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Explore the process and decisions involved in architectural design for software systems, including distributed system models and advantages of explicit architecture. Learn about architectural models and characteristics to guide your system design.
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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.
Architectural design • An 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 involves identifying major system components and their communications.
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.
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.
Architectural design decisions • 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?
Architectural models • Used to document an architectural design. • Static structural model that shows the major system components. • Dynamic process model that shows the process structure of the system. • Interface model that defines sub-system interfaces. • Relationships model such as a data-flow model that shows sub-system relationships. • Distribution model that shows how sub-systems are distributed across computers.
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 • 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.
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.
Multiprocessor architectures • Simplest distributed system model. • System composed of multiple processes which may (but need not) execute on different processors. • Architectural model of many large real-time systems. • Distribution of process to processor may be pre-ordered or may be under the control of a dispatcher.
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.
Client-server model • Distributed system model which shows how data and processing is distributed across a range of components. • 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.
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.
Client-server characteristics • Advantages • Distribution of data is straightforward; • Makes effective use of networked systems. May require cheaper hardware; • Easy to add new servers or upgrade existing servers. • Disadvantages • No shared data model so sub-systems use different data organisation. Data interchange may be inefficient; • Redundant management in each server; • No central register of names and services - it may be hard to find out what servers and services are available.
Abstract machine (layered) model • 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.
Object models • Structure the system into a set of loosely coupled objects with well-defined interfaces. • Object-oriented decomposition is concerned with identifying object classes, their attributes and operations. • When implemented, objects are created from these classes and some control model used to coordinate object operations.
Object model advantages • Objects are loosely coupled so their implementation can be modified without affecting other objects. • The objects may reflect real-world entities. • OO implementation languages are widely used. • However, object interface changes may cause problems and complex entities may be hard to represent as objects.
Function-oriented pipelining • Functional 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 transformations are sequential, this is a batch sequential model which is extensively used in data processing systems. • Not really suitable for interactive systems.
Event-driven systems • Driven by externally generated events where the timing of the event is outwith the control of the sub-systems which process the event. • Two principal event-driven models • Broadcast models. An event is broadcast to all sub-systems. Any sub-system which can handle the event may do so; • Interrupt-driven models. Used in real-time systems where interrupts are detected by an interrupt handler and passed to some other component for processing. • Other event driven models include spreadsheets and production systems.
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.
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 • 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.
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.
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.
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.
Editing system components • Editing systems are naturally object-oriented: • Screen - monitors screen memory and detects events; • Event - recognises events and passes them for processing; • Command - executes a user command; • Editor data - manages the editor data structure; • Ancillary data - manages other data such as styles and preferences; • File system - manages file I/O; • Display - updates the screen display.
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.
Language processing components • Lexical analyser • Symbol table • Syntax analyser • Syntax tree • Semantic analyser • Code generator
