Chapter 6 Architectural Design PDF

Chapter 6 – Architectural Design 01/11/2020 Chapter 6 Architectural Design 1 Topics covered Architectural design decisions Architectural patterns 01/11/2020 Chapter 6 Architectural Design 2 Architectural design Architectural design is concerned with understanding...

Chapter 6 – Architectural Design 01/11/2020 Chapter 6 Architectural Design 1 Topics covered Architectural design decisions Architectural patterns 01/11/2020 Chapter 6 Architectural Design 2 Architectural design Architectural design is concerned with understanding how a software system should be organized and designing the overall structure of that system. Architectural design is the critical link between design and requirements engineering, as it identifies the main structural components in a system and the relationships between them. The output of the architectural design process is an architectural model that describes how the system is organized as a set of communicating components. 01/11/2020 Chapter 6 Architectural Design 3 01/11/2020 Chapter 6 Architectural Design 4 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. 01/11/2020 Chapter 6 Architectural Design 5 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. 01/11/2020 Chapter 6 Architectural Design 6 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. 01/11/2020 Chapter 6 Architectural Design 7 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. 01/11/2020 Chapter 6 Architectural Design 8 Architectural design decisions Architectural design is a creative process so the process differs depending on the type of system being developed. However, a number of common decisions span all design processes and these decisions affect the non- functional characteristics of the system. 01/11/2020 Chapter 6 Architectural Design 9 Architectural design decisions 01/11/2020 Chapter 6 Architectural Design 10 Architecture reuse Systems in the same domain often have similar architectures that reflect 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 instantiated in different ways. 01/11/2020 Chapter 6 Architectural Design 11 Architecture and system characteristics Performance Localise critical operations and minimise communications. Use large rather than fine-grain components. Security Use a layered architecture with critical assets in the inner layers. Safety Localise safety-critical features in a small number of sub- systems. Availability Include redundant components and mechanisms for fault tolerance. Maintainability Use fine-grain, replaceable components. 01/11/2020 Chapter 6 Architectural Design 12 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 used and when the are not useful. Patterns may be represented using tabular and graphical descriptions. 01/11/2020 Chapter 6 Architectural Design 13 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. 01/11/2020 Chapter 6 Architectural Design 14 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. 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 01/11/2020 Chapter 6 Architectural Design 15 each layer. A generic layered architecture 01/11/2020 Chapter 6 Architectural Design 16 Repository architecture 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. 01/11/2020 Chapter 6 Architectural Design 17 The Repository pattern Name Repository Description All data in a system is managed in a central repository that is accessible to all system components. Components do not interact directly, only through the repository. When used You should use this pattern when you have a system in which large volumes of information are generated that has to be stored for a long time. Advantages Components can be independent—they do not need to know of the existence of other components. Changes made by one component can be propagated to all components. All data can be managed consistently (e.g., backups done at the same time) as it is all in one place. Disadvantages The repository is a single point of failure so problems in the repository affect the whole system. May be inefficiencies in organizing all communication through the repository. Distributing the repository across several computers 01/11/2020 Chapter 6 Architectural Design 18 may be difficult. 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. 01/11/2020 Chapter 6 Architectural Design 19 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. When used Used when data in a shared database has to be accessed from a range of locations. 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 01/11/2020 Chapter 6 Architectural Design 20 different organizations. A client–server architecture for a film library 01/11/2020 Chapter 6 Architectural Design 21 Pipe and filter architecture Functional transformations process their inputs to produce outputs. May be referred to as a pipe and filter model. 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. 01/11/2020 Chapter 6 Architectural Design 22 The pipe and filter pattern Name Pipe and filter Description The processing of the data in a system is organized so that each processing component (filter) is discrete and carries out one type of data transformation. The data flows (as in a pipe) from one component to another for processing. When used Commonly used in data processing applications (both batch- and transaction-based) where inputs are processed in separate stages to generate related outputs. Advantages Easy to understand and supports transformation reuse. Workflow style matches the structure of many business processes. Evolution by adding transformations is straightforward. Can be implemented as either a sequential or concurrent system. Disadvantages The format for data transfer has to be agreed upon between communicating transformations. Each transformation must parse its input and unparse its output to the agreed form. 01/11/2020 Chapter 6 Architectural Design 23 This increases system overhead and may mean that it is impossible to reuse functional transformations that use 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. 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. 01/11/2020 Chapter 6 Architectural Design 24