06

  • Topics covered

    Architectural design decisions
    Architectural views
    Architectural patterns
    Application architectures

  • Architectural design decisions

  • Architectural design

    Architectural design is
    concerned with understanding how a software system should be organised and designing the overall structure of that system.
    the critical link between design and requirements engineering, as it identifies the main structural components in a system and the relationships between them.
    Architectural design is the output of the architectural design process is an architectural model that describes how the system is organised as a set of communicating components.

    Agility and architecture

    It is generally accepted that an early stage of agile processes is to design an overall systems architecture.
    Refactoring the system architecture is usually expensive because it affects so many components in the system

    Architectural abstraction

    Architecture in the small is concerned with the architecture of individual programs.
    At this level, we are concerned with the way that an individual program is decomposed into components.
    Architecture in the large is concerned with the architecture of complex enterprise systems that include other systems, programs, and program components.
    These enterprise systems are distributed over different computers, which may be owned and managed by different companies.

    Advantages of explicit architecture

    Stakeholder communication
    Architecture may be used as a focus of discussion by system stakeholders.
    System analysis
    Means that analysis of whether the system can meet its non-functional requirements is possible.
    Large-scale reuse
    The architecture may be reusable across a range of systems
    Product-line architectures may be developed.

  • Architectural representations

    Simple, informal block diagrams showing entities and relationships are the most frequently used method for documenting software architectures.
    But these have been criticized because they lack semantics, do not show the types of relationships between entities nor the visible properties of entities in the architecture.
    Depends on the use of architectural models. The requirements for model semantics depends on how the models are used.

    Useful link :

    📹 What is Software Architecture?
    📹 Architecting Software the SEI Way - Software Architecture Fundamentals: Technical, Business, and Social Influences.

    Box and line diagrams

    Very abstract - they do not show the nature of component relationships nor the externally visible properties of the sub-systems.
    However, useful for communication with stakeholders and for project planning.

    Use of architectural models

    As a way of facilitating discussion about the system design
    A high-level architectural view of a system is useful for communication with system stakeholders and project planning because it is not cluttered with detail. Stakeholders can relate to it and understand an abstract view of the system. They can then discuss the system as a whole without being confused by detail.
    As a way of documenting an architecture that has been designed
    The aim here is to produce a complete system model that shows the different components in a system, their interfaces and their connections.
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  • Architectural patterns

  • Architectural patterns

    Patterns are a means of representing, sharing and reusing knowledge.
    An architectural pattern is a stylized description of good design practice, which has been tried and tested in different environments.
    Patterns should include information about when they are and when the are not useful.
    Patterns may be represented using tabular and graphical descriptions.

    Layered architecture

    Used to model the interfacing of sub-systems.
    Organises the system into a set of layers (or abstract machines) each of which provide a set of services.
    Supports the incremental development of sub-systems in different layers. When a layer interface changes, only the adjacent layer is affected.
    However, often artificial to structure systems in this way.

    The Layered architecture pattern

    Name Layered architecture
    Description Organizes the system into layers with related functionality associated with each layer. A layer provides services to the layer above it so the lowest-level layers represent core services that are likely to be used throughout the system. See Figure 6.6.
    Example A layered model of a system for sharing copyright documents held in different libraries, as shown in Figure 6.7.
    When used Used when building new facilities on top of existing systems; when the development is spread across several teams with each team responsibility for a layer of functionality; when there is a requirement for multi-level security.
    Advantages Allows replacement of entire layers so long as the interface is maintained. Redundant facilities (e.g., authentication) can be provided in each layer to increase the dependability of the system.
    Disadvantages In practice, providing a clean separation between layers is often difficult and a high-level layer may have to interact directly with lower-level layers rather than through the layer immediately below it. Performance can be a problem because of multiple levels of interpretation of a service request as it is processed at each layer.

    A generic layered architecture

    User interface

    User interface management
    Authentication and authorisation

    Core business logic/application functionality
    System utilities

    System support (OS, database etc.)

  • The architecture of the iLearn system

    Browser-based user interface     iLearn app

    Configuration services
    Group management   Application management
    Identity management

    Application services
    Email, Messaging, Video conferencing, Newspaper archive, Word processing, Simulation, Video storage, Resource finder, Spreadsheet, Virtual learning environment, History archive

    Utility services
    Authentication, Logging and monitoring, Interfacing, User storage, Application storage, Search

    Client-server architecture

    Distributed system model which shows how data and processing is distributed across a range of components.
    Can be implemented on a single computer.
    Set of stand-alone servers which provide specific services such as printing, data management, etc.
    Set of clients which call on these services.
    Network which allows clients to access servers.

    The Client–server pattern

    Name Client-server
    Description In a client–server architecture, the functionality of the system is organized into services, with each service delivered from a separate server. Clients are users of these services and access servers to make use of them.
    Example Figure 6.11 is an example of a film and video/DVD library organized as a client–server system.
    When used Used when data in a shared database has to be accessed from a range of locations. Because servers can be replicated, may also be used when the load on a system is variable.
    Advantages The principal advantage of this model is that servers can be distributed across a network. General functionality (e.g., a printing service) can be available to all clients and does not need to be implemented by all services.
    Disadvantages Each service is a single point of failure so susceptible to denial of service attacks or server failure. Performance may be unpredictable because it depends on the network as well as the system. May be management problems if servers are owned by different organizations.

    📷 A client–server architecture for a film library
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    A client–server architecture for a film library

  • Application architectures

  • Application type examples

    Two very widely used generic application architectures are transaction processing systems and language processing systems.
    Transaction processing systems
    E-commerce systems;
    Reservation systems.
    Language processing systems
    Compilers;
    Command interpreters.

    Application architectures

    Application systems are designed to meet an organizational need.
    As businesses have much in common, their application systems also tend to have a common architecture that reflects the application requirements.
    A generic application architecture is an architecture for a type of software system that may be configured and adapted to create a system that meets specific requirements.

    Use of application 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.
    As a vocabulary for talking about application types.

    Examples of 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.

  • 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.

    The structure of transaction processing applications

    The structure of transaction processing applications diagram

    The software architecture of an ATM system

    The software architecture of an ATM system

    Information systems architecture

    Information systems have a generic architecture that can be organised as a layered architecture.
    These are transaction-based systems as interaction with these systems generally involves database transactions.
    Layers include:
    The user interface
    User communications
    Information retrieval
    System database

    Layered information system architecture

    User interface

    User communication
    Authentication and authorization

    Information retrieval and modification

    Transaction management
    Database

    The architecture of the Mentcare system

    Web browser

    Login, Role checking, Form and menu manager, Data validation

    Security management, Patient info manager, Data import and export, Report generation

    Transaction management
    Patient database

  • Web-based information systems

    Information and resource management systems are now usually web-based systems where the user interfaces are implemented using a web browser.
    For example, e-commerce systems are Internet-based resource management systems that accept electronic orders for goods or services and then arrange delivery of these goods or services to the customer.
    In an e-commerce system, the application-specific layer includes additional functionality supporting a ‘shopping cart’ in which users can place a number of items in separate transactions, then pay for them all together in a single transaction.

    Server implementation

    These systems are often implemented as multi-tier client server/architectures (discussed in Chapter 17)
    The web server is responsible for all user communications, with the user interface implemented using a web browser;
    The application server is responsible for implementing application-specific logic as well as information storage and retrieval requests;
    The database server moves information to and from the database and handles transaction management.

    Useful link:

    📹 Client Server Architecture video v 1 0 Java tutorials
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  • Key Points

    A software architecture is a description of how a software system is organized.
    Architectural design decisions include decisions on the type of application, the distribution of the system, the architectural styles to be used.
    Architectures may be documented from several different perspectives or views such as a conceptual view, a logical view, a process view, and a development view.
    Architectural patterns are a means of reusing knowledge about generic system architectures. They describe the architecture, explain when it may be used and describe its advantages and disadvantages.
    Models of application systems architectures help us understand and compare applications, validate application system designs and assess large-scale components for reuse.
    Transaction processing systems are interactive systems that allow information in a database to be remotely accessed and modified by a number of users.

    Useful Links

    📹 Architectural patterns for real-time systems

  • Glossary

    ACDEFHILMOPSUWX

    A

    application family

    A set of software application programs that have a common architecture and generic functionality. There can be tailored to the needs of specific customers by modifying components and program parameters.

    Data Source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.734

    application framework

    A set of reusable concrete and abstract classes that implement features common to many applications in a domain (e.g. user interface). The classes in the application framework are specialised and instantiated to create an application.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.734

    architectural pattern (style)

    An abstract description of a software architecture that has been tried and tested in a number of different software systems. The pattern description includes information about where it is appropriate to use the pattern and the organisation of the components of the architecture.

    architectural view

    A description of a software architecture from a particular perspective

    Data source: Sommerville I. 2016"Software Engineering" 10th Edition. Pearson Pp.758

    architecture

    Informally, a description of the organisation, motivation, and structure of a system. Many different levels of architectures are involved in developing software systems from physical hardware architecture to the logical architecture of application framework.

    Data source: Larman c. 2002 "Applying UML and Patterns", 2nd, Pearson. Pp.615

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    C

    class diagram

    A UML diagram types that shows the object classes in a system and their relationships. Entity classes define the functional requirements in software system.

    Data source: Sommerville I. 2016"Software Engineering" 10th Edition. Pearson Pp.759

    class hierarchy

    A description of the inheritance relations between classes.

    Data source: Larman c. 2002 "Applying UML and Patterns", 2nd, Pearson. Pp.615

    Client

    Note that some user-agents interpret HTML but don’t display it

    Client-Server architecture

    An architectural model for distributed systems where the system functionality is offered as as set of services provided by a server. There ae accessed by client computers that make use of the services. Variants of this approach, such as three-tier client-server architectures, use multiple servers.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.736

    component

    A deployable, independent unit of software that is completely defined and accessed through a set of interfaces.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.736

    component model

    A set of standards for component implementation, documentation and deployment. These cover the specific interfaces that may be provided by a component, component naming, component interoperation, and component composition. Component models provide the basis for middleware to support executing components.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.736-377

    conceptural design

    The development of a high-level vision of a complex system and a description of its essential capabilities. Designed to be understood by people who are not systems engineers.

    Data source: Sommerville I. 2016"Software Engineering" 10th Edition. Pearson Pp.761

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    D

    design pattern

    A well-tried solution to a common problem that captures experience and good practice in a form that can be reused. It is an abstract representation than can be instantiated in a number of ways.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp. 378

    domain

    A specific problem or business area where software systems are used. Examples of domains include real-time control, business data processing and telecommunications switching.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.738

    domain model

    A definition of domain abstractions, such as policies, procedures, objects, relationships and events. It serves as a base of knowledge about some problem area.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.738

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    E

    embedded system

    A software system that tis embedded in a hardware device (e.g. the software system in a mobile phone). Embedded systems are usually real-time systems and so have to respond in a timely way to events occurring in heir environment.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.738

    event-based systems

    Systems where the control of operation is determined by events that are generated in the system's environment. Most real-time systems are event-based systems.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.739

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    F

    FTP

    File Transfer Protocol, used on some servers that host downloadable materials

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    H

    HTML

    "Hyper Text Markup Language (HTML). Describes the logical structure of the documents instead of its formatting. This allows it to be displayed optimally on different platforms using different fonts and conventions."

    Data source: Cook c. and Carber J, 2012. "Foundation HTML5 with CSS3". Springer Pp.

    HTML5

    HTML5 is a markup language for describing web documents (web pages). HTML stands for Hyper Text Markup Language; HTML5 is supported in all modern browsers.

    Data source: Cook c. and Carber J, 2012. "Foundation HTML5 with CSS3". Springer Pp.

    HTTP Protocol

    "Hypertext Transport Protocol (set as the default). The Hypertext Transfer Protocol (HTTP) defines how browsers should request Web pages and how Web servers should respond to those requests."

    HTTPS

    Secure HTTP (used to exchange confidential information such as credit card numbers)

    Hyperlinks

    Hyperlinks are the backbone of web pages: they allow you to make connections to places within a web page and to other web pages.

    Hypertext

    "Text with links.The computer makes following references as easy as turning the page."

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    I

    Information system

    An informtion system is a system of communication between people. Information system are systems involved in the gathering, processing, distrution and use of information.

    Data source: Cook c. and Carber J, 2012. "Foundation HTML5 with CSS3". Springer Pp.

    interface

    A specification of the attributes and operations associated with a software component. The interface is used as the means of accessing the component's functionality.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.739

    internet

    A set of interconnected computer newtworks distributed around the globe

    Data source: Cook c. and Carber J, 2012. "Foundation HTML5 with CSS3". Springer Pp.

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    L

    legacy system

    A socio-technical system that is useful or essential to an organisation but which has been developed using obsolete technology or methods. Because legacy systems often perform critical business functions , they have to be maintained.

    Data source: Sommerville I. 2016"Software Engineering" 10th Edition. Pearson Pp.765

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    M

    Meta-information

    information about the document: its structure, formatting, etc.

    model-driven architecture (MDA)

    An approach to software development based on the construction of a set of system models, which can be automatically or semi-automatically processed to generate an executable system.

    Data source: Sommerville I. 2016"Software Engineering" 10th Edition. Pearson Pp.765

    model-driven development (MDD)

    An approach to software engineering centred around system models that are expressed in the UML. Rather than programming language code. This extends MDA to consider activities other than development such as requirements engineering and testing.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.741

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    O

    object class

    An object class defines that attributes and operations of objects. Objects are created at run-time by instantiating the class definition. The object class name can be used as a type name in some object-oriented language.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.741

    object model

    A model of a software system that is structured and organised as a set of object classes and the relationships between these classes. Various different perspectives on the model may exist such s a state perspective and a sequence perspective.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.742

    object-oriented (OO) development

    An approach to software development where the fundamental abstractions in the system are independent objects. The same type of abstraction is used during specification, design and development.

    Data source: Sommerville I. 2016"Software Engineering" 10th Edition. Pearson Pp.766

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    P

    POP

    Post Office Protocol (POP) make e-mail exchange possible. It is an application-layer Internet standard protocol which is used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP. connection.

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    S

    server

    "The computer that serves data to the client is, not surprisingly, known as a server. The Internet is riddled with servers, all storing and processing data and delivering it in response to client requests."

    single cell

    A single cell can span multiple columns or multiple rows.

    SMTP

    Simple Mail Transfer Protocol (SMTP). It is one of type of protocols at work on the Internet. It is an Internet standard for electronic mail (email) transmission.

    software architecture

    A model of the fundamental structure and organisation of a software system

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.747

    software development life cycle

    often used as another name for the software process; originally coined to refer to the watherfall model of the software process.

    Data source: Cook c. and Carber J, 2012. "Foundation HTML5 with CSS3". Springer Pp.

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    U

    URL

    URL: Uniform Resource Locator.

    URL address

    The Internet address portion of a URL can be either a name, e.g. www.gcu.ac.uk, or a number, e.g. 152.118.13.123

    URL protocol

    The protocol portion of a URL specifies the way that the web browser will interact with the server to obtain the resource.

    user interface design

    The process of designing the way in which system users can access system functionality, and the way that information produced by the system is displayed.

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    W

    web service

    An independent software component that can be accessed through the internet using standard protocols. It is completely self-contained without external dependencies. XML-based standards such as SOAP (standard Object Access Protocol), for web service information exchange, and WSDL (Web Service Definition language), for the definition of web service interfaces, have been developed. However, the REST approach may also be used for web service implementation.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.748

    workflow

    A detailed definition of a business process that is intended to accomplish a certain task. The workflow is usually expressed graphically and shows the individual process activities and the information that is produced and consumed by each activity.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.748

    WWW

    World Wide Web. A set of standards for hypermedia documentation. It now has become synonymous with the Internet.

    Data source: Cook c. and Carber J, 2012. "Foundation HTML5 with CSS3". Springer Pp.

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    X

    XML

    Extended Markup Language. XML is a text markup language that supports the interchange of structured data. Each data field is delimited by tags that five information about that field. XML is now very widely used and has become the basis of protocols for web services.

    Data source: Sommerville I. 2011"Software Engineering" 9th Edition. Pearson Pp.748

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