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Models of Software Architectures. Topics covered. Distributed architectures (Chapter 12) Client-server architectures Distributed object architectures Peer-to-peer architectures Service-oriented architectures Application architectures (Chapter 13) Data processing systems

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topics covered
Topics covered
  • Distributed architectures (Chapter 12)
    • Client-server architectures
    • Distributed object architectures
    • Peer-to-peer architectures
    • Service-oriented architectures
  • Application architectures (Chapter 13)
    • Data processing systems
    • Transaction processing systems
    • Event processing systems
    • Language processing systems
use of architectures
Use of architectures
  • As a starting point for architectural design.
  • As a design checklist.
  • As a way of organising the work of the development team.
  • As a means of assessing components for reuse.
  • We will examine some important architectural models from two perspectives:
    • Distribution strategy – how subsystems are decomposed and distributed over several computers
    • Application types – how subsystems are decomposed based on application-specific functionality
computer systems
Computer systems
  • Personal systems that are not distributed and that are 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 software runs on a loosely integrated group of cooperating processors linked by a network.
distributed systems
Distributed systems
  • Information processing is distributed over several computers rather than confined to a single machine.
  • Virtually all large computer-based systems are now distributed systems.
  • Distributed software engineering is therefore very important for enterprise computing systems.
distributed system characteristics
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.
distributed system disadvantages
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.
intra organizational distributed systems architectures
Intra-organizational distributed systems architectures
  • Client-server architectures
    • Distributed services which are called on by clients. Servers that provide services are treated differently from clients that use services.
  • Distributed object architectures
    • No distinction between clients and servers. Any object on the system may provide and use services from other objects.
middleware
Middleware
  • Software that manages and supports the different components of a distributed system. In essence, it sits in the middle of the system.
  • Middleware is usually off-the-shelf rather than specially written software.
  • Examples
    • Transaction processing monitors;
    • Data converters;
    • Communication controllers.
topics covered1
Topics covered
  • Distributed architectures (Chapter 12)
    • Client-server architectures
    • Distributed object architectures
    • Peer-to-peer architectures
    • Service-oriented architectures
  • Application architectures (Chapter 13)
    • Data processing systems
    • Transaction processing systems
    • Event processing systems
    • Language processing systems
client server architectures
Client-server architectures
  • The application is modelled as a set of services that are provided by servers and a set of clients that use these services.
  • Clients know of servers but servers need not know of clients.
  • Clients and servers are logical processes
  • The mapping of processors to processes is not necessarily 1 : 1.
layered application architecture
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 open account, close account, etc.
  • Data management layer
    • Concerned with managing the system databases.
thin and fat clients
Thin and fat clients
  • Thin-client model
    • In a thin-client model, all of the application processing and data management is carried out on the server. The client is simply responsible for running the presentation software.
  • Fat-client model
    • In this model, the server is only responsible for data management. The software on the client implements the application logic and the interactions with the system user.
thin client model
Thin client model
  • Used when legacy systems are migrated to client server 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 the network.
fat client model
Fat client model
  • More processing is delegated to the client as the application processing is locally executed.
  • Most suitable for new C/S systems where the capabilities of the client system are known in advance.
  • More complex than a thin client model especially for management. New versions of the application have to be installed on all clients.
three tier architectures
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.
topics covered2
Topics covered
  • Distributed architectures (Chapter 12)
    • Client-server architectures
    • Distributed object architectures
    • Peer-to-peer architectures
    • Service-oriented architectures
  • Application architectures (Chapter 13)
    • Data processing systems
    • Transaction processing systems
    • Event processing systems
    • Language processing systems
distributed object architectures
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.
  • However, distributed object architectures are more complex to design than C/S systems.
advantages of distributed object architecture
Advantages of distributed object architecture
  • 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.
uses of distributed object architecture
Uses of 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 common communication framework.
corba
CORBA
  • CORBA is an international standard for an Object Request Broker - middleware to manage communications between distributed objects.
  • Middleware for distributed computing is required at 2 levels:
    • At the logical communication level, the middleware allows objects on different computers to exchange data and control information;
    • At the component level, the middleware provides a basis for developing compatible components. CORBA component standards have been defined.
corba standards
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.
corba objects
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.
object request broker orb
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.
inter orb communications
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.
corba services
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.
topics covered3
Topics covered
  • Distributed architectures (Chapter 12)
    • Client-server architectures
    • Distributed object architectures
    • Peer-to-peer architectures
    • Service-oriented architectures
  • Application architectures (Chapter 13)
    • Data processing systems
    • Transaction processing systems
    • Event processing systems
    • Language processing systems
inter organisational computing
Inter-organisational computing
  • For security and inter-operability reasons, most distributed computing has been implemented at the enterprise level.
  • Local standards, management and operational processes apply.
  • Newer models of distributed computing have been designed to support inter-organisational computing where different nodes are located in different organisations.
peer to peer architectures
Peer-to-peer architectures
  • Peer to peer (p2p) systems are decentralised systems where computations may be carried out by any node in the network.
  • The overall system is designed to take advantage of the computational power and storage of a large number of networked computers.
  • Most p2p systems have been personal systems but there is increasing business use of this technology.
p2p architectural models
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.
topics covered4
Topics covered
  • Distributed architectures (Chapter 12)
    • Client-server architectures
    • Distributed object architectures
    • Peer-to-peer architectures
    • Service-oriented architectures
  • Application architectures (Chapter 13)
    • Data processing systems
    • Transaction processing systems
    • Event processing systems
    • Language processing systems
service oriented architectures
Service-oriented architectures
  • Based around the notion of externally provided services (web services).
  • A web service is a standard approach to making a reusable component available and accessible across the web
    • A tax filing service could provide support for users to fill in their tax forms and submit these to the tax authorities.
a generic service
A generic service
  • An act or performance offered by one party to another. Although the process may be tied to a physical product, the performance is essentially intangible and does not normally result in ownership of any of the factors of production.
  • Service provision is therefore independent of the application using the service.
services and distributed objects
Services and distributed objects
  • Provider independence.
  • Public advertising of service availability.
  • Potentially, run-time service binding.
  • Opportunistic construction of new services through composition.
  • Pay for use of services.
  • Smaller, more compact applications.
  • Reactive and adaptive applications.
services standards
Services standards
  • Services are based on agreed, XML-based standards so can be provided on any platform and written in any programming language.
  • Key standards
    • SOAP - Simple Object Access Protocol;
    • WSDL - Web Services Description Language;
    • UDDI - Universal Description, Discovery and Integration.
services scenario
Services scenario
  • An in-car information system provides drivers with information on weather, road traffic conditions, local information etc. This is linked to car radio so that information is delivered as a signal on a specific radio channel.
  • The car is equipped with GPS receiver to discover its position and, based on that position, the system accesses a range of information services. Information may be delivered in the driver’s specified language.
topics covered5
Topics covered
  • Distributed architectures (Chapter 12)
    • Client-server architectures
    • Distributed object architectures
    • Peer-to-peer architectures
    • Service-oriented architectures
  • Application architectures (Chapter 13)
    • Data processing systems
    • Transaction processing systems
    • Event processing systems
    • Language processing systems
generic application architectures
Generic application architectures
  • Application systems are designed to meet an organisational need.
  • As businesses have much in common, their application systems also tend to have a common architecture that reflects the application requirements.
  • A generic architecture is configured and adapted to create a system that meets specific requirements.
  • A generic architecture can be used as a basic vocabulary for talking about application types.
application types
Application types
  • Data processing applications
    • Data driven applications that process data in batches without explicit user intervention during the processing.
  • Transaction processing applications
    • Data-centred applications that process user requests and update information in a system database.
  • Event processing systems
    • Applications where system actions depend on interpreting events from the system’s environment.
  • Language processing systems
    • Applications where the users’ intentions are specified in a formal language that is processed and interpreted by the system.
application type examples
Application type examples
  • Data processing systems
    • Billing systems;
    • Payroll systems.
  • Transaction processing systems
    • E-commerce systems;
    • Reservation systems.
  • Event processing systems
    • Word processors;
    • Real-time systems.
  • Language processing systems
    • Compilers;
    • Command interpreters.
topics covered6
Topics covered
  • Distributed architectures (Chapter 12)
    • Client-server architectures
    • Distributed object architectures
    • Peer-to-peer architectures
    • Service-oriented architectures
  • Application architectures (Chapter 13)
    • Data processing systems
    • Transaction processing systems
    • Event processing systems
    • Language processing systems
data processing systems
Data processing systems
  • Systems that are data-centred where the databases used are usually orders of magnitude larger than the software itself.
  • Data is input and output in batches
    • Input: A set of customer numbers and associated readings of an electricity meter;
    • Output: A corresponding set of bills, one for each customer number.
  • Data processing systems usually have an input-process-output structure.
input process output
Input-process-output
  • The input component reads data from a file or database, checks its validity and queues the valid data for processing.
  • The process component takes a transaction from the queue (input), performs computations and creates a new record with the results of the computation.
  • The output component reads these records, formats them accordingly and writes them to the database or sends them to a printer.
data flow diagrams
Data-flow diagrams
  • Show how data is processed as it moves through a system.
  • Transformations are represented as round-edged rectangles, data-flows as arrows between them and files/data stores as rectangles.
topics covered7
Topics covered
  • Distributed architectures (Chapter 12)
    • Client-server architectures
    • Distributed object architectures
    • Peer-to-peer architectures
    • Service-oriented architectures
  • Application architectures (Chapter 13)
    • Data processing systems
    • Transaction processing systems
    • Event processing systems
    • Language processing systems
transaction processing systems
Transaction processing systems
  • Process user requests for information from a database or requests to update the database.
  • From a user perspective a transaction is:
    • Any coherent sequence of operations that satisfies a goal;
    • For example - find the times of flights from London to Paris.
  • Users make asynchronous requests for service which are then processed by a transaction manager.
  • Transaction processing systems are often also client-server systems.
transaction processing middleware
Transaction processing middleware
  • Transaction management middleware or teleprocessing monitors handle communications with different terminal types (e.g. ATMs and counter terminals), serialises data and sends it for processing.
  • Query processing takes place in the system database and results are sent back through the transaction manager to the user’s terminal.
layered system implementation
Layered system implementation
  • Each layer can be implemented as a large scale component running on a separate server. This is the most commonly used architectural model for web-based systems.
  • On a single machine, the middle layers are implemented as a separate program that communicates with the database through its API.
  • Fine-grain components within layers can be implemented as web services.
e commerce system architecture
E-commerce system architecture
  • E-commerce systems are Internet-based resource management systems that accept electronic orders for goods or services.
  • They are usually organised using a multi-tier architecture with application layers associated with each tier.
topics covered8
Topics covered
  • Distributed architectures (Chapter 12)
    • Client-server architectures
    • Distributed object architectures
    • Peer-to-peer architectures
    • Service-oriented architectures
  • Application architectures (Chapter 13)
    • Data processing systems
    • Transaction processing systems
    • Event processing systems
    • Language processing systems
event processing systems
Event processing systems
  • These systems respond to events in the system’s environment.
  • Their key characteristic is that event timing is unpredictable so the architecture has to be organised to handle this.
  • Many common systems such as word processors, games, etc. are event processing systems.
topics covered9
Topics covered
  • Distributed architectures (Chapter 12)
    • Client-server architectures
    • Distributed object architectures
    • Peer-to-peer architectures
    • Service-oriented architectures
  • Application architectures (Chapter 13)
    • Data processing systems
    • Transaction processing systems
    • Event processing systems
    • Language processing systems
language processing systems
Language processing systems
  • Accept a natural or artificial language as input and generate some other representation of that language.
  • May include an interpreter to act on the instructions in the language that is being processed.
  • Used in situations where the easiest way to solve a problem is to describe an algorithm or describe the system data
    • Meta-case tools process tool descriptions, method rules, etc and generate tools.
key points
Key points
  • Architectural models are useful starting points for system design.
  • In distributed systems, subsystems are distributed over several computers.
  • Generic models of application architectures help us understand and compare certain types of applications.
key points1
Key points
  • Client-server architectures
    • Involve services being delivered by servers to programs operating on clients.
    • Three layers distributed over client and server: user interface, application processing, data management.
  • Distributed object architectures
    • No distinction between clients and servers.
    • Require middleware to handle object communications and to add and remove system objects.
    • CORBA – a set of middleware standards.
  • Peer to peer architectures
    • Decentralised systems where computations may be carried out by any node in the network.
  • Service-oriented architectures
    • Make a service available externally.
key points2
Key points
  • Data processing systems
    • Data is input and output in batches.
  • Transaction processing systems
    • Users interact with the system through atomic data transactions.
    • Includes a transaction management subsystem.
  • Event processing systems
    • Respond to events in a timely manner.
    • No central controller; critical components respond directly to events.
  • Language processing systems
    • Process languages (natural or artificial) as input to accomplish some complex data processing function.
    • Includes a translator and an interpreter.