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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 1
Distributed Systems Architectures
Architectural design for software
that executes on more than one processor
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 2
Objectives
To explain the advantages and disadvantages of distributed systems architecturesTo describe different approaches to thedevelopment of client-server systemsTo explain the differences between client-server and distributed object architectures
To describe object request brokers and the principles underlying the CORBA standards
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 3
Topics covered
Multiprocessor architecturesClient-server architectures
Distributed object architecturesCORBA
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 4
Distributed systems
Virtually all large computer-based systems arenow distributed systemsInformation processing is distributed over severalcomputers rather than confined to a singlemachineDistributed software engineering is now very
important
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 5
System types
Personal systems that are not distributed and thatare designed to run on a personal computer or workstation.
Embedded systems that run on a single processor or on an integrated group of processors.
Distributed systems where the system softwareruns on a loosely integrated group of cooperating
processors linked by a network.
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 6
Distributed system characteristics
Resource sharingOpenness
ConcurrencyScalabilityFault toleranceTransparency
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 7
Distributed system disadvantages
ComplexitySecurity
ManageabilityUnpredictability
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Issues in distributed system design
Design issue Description Resourceidentification
The resources in a distributed system are spread across differentcomputers and a naming scheme has to be devised so that users candiscover and refer to the resources that they need. An example of such a naming scheme is the URL (Uniform Resource Locator) thatis used to identify WWW pages. If a meaningful and universallyunderstood identification scheme is not used then many of theseresources will be inaccessible to system users.
Communications The universal availability of the Internet and the efficientimplementation of Internet TCP/IP communication protocols meansthat, for most distributed systems, these are the most effective way
for the computers to communicate. However, where there arespecific requirements for performance, reliability etc. alternativeapproaches to communications may be used.
Quality of service The quality of service offered by a system reflects its performance,availability and reliability. It is affected by a number of factors suchas the allocation of processes to processes in the system, thedistribution of resources across the system, the network and thesystem hardware and the adaptability of the system.
Softwarearchitectures
The software architecture describes how the applicationfunctionality is distributed over a number of logical components andhow these components are distributed across processors. Choosingthe right architecture for an application is essential to achieve thedesired quality of service.
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 9
Distributed systems archiectures
Client-server architecturesDistributed services which are called on by clients. Servers that
provide services are treated differently from clients that use
servicesDistributed object architectures
No distinction between clients and servers. Any object on thesystem may provide and use services from other objects
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 10
Middleware
Software that manages and supports the differentcomponents of a distributed system. In essence, itsits in the middle of the systemMiddleware is usually off-the-shelf rather thanspecially written softwareExamples
Transaction processing monitorsData convertorsCommunication controllers
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 11
Multiprocessor architectures
Simplest distributed system modelSystem composed of multiple processes whichmay (but need not) execute on different
processorsArchitectural model of many large real-timesystems
Distribution of process to processor may be pre-ordered or may be under the control of adespatcher
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 12
A multiprocessor traffic control system
Traffic lights
Lightcontrolprocess
Traffic light controlprocessor
Traffic flowprocessor
Operator consolesTraffic flow sensors
and cameras
Sensorprocessor
Sensorcontrolprocess
Displayprocess
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 13
Client-server architectures
The application is modelled as a set of servicesthat are provided by servers and a set of clientsthat use these servicesClients know of servers but servers need notknow of clientsClients and servers are logical processes
The mapping of processors to processes is notnecessarily 1 : 1
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 14
A client-server system
s1
s2 s3
s4c1
c2 c3 c4
c5
c6c7 c8
c9
c10
c11
c12
Client process
Server process
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 15
Computers in a C/S network
Network
SC1SC2
CC1 CC2 CC3
CC5 CC6CC4
Servercomputer
Clientcomputer
s1, s2 s3, s4
c5, c6, c7
c1 c2 c3, c4
c8, c9 c10, c11, c12
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 16
Layered application architecture
Presentation layer Concerned with presenting the results of a computation tosystem users and with collecting user inputs
Application processing layer Concerned with providing application specific functionality e.g.,in a banking system, banking functions such as open account,close account, etc.
Data management layer Concerned with managing the system databases
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 17
Application layers
Presentation layer
Application processinglayer
Data managementlayer
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 18
Thin and fat clientsT hin-client model
In a thin-client model, all of the application processing and datamanagement is carried out on the server. The client is simply
responsible for running the presentation software.F at-client model
In this model, the server is only responsible for datamanagement. The software on the client implements theapplication logic and the interactions with the system user.
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 19
Thin and fat clients
Thin-clientmodel
Fat-clientmodel Client
Client
Server
Data managementApplicationprocessing
Presentation
Server
Data
management
PresentationApplication processing
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 20
Thin client model
Used when legacy systems are migrated to clientserver architectures.
The legacy system acts as a server in its own right with a
graphical interface implemented on a client
A major disadvantage is that it places a heavy processing load on both the server and thenetwork
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 21
Fat client model
More processing is delegated to the client as theapplication processing is locally executedMost suitable for new C/S systems where thecapabilities of the client system are known inadvanceMore complex than a thin client model especially
for management. New versions of the applicationhave to be installed on all clients
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 22
A client-server ATM system
Account server
Customeraccountdatabase
Tele-processing
monitor
ATM
ATM
ATM
ATM
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 23
Three-tier architectures
In a three-tier architecture, each of theapplication architecture layers may execute on aseparate processor Allows for better performance than a thin-clientapproach and is simpler to manage than a fat-client approach
A more scalable architecture - as demandsincrease, extra servers can be added
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 24
A 3-tier C/S architecture
Client
Server
Datamanagement
PresentationServer
Applicationprocessing
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 25
An internet banking system
Database server
Customeraccountdatabase
Web server
Client
Client
Client
Client
Account serviceprovision
SQLSQL query
HTTP interaction
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 26
Use of C/S architecturesArchitect re Appl icatio sTwo-tier C/Sarchitecture withthin clients
egacy system applications where separating application processing and data management is impracticalComputationally-intensive applications such as compilers withlittle or no data management
ata-intensive applications (browsing and querying) with littleor no application processing.Two-tier C/Sarchitecture withfat clients
pplications where application processing is provided byC TS (e.g. icrosoft Excel) on the client
pplications where computationally-intensive processing of data (e.g. data visualisation) is required.
pplications with relatively stable end-user functionality usedin an environment with well-established system management
Three-tier or multi-tier C/Sarchitecture
arge scale applications with hundreds or thousands of clients pplications where both the data and the application are
volatile. pplications where data from multiple sources are integrated
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 27
Distributed object architectures
There is no distinction in a distributed objectarchitectures between clients and serversEach distributable entity is an object that providesservices to other objects and receives servicesfrom other objectsObject communication is through a middleware
system called an object request broker (software bus)However, more complex to design than C/Ssystems
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 28
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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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 29
Advantages of distributed object architecture
It allows the system designer to delay decisionson where and how services should be providedIt is a very open system architecture that allowsnew resources to be added to it as requiredThe system is flexible and scaleableIt is possible to reconfigure the system
dynamically with objects migrating across thenetwork as required
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 30
Uses of distributed object architecture
As a logical model that allows you to structureand organise the system. In this case, you think about how to provide application functionalitysolely in terms of services and combinations of servicesAs a flexible approach to the implementation of client-server systems. The logical model of thesystem is a client-server model but both clientsand servers are realised as distributed objectscommunicating through a software bus
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 31
A data mining system
Database 1
Database 2
Database 3
Integrator 1
Integrator 2
Visualiser
Display
Report gen.
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 32
Data mining system
The logical model of the system is not one of service provision where there are distinguisheddata management servicesIt allows the number of databases that areaccessed to be increased without disrupting thesystem
It allows new types of relationship to be mined byadding new integrator objects
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 33
CORBA
CORBA is an international standard for an ObjectRequest Broker - middleware to managecommunications between distributed objectsSeveral implementation of CORBA are availableDCOM is an alternative approach by Microsoft toobject request brokers
CORBA has been defined by the ObjectManagement Group
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 34
Application structure
Application objectsStandard objects, defined by the OMG, for aspecific domain e.g. insuranceFundamental CORBA services such as directoriesand security managementHorizontal (i.e. cutting across applications)
facilities such as user interface facilities
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 35
CORBA application structure
CORBA services
Object request broker
Domainfacilities
HorizontalCORBA facilities
Applicationobjects
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 36
CORBA standards
An object model for application objectsA CORBA object is an encapsulation of state with a well-defined, language-neutral interface defined in an IDL (interfacedefinition language)
An object request broker that manages requestsfor object servicesA set of general object services of use to many
distributed applicationsA set of common components built on top of these services
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 37
CORBA objects
CORBA objects are comparable, in principle, toobjects in C++ and JavaThey MUST have a separate interface definitionthat 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 languagescan communicate with each other
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 38
Object request broker (ORB)
The ORB handles object communications. Itknows of all objects in the system and their interfacesUsing an ORB, the calling object binds an IDLstub that defines the interface of the called objectCalling this stub results in calls to the ORB which
then calls the required object through a publishedIDL skeleton that links the interface to the serviceimplementation
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 39
ORB-based object communications
o1 o2
S (o1) S (o2)
IDLstub
IDLskeleton
Object Request Broker
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 40
Inter-ORB communications
ORBs are not usually separate programs but are aset of objects in a library that are linked with anapplication when it is developedORBs handle communications between objectsexecuting on the sane machineSeveral ORBS may be available and each
computer in a distributed system will have itsown ORBInter-ORB communications are used for distributed object calls
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 41
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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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 42
CORBA services
Naming and trading servicesThese allow objects to discover and refer to other objects on thenetwork
Notification servicesThese allow objects to notify other objects that an event hasoccurred
Transaction services
These support atomic transactions and rollback on failure
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 43
Almost all new large systems are distributedsystemsDistributed systems support resource sharing,openness, concurrency, scalability, fault toleranceand transparencyClient-server architectures involve services being
delivered by servers to programs operating onclientsUser interface software always runs on the clientand data management on the server
Key points
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Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Slide 44
Key points
In a distributed object architecture, there is nodistinction between clients and serversDistributed object systems require middleware tohandle object communicationsThe CORBA standards are a set of middlewarestandards that support distributed object
architectures
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