORBA Standards
An object model for application objects A CORBA object is an encapsulation of state with a
well-defined, language-neutral interface defined in an IDL (interface definition language)
An object request broker that manages requests for object services
A set of general object services of use to many distributed applications
A set of common components built on top of these services
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CORBA objects
CORBA objects are comparable, in principle, to objects in C++ and Java
They MUST have a separate interface definition that is expressed using a common language (IDL) similar to C++
There is a mapping from this IDL to programming languages (C++, Java, etc.)
Therefore, objects written in different languages can communicate with each other
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Transparence Through IDL
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CORBA Services
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Structure of ORB
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CORBA Business Objects
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What does IDL look like?
IDL interface definitions are elegant and (usually!) easy to write and understand. interface salestax { float calculate_tax ( in float taxable_amount );}
Interface to an object that calculates sales tax in a store's computing system. The object's type is salestax, and it can perform one
operation: calculate_tax. The object takes one input parameter, taxable_amount,
which is a float. The return value is also a float.
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Object Request Broker (ORB)
The ORB handles object communications. It knows of all objects in the system and their interfaces
Using an ORB, the calling object binds an IDL stub that defines the interface of the called object
Calling this stub results in calls to the ORB which then calls the required object through a published IDL skeleton that links the interface to the service implementation
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ORB-based Object Comm’tions
o1 o2
S (o1) S (o2)
IDLstub
IDLskeleton
Object Request Broker
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Inter-ORB Communications
ORBs are not usually separate programs but are a set of objects in a library that are linked with an application when it is developed
ORBs handle communications between objects executing on the sane machine
Several ORBS may be available and each computer in a distributed system will have its own ORB
Inter-ORB communications are used for distributed object calls
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Inter-ORB Communications
o1 o2
S (o1) S (o2)
IDL IDL
Object Request Broker
o3 o4
S (o3) S (o4)
IDL IDL
Object Request Broker
Network
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CORBA Services
Naming and trading services These allow objects to discover and refer to other
objects on the network
Notification services These allow objects to notify other objects that an
event has occurred
Transaction services These support atomic transactions and rollback on
failure