1. Model-View-Controller (MVC):
   • Explanation: MVC is an architectural pattern commonly used in software development for designing user interfaces. It divides an application into three interconnected components: Model (data and business logic), View (presentation layer), and Controller (user input handling and application flow control). The Model manages the application's data and behavior, the View represents the presentation layer visible to users, and the Controller handles user input and updates the Model or View accordingly.
   • Example: Web development frameworks like Ruby on Rails, Django, and Spring MVC often follow the MVC pattern to separate concerns and facilitate modular development.
2. Layered Architecture:
   • Explanation: Layered architecture divides a system into multiple layers, each responsible for a specific aspect of functionality. Each layer only communicates with adjacent layers, promoting encapsulation and separation of concerns. Common layers include presentation/UI layer, business logic layer, and data access layer.
   • Example: The OSI model in networking, where protocols and services are organized into distinct layers such as the application layer, transport layer, and network layer, illustrates the layered architecture concept.
3. Repository Pattern:
   • Explanation: The repository pattern provides a way to abstract data access logic and encapsulate it within a single component. It separates the business logic from the underlying data storage implementation, allowing the application to interact with data through a common interface. This pattern facilitates code maintainability, testability, and flexibility in choosing different data storage technologies.
   • Example: Object-Relational Mapping (ORM) frameworks like Hibernate in Java or Entity Framework in .NET implement the repository pattern to provide a unified interface for database operations.
4. Client-Server Architecture:
   • Explanation: In a client-server architecture, the system is divided into two distinct components: clients and servers. Clients request services or resources from servers, which fulfill these requests. This architecture allows for distributed computing, scalability, and centralization of resources and data management.
   • Example: Web applications follow a client-server architecture, where web browsers (clients) interact with web servers to request and display web pages or access services like databases.
5. Pipe and Filter Pattern:
   • Explanation: The pipe and filter pattern consists of a series of processing elements (filters) connected by channels (pipes). Each filter performs a specific transformation on input data and passes the result to the next filter through the pipe. This pattern promotes modularity, reusability, and composability of components.
   • Example: Unix command-line utilities, where commands like grep, sort, and sed are connected by pipes (|) to perform sequential data processing tasks, demonstrate the pipe and filter pattern in action.

Key Points

■ A software architecture is a description of how a software system is organized. Properties of a system such as performance, security, and availability are influenced by the architecture used. ■ Architectural design decisions include decisions on the type of application, the distribution of the system, the architectural styles to be used, and the ways in which the architecture should be documented and evaluated. ■ Architectures may be documented from several different perspectives or views. Possible views include a conceptual view, a logical view, a process view, a development view, and a physical view. ■ Architectural patterns are a means of reusing knowledge about generic system architectures. They describe the architecture, explain when it may be used, and point out its advantages and disadvantages. ■ Commonly used Architectural patterns include model-view-controller, layered architecture, repository, client–server, and pipe and filter. ■ Generic models of application systems architectures help us understand the operation of applications, compare applications of the same type, 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. Information systems and resource management systems are examples of transaction processing systems. ■ 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.

Exercise

6.1. When describing a system, explain why you may have to start the design of the system architecture before the requirements specification is complete.

6.2. You have been asked to prepare and deliver a presentation to a nontechnical manager to justify the hiring of a system architect for a new project. Write a list of bullet points setting out the key points in your presentation in which you explain the importance of software architecture.

6.3. Performance and security may pose to be conflicting non-functional requirements when architecting software systems. Make an argument in support of this statement.

6.4. Draw diagrams showing a conceptual view and a process view of the architectures of the following systems: → A ticket machine used by passengers at a railway station. → A computer-controlled video conferencing system that allows video, audio, and computer data to be visible to several participants at the same time. → A robot floor-cleaner that is intended to clean relatively clear spaces such as corridors. The cleaner must be able to sense walls and other obstructions.

6.5. A software system will be built to allow drones to autonomously herd cattle in farms. These drones can be remotely controlled by human operators. Explain how multiple architectural patterns can fit together to help build this kind of system.

6.6. Suggest an architecture for a system (such as iTunes) that is used to sell and distribute music on the Internet. What Architectural patterns are the basis for your proposed architecture?

6.7. An information system is to be developed to maintain information about assets owned by a utility company such as buildings, vehicles, and equipment. It is intended that this will be updatable by staff working in the field using mobile devices as new asset information becomes available. The company has several existing asset databases that should be integrated through this system. Design a layered architecture for this asset management system based on the generic information system architecture shown in Figure 6.18.

6.8. Using the generic model of a language processing system presented here, design the architecture of a system that accepts natural language commands and translates these into database queries in a language such as SQL.

6.9. Using the basic model of an information system, as presented in Figure 6.18, suggest the components that might be part of an information system that allows users to view box office events, available tickets and prices, and to eventually buy tickets.

6.10. Should there be a separate profession of ’software architect’ whose role is to work independently with a customer to design the software system architecture? A separate software company would then implement the system. What might be the difficulties of establishing such a profession?