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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 1
Distributed Systems Architectures
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 2
Objectives
● To explain the advantages and disadvantages ofdifferent distributed systems architectures
● To discuss client-server and distributed objectarchitectures
● To describe object request brokers and theprinciples underlying the CORBA standards
● To introduce peer-to-peer and service-orientedarchitectures as new models of distributedcomputing.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 3
Topics covered
● Multiprocessor architectures
● Client-server architectures
● Distributed object architectures
● Inter-organisational computing
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 4
Distributed systems
● Virtually all large computer-based systemsare now distributed systems.
● Information processing is distributed overseveral computers rather than confined to asingle machine.
● Distributed software engineering is thereforevery important for enterprise computingsystems.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 5
System types
● Personal systems that are not distributed and thatare designed to run on a personal computer orworkstation.
● Embedded systems that run on a single processoror on an integrated group of processors.
● Distributed systems where the system software runson a loosely integrated group of cooperatingprocessors linked by a network.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 6
Distributed system characteristics
● Resource sharing• Sharing of hardware and software resources.
● Openness• Use of equipment and software from different vendors.
● Concurrency• Concurrent processing to enhance performance.
● Scalability• Increased throughput by adding new resources.
● Fault tolerance• The ability to continue in operation after a fault has
occurred.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 7
Distributed system disadvantages
● Complexity• Typically, distributed systems are more complex than
centralised systems.
● Security• More susceptible to external attack.
● Manageability• More effort required for system management.
● Unpredictability• Unpredictable responses depending on the system
organisation and network load.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 8
Distributed systems architectures
● Client-server architectures• Distributed services which are called on by
clients. Servers that provide services are treateddifferently from clients that use services.
● Distributed object architectures• No distinction between clients and servers. Any
object on the system may provide and useservices from other objects.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 9
Middleware
● Software that manages and supports the differentcomponents of a distributed system. In essence, itsits in the middle of the system.
● Middleware is usually off-the-shelf rather thanspecially written software.
● Examples• Transaction processing monitors;
• Data converters;
• Communication controllers.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 10
Multiprocessor architectures
● Simplest distributed system model.● System composed of multiple processes
which may (but need not) execute ondifferent processors.
● Architectural model of many large real-timesystems.
● Distribution of process to processor may bepre-ordered or may be under the control of adispatcher.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 11
A multiprocessor traffic control system
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 12
Client-server architectures
● The application is modelled as a set ofservices that are provided by servers and aset of clients that use these services.
● Clients know of servers but servers need notknow of clients.
● Clients and servers are logical processes
● The mapping of processors to processes isnot necessarily 1 : 1.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 13
A client-server system
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 14
Computers in a C/S network
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 15
Layered application architecture
● Presentation layer• Concerned with presenting the results of a computation to
system 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 openaccount, close account, etc.
● Data management layer• Concerned with managing the system databases.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 16
Application layers
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 17
Thin and fat clients
● Thin-client model• In a thin-client model, all of the application
processing and data management is carried outon the server. The client is simply responsiblefor running the presentation software.
● Fat-client model• In this model, the server is only responsible for
data management. The software on the clientimplements the application logic and theinteractions with the system user.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 18
Thin and fat clients
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 19
Thin client model
● Used when legacy systems are migrated toclient server architectures.• The legacy system acts as a server in its own
right with a graphical interface implemented ona client.
● A major disadvantage is that it places aheavy processing load on both the serverand the network.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 20
Fat client model
● More processing is delegated to the client asthe application processing is locallyexecuted.
● Most suitable for new C/S systems where thecapabilities of the client system are known inadvance.
● More complex than a thin client modelespecially for management. New versions ofthe application have to be installed on allclients.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 21
A client-server ATM system
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 22
Three-tier architectures
● In a three-tier architecture, each of theapplication architecture layers may executeon a separate processor.
● Allows for better performance than a thin-client approach and is simpler to managethan a fat-client approach.
● A more scalable architecture - as demandsincrease, extra servers can be added.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 23
A 3-tier C/S architecture
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 24
An internet banking system
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 25
Use of C/S architectures
Architecture ApplicationsTwo-tier C/Sarchitecture withthin clients
Legacy system applications where separating application processing anddata management is impractical.Computationally-intensive applications such as compilers with little orno data management.Data-intensive applications (browsing and querying) with little or noapplication processing.
Two-tier C/Sarchitecture withfat clients
Applications where application processing is provided by off-the-shelfsoftware (e.g. Microsoft Excel) on the client.Applications where computationally-intensive processing of data (e.g.data visualisation) is required.Applications with relatively stable end-user functionality used in anenvironment 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 are volatile.Applications where data from multiple sources are integrated.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 26
Distributed object architectures
● There is no distinction in a distributed objectarchitectures between clients and servers.
● Each distributable entity is an object that providesservices to other objects and receives services fromother objects.
● Object communication is through a middlewaresystem called an object request broker.
● However, distributed object architectures are morecomplex to design than C/S systems.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 27
Distributed object architecture
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 28
Advantages of distributed object architecture
● It allows the system designer to delay decisions onwhere and how services should be provided.
● It is a very open system architecture that allows newresources to be added to it as required.
● The system is flexible and scaleable.
● It is possible to reconfigure the system dynamicallywith objects migrating across the network asrequired.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 29
Uses of distributed object architecture
● As a logical model that allows you to structure andorganise the system. In this case, you think abouthow to provide application functionality solely interms of services and combinations of services.
● As a flexible approach to the implementation ofclient-server systems. The logical model of thesystem is a client-server model but both clients andservers are realised as distributed objectscommunicating through a common communicationframework.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 30
A data mining system
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 31
Data mining system
● The logical model of the system is not one ofservice provision where there aredistinguished data management services.
● It allows the number of databases that areaccessed to be increased without disruptingthe system.
● It allows new types of relationship to bemined by adding new integrator objects.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 32
CORBA
● CORBA is an international standard for an ObjectRequest Broker - middleware to managecommunications between distributed objects.
● Middleware for distributed computing is required at 2levels:• At the logical communication level, the middleware allows
objects on different computers to exchange data andcontrol information;
• At the component level, the middleware provides a basisfor developing compatible components. CORBAcomponent standards have been defined.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 33
CORBA application structure
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 34
Application structure
● Application objects.
● Standard objects, defined by the OMG, for aspecific domain e.g. insurance.
● Fundamental CORBA services such asdirectories and security management.
● Horizontal (i.e. cutting across applications)facilities such as user interface facilities.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 35
CORBA standards
● An object model for application objects• A CORBA object is an encapsulation of state
with a well-defined, language-neutral interfacedefined in an IDL (interface definition language).
● An object request broker that managesrequests for object services.
● A set of general object services of use tomany distributed applications.
● A set of common components built on top ofthese services.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 36
CORBA objects
● CORBA objects are comparable, in principle, toobjects in C++ and Java.
● They MUST have a separate interface definition thatis expressed using a common language (IDL) similarto C++.
● There is a mapping from this IDL to programminglanguages (C++, Java, etc.).
● Therefore, objects written in different languages cancommunicate with each other.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 37
Object request broker (ORB)
● The ORB handles object communications. It knowsof all objects in the system and their interfaces.
● Using an ORB, the calling object binds an IDL stubthat defines the interface of the called object.
● Calling this stub results in calls to the ORB whichthen calls the required object through a publishedIDL skeleton that links the interface to the serviceimplementation.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 38
ORB-based object communications
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 39
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 developed.
● ORBs handle communications between objectsexecuting on the sane machine.
● Several ORBS may be available and each computerin a distributed system will have its own ORB.
● Inter-ORB communications are used for distributedobject calls.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 40
Inter-ORB communications
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 41
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.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 42
Inter-organisational computing
● For security and inter-operability reasons,most distributed computing has beenimplemented at the enterprise level.
● Local standards, management andoperational processes apply.
● Newer models of distributed computing havebeen designed to support inter-organisational computing where differentnodes are located in different organisations.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 43
Peer-to-peer architectures
● Peer to peer (p2p) systems are decentralisedsystems where computations may be carried out byany node in the network.
● The overall system is designed to take advantage ofthe computational power and storage of a largenumber of networked computers.
● Most p2p systems have been personal systems butthere is increasing business use of this technology.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 44
P2p architectural models
● The logical network architecture• Decentralised architectures;
• Semi-centralised architectures.
● Application architecture• The generic organisation of components making
up a p2p application.
● Focus here on network architectures.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 45
Decentralised p2p architecture
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 46
Semi-centralised p2p architecture
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 47
Service-oriented architectures
● Based around the notion of externallyprovided services (web services).
● A web service is a standard approach tomaking a reusable component available andaccessible across the web• A tax filing service could provide support for
users to fill in their tax forms and submit theseto the tax authorities.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 48
A generic service
● An act or performance offered by one partyto another. Although the process may be tiedto a physical product, the performance isessentially intangible and does not normallyresult in ownership of any of the factors ofproduction.
● Service provision is therefore independent ofthe application using the service.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 49
Web services
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 50
Services and distributed objects
● Provider independence.
● Public advertising of service availability.
● Potentially, run-time service binding.
● Opportunistic construction of new services throughcomposition.
● Pay for use of services.
● Smaller, more compact applications.
● Reactive and adaptive applications.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 51
Services standards
● Services are based on agreed, XML-basedstandards so can be provided on anyplatform and written in any programminglanguage.
● Key standards• SOAP - Simple Object Access Protocol;
• WSDL - Web Services Description Language;
• UDDI - Universal Description, Discovery andIntegration.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 52
Services scenario
● An in-car information system provides drivers withinformation on weather, road traffic conditions, localinformation etc. This is linked to car radio so thatinformation is delivered as a signal on a specificradio channel.
● The car is equipped with GPS receiver to discoverits position and, based on that position, the systemaccesses a range of information services.Information may be delivered in the driver’s specifiedlanguage.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 53
Automotive system
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 54
● Distributed systems support resource sharing,openness, concurrency, scalability, fault toleranceand transparency.
● Client-server architectures involve services beingdelivered by servers to programs operating onclients.
● User interface software always runs on the clientand data management on the server. Applicationfunctionality may be on the client or the server.
● In a distributed object architecture, there is nodistinction between clients and servers.
Key points
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 12 Slide 55
Key points
● Distributed object systems require middleware tohandle object communications and to add andremove system objects.
● The CORBA standards are a set of middlewarestandards that support distributed objectarchitectures.
● Peer to peer architectures are decentralisedarchitectures where there is no distinction betweenclients and servers.
● Service-oriented systems are created by linkingsoftware services provided by different servicesuppliers.