Chapter 6 – Architectural Design

1. Chapter 6 – Architectural Design Lecture 1 Chapter 6 Architectural design

2. Topics covered • Architectural design decisions • Architectural views • Architectural patterns • Application architectures Chapter 6 Architectural design

3. Software architecture • The design process for identifying the sub-systems making up a system and the framework for sub-system control and communication is architectural design. • The output of this design process is a description of thesoftware architecture. Chapter 6 Architectural design

4. Architectural design • Architectural design is concerned with understanding how a system should be organized and designing the overall structure of the system • An early stage of the system design process. • Represents the link between specification and design processes • In practice there is overlap between the processes of requirements and architectural design and is carried out in parallel with some specification activities. • It involves identifying major system components and their communications. Chapter 6 Architectural design

5. The architecture of a packing robot control system Chapter 6 Architectural design

6. Architectural abstraction You can design software architectures at 2 abstraction levels : • 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. Chapter 6 Architectural design

7. Non functionalrequirements and software architecture • Non functionalrequirementsdepends on the system architecture. • Software architecture is important because it affects : • Performance • Robustness • Distributivity • Manitainbility Chapter 6 Architectural design

8. Advantages of explicit architecture Explicitely designing and documenting a software architecture has 3 main advantages : • Stakeholder communication • Architecture may be used as a focus of discussion by system stakeholders. • System analysis • Architecture design requires some analysis , so It is a mean of whether or not the system can meet its non-functional requirements ( such as performance , reliability and maintainbility ). • Large-scale reuse • The architecture may be reusable across a range of systems that are similar • Product-line architectures may be developed where the same architecture is reused across a range of related system Chapter 6 Architectural design

9. Architectural representations System architecture are often modeled using: • Simple, informal block diagrams showing entities and relationships are the most frequently used method for documenting software architectures. • But these have been critisisedbecause 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. Chapter 6 Architectural design

10. 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. Chapter 6 Architectural design

11. 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. Chapter 6 Architectural design

12. Architectural design decisions • Architectural design is a creative process , where you design a system organization that will satisfy the functional and non functional requirements. • So the process differs depending on the type of system being developed. • Think of architectural design as a series of decisions to be made rather than a sequence of activities. • However, a number of common decisions span all design processes and these decisions affect the non-functional characteristics of the system. Chapter 6 Architectural design

13. Architectural design decisions Based on their experience and knowledge , system architects have to consider these fundamental questions: • Is there a generic application architecture that can be used? • How will the system be distributed? • What architectural styles are appropriate? • What approach will be used to structure the system? • How will the system be decomposed into modules? • What control strategy should be used? • How will the architectural design be evaluated? • How should the architecture be documented? Chapter 6 Architectural design

14. Architecture reuse • Systems in the same domain often have similar architectures that reflect domain concepts. • Application product lines are application that are built around a core architecture with variants that satisfy particular customer requirements. So when designing an architecture , you have to decide what your system and broader application classes have in common, then decide how much knowledge can be reused. • The architecture of a system may be designed around one of more architectural patterns or ‘styles’. • These capture the essence of an architecture and can be instantiated in different ways. • Discussed later in this lecture. Chapter 6 Architectural design

15. Architecture and system characteristics For distributed systems across many different computers, the choice of distribution architecture is a key decision that affects : • Performance • Security • Safety • Availability • Maintainability Chapter 6 Architectural design

16. Architecture and system characteristics Because of the close relationship between non functional requirements and software architecture , the particular architectural style and structure that you choose for a system should depend on the non functional system requirements: • Performance • If performance is a critical requirement , localize critical operations within a small number of components , deployed on the same computer , and minimize communications. • Use large rather than fine-grain components. • Security • If security is a critical requirement , Use a layered architecture with critical assets in the innermost layers, with a high level of security validation applied to these layers. Chapter 6 Architectural design

17. Architecture and system characteristics • Safety • Localise safety-critical features in a single component or in small number of sub-systems. • Availability • Include redundant components so that it is possible to replace and update component without stopping the system. • Maintainability • Use fine-grain, replaceable components. • Producers of data should be separated from consumers and shared data structure should be avoided Chapter 6 Architectural design

18. Architectural views Architecture models may be used to document a design so it can be used as a basis for more detailed design and implementation. 2 issues may be raised : • What views or perspectives are useful when designing and documenting a system’s architecture? • What notations should be used for describing architectural models? Each architectural model only shows one view or perspective of the system. • It might show how a system is decomposed into modules, how the run-time processes interact or the different ways in which system components are distributed across a network. For both design and documentation, you usually need to present multiple views of the software architecture. Chapter 6 Architectural design

19. 4 + 1 view model of software architecture There are 4 fundamental architectural views , which are related using use cases or scenarios : • A logical view, which shows the key abstractions in the system as objects or object classes. It should be possible to relate requirements to entities • A process view, which shows how, at run-time, the system is composed of interacting processes. Useful for making judgment about non functional req such as performance. • A development view, which shows how the software is decomposed for development, that is the breakdown of the software into components that should be implemented by a single developer. • A physical view, which shows the system hardware and how software components are distributed across the processors in the system. Useful for planning development • Related using use cases or scenarios (+1) Chapter 6 Architectural design

20. 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. Chapter 6 Architectural design

21. The Model-View-Controller (MVC) pattern Chapter 6 Architectural design

22. The organization of the Model-View-Controller • Separation and independence are fundamental to architectural design • It allows changes to be localized. • MVC pattern separate elements , allowing them to change independently. • Adding a view or changing an existing view can be done without any change to the underlying data in the model Chapter 6 Architectural design

23. Web application architecture using the MVC pattern Chapter 6 Architectural design

24. Layered architecture • Achieve separations and independence • 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. Chapter 6 Architectural design

25. The Layered architecture pattern Chapter 6 Architectural design

26. Layered architecture • Lowest layer include system support software-ex: DB support • Application layer: components concerned with the application functionalities and utility components used by other application components. • User Interface management , authentication and authorization. • User interface NB: any layer can split into two or more layers Chapter 6 Architectural design

27. The architecture of the LIBSYS system Chapter 6 Architectural design

28. 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. Chapter 6 Architectural design

29. Chapter 6 – Architectural Design Lecture 2 Chapter 6 Architectural design

30. Repository architecture Used for systems that use a large amount of data (shared database) • Sub-systems must exchange data. This may be done in two ways: • Shared data is held in a central database or repository and may be accessed by all sub-systems; • Each sub-system maintains its own database and passes data explicitly to other sub-systems. • When large amounts of data are to be shared, the repository model of sharing is most commonly used a this is an efficient data sharing mechanism. Chapter 6 Architectural design

31. The Repository pattern Chapter 6 Architectural design

32. A repository architecture for an IDE Organizing tools around a repository is an efficient way to share large amount of data Chapter 6 Architectural design

33. Client-server architecture A system that follows the client-server pattern is organized as a set of services and associated servers, and clients that access and use the services. Major components are : • Distributed system model which shows how data and processing is distributed across a range of components. • But 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. Clients may not have to know name the available servers /services Access is done through remote calls using a request-reply protocol (ex:www) Chapter 6 Architectural design

34. The Client–server pattern Chapter 6 Architectural design

35. A client–server architecture for a film library Chapter 6 Architectural design

36. Pipe and filter architecture • System where functional transformations process their inputs to produce outputs. • Data flows from one to another and is transformed as it moves through the sequence. • May be referred to as a pipe and filter model (as in UNIX shell). • Unix programs. The output of one program can be linked to the input of another program. • Compilers. The consecutive filters perform lexical analysis, parsing, semantic analysis, and code generation. • Variants of this approach are very common • Not really suitable for interactive systems. Chapter 6 Architectural design

37. . When should you use it? This architecture is great if you have a lot of transformations to perform and you need to be very flexible in using them, yet you want them to be robust. Chapter 6 Architectural design

38. The pipe and filter pattern Chapter 6 Architectural design

39. An example of the pipe and filter architecture • An organization has issued invoices to customers • Once a week, payments that have been made are reconciled with the invoices • A receipt is issued for those invoices that have been paid • A reminder is issued for invoices not paid within the allowed payment time Chapter 6 Architectural design

40. Application systems are designed to meet a business or an organizational need. As businesses have much in common ( need to hire people, issue invoices …etc ), 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. Application reuse is possible without re-implementation A system forsupply chain management can be adapted for any type of supplier , goods. Application architectures Chapter 6 Architectural design

41. As a software designer, you can use models of application architectures in a number of ways : As a starting point for architectural design, if you are not familiar with the type of application you are developing. As a design checklist. If you have developed an architectural design , you can compare this with the generic design. As a way of organizing the work of the development team. As a means of assessing components for reuse, by comparing with the generic structure. As a vocabulary for talking about application types , using the concepts identified in the generic architecture. Use of application architectures Chapter 6 Architectural design

42. Data processing applications Data driven applications that process data in batches without explicit user intervention during the processing. Transaction processing applications Data-centered applications that process user requests and update information in a system database. These are most common type of interactive business systems. (banking systems , e-commerce ..etc) 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 ( Java) that is processed and interpreted by the system ( compiler). Examples of application types Chapter 6 Architectural design

43. Focus here is on transaction processing and language processing systems. Transaction processing systems E-commerce systems; Reservation systems. Language processing systems Compilers; Command interpreters. Application type examples Chapter 6 Architectural design

44. 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. An example of a transaction is a customer request to withdraw money from a bank account using an ATM. This involve getting his/her details , checking the balance, modifying the balance by the amount withdrawn and sending command to the ATM to deliver the cash . Until all of these steps have been completed , the transaction is incomplete Users make asynchronous requests for service which are then processed by a transaction manager. Transaction processing systems Chapter 6 Architectural design

45. The structure of transaction processing applications • User makes request through I/O processing component. • Request processed by some application logic ,then a transaction is created and passed to the TM • TM ensure that the transaction is properly completed, then it signals to the application that processing has finished. Chapter 6 Architectural design

46. The software architecture of an ATM system Chapter 6 Architectural design

47. These are transaction-based systems as interaction with these systems generally involves database transactions Information systems have a generic architecture that can be organized as a layered architecture. Layers include: The user interface : implementing the user interface User communications : component allowing the user to login to the system and checking components that ensure that the operations are allowed by the user Information retrieval System database : provide transaction management and persistent data storage Information systems architecture Chapter 6 Architectural design

48. Layered information system architecture Chapter 6 Architectural design

49. The architecture of the MHC-PMS Chapter 6 Architectural design

50. 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. Chapter 6 Architectural design