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1、Chapter 6 Architectural DesignLecture 1Topics coveredArchitectural design decisionsArchitectural viewsArchitectural patternsApplication architecturesSoftware architectureThe design process for identifying the sub-systems making up a system and the framework for sub-system control and communication i
2、s architectural design.The output of this design process is a description of the software architecture.Architectural designAn early stage of the system design process.Represents the link between specification and design processes.Often carried out in parallel with some specification activities.It in
3、volves identifying major system components and their communications.The architecture of a packing robot control systemArchitectural abstractionArchitecture 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
4、 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.Ad
5、vantages of explicit architectureStakeholder communicationArchitecture may be used as a focus of discussion by system stakeholders.System analysisMeans that analysis of whether the system can meet its non-functional requirements is possible.Large-scale reuseThe architecture may be reusable across a
6、range of systemsProduct-line architectures may be developed.Architectural representationsSimple,informal block diagrams showing entities and relationships are the most frequently used method for documenting software architectures.But these have been criticised because they lack semantics,do not show
7、 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.Box and line diagramsVery abstract-they do not show the nature of component relat
8、ionships nor the externally visible properties of the sub-systems.However,useful for communication with stakeholders and for project planning.Use of architectural modelsAs a way of facilitating discussion about the system design A high-level architectural view of a system is useful for communication
9、 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 design
10、ed The aim here is to produce a complete system model that shows the different components in a system,their interfaces and their connections.Architectural design decisionsArchitectural design is a creative process so the process differs depending on the type of system being developed.However,a numbe
11、r of common decisions span all design processes and these decisions affect the non-functional characteristics of the system.Architectural design decisionsIs there a generic application architecture that can be used?How will the system be distributed?What architectural styles are appropriate?What app
12、roach 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?Architecture reuseSystems in the same domain often have similar architectures that r
13、eflect domain concepts.Application product lines are built around a core architecture with variants that satisfy particular customer requirements.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 i
14、nstantiated in different ways.Discussed later in this lecture.Architecture and system characteristicsPerformanceLocalise critical operations and minimise communications.Use large rather than fine-grain components.SecurityUse a layered architecture with critical assets in the inner layers.SafetyLocal
15、ise safety-critical features in a small number of sub-systems.AvailabilityInclude redundant components and mechanisms for fault tolerance.MaintainabilityUse fine-grain,replaceable components.Architectural viewsWhat views or perspectives are useful when designing and documenting a systems architectur
16、e?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 acr
17、oss a network.For both design and documentation,you usually need to present multiple views of the software architecture.4+1 view model of software architectureA logical view,which shows the key abstractions in the system as objects or object classes.A process view,which shows how,at run-time,the sys
18、tem is composed of interacting processes.A development view,which shows how the software is decomposed for development.A physical view,which shows the system hardware and how software components are distributed across the processors in the system.Related using use cases or scenarios(+1)Architectural
19、 patternsPatterns 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.Pattern
20、s may be represented using tabular and graphical descriptions.The Model-View-Controller(MVC)pattern The organization of the Model-View-Controller Web application architecture using the MVC pattern Layered architectureUsed to model the interfacing of sub-systems.Organises the system into a set of lay
21、ers(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 A gen
22、eric layered architecture The architecture of the LIBSYS system Key pointsA 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.Arch
23、itectures may be documented from several different perspectives or viewssuch 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
24、 and describe its advantages and disadvantages.Chapter 6 Architectural DesignLecture 2Repository architectureSub-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 da
25、tabase 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.The Repository pattern A repository architecture for an IDE Client-server architectureDistributed sys
26、tem 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 t
27、o access servers.The Clientserver pattern A clientserver architecture for a film library Pipe and filter architectureFunctional transformations process their inputs to produce outputs.May be referred to as a pipe and filter model(as in UNIX shell).Variants of this approach are very common.When trans
28、formations are sequential,this is a batch sequential model which is extensively used in data processing systems.Not really suitable for interactive systems.The pipe and filter pattern An example of the pipe and filter architecture Application architecturesApplication systems are designed to meet an
29、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 application architecture is an architecture for a type of software system that may be configured and adapted to create a system
30、 that meets specific requirements.Use of application architecturesAs 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 ap
31、plication typesData processing applicationsData driven applications that process data in batches without explicit user intervention during the processing.Transaction processing applicationsData-centred applications that process user requests and update information in a system database.Event processi
32、ng systemsApplications where system actions depend on interpreting events from the systems environment.Language processing systemsApplications where the users intentions are specified in a formal language that is processed and interpreted by the system.Application type examplesFocus here is on trans
33、action processing and language processing systems.Transaction processing systemsE-commerce systems;Reservation systems.Language processing systemsCompilers;Command interpreters.Transaction processing systemsProcess user requests for information from a database or requests to update the database.From
34、 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 application
35、s The software architecture of an ATM system Information systems architectureInformation 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:T
36、he user interfaceUser communicationsInformation retrievalSystem databaseLayered information system architecture The architecture of the MHC-PMS Web-based information systemsInformation and resource management systems are now usually web-based systems where the user interfaces are implemented using a
37、 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
38、 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 implementationThese systems are often implemented as multi-tier client server/architectures(discussed in Chapter 18)The web server is responsi
39、ble 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 t
40、ransaction management.Language processing systemsAccept 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
41、problem is to describe an algorithm or describe the system dataMeta-case tools process tool descriptions,method rules,etc and generate tools.The architecture of a language processing system Compiler componentsA lexical analyzer,which takes input language tokens and converts them to an internal form.
42、A symbol table,which holds information about the names of entities(variables,class names,object names,etc.)used in the text that is being translated.A syntax analyzer,which checks the syntax of the language being translated.A syntax tree,which is an internal structure representing the program being
43、compiled.Compiler componentsA semantic analyzer that uses information from the syntax tree and the symbol table to check the semantic correctness of the input language text.A code generator that walks the syntax tree and generates abstract machine code.A pipe and filter compiler architecture A repos
44、itory architecture for a language processing systemKey pointsModels 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 infor
45、mation in a database to be remotely accessed and modified by a number of users.Language processing systems are used to translate texts from one language into another and to carry out the instructions specified in the input language.They include a translator and an abstract machine that executes the generated language.