Software Engineering Chapter 6: Architectural Design

Architectural Design

• Architectural design is concerned with understanding how a software system should be organized and designing the overall structure of that system.
• It is a 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.

Advantages of Architecture Design

• Stakeholder communication: Architecture may be used as a focus of discussion by system stakeholders.
• System analysis: It 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 with similar requirements.

Architectural Representations

• Informal block diagrams showing entities and relationships are the most frequently used method for documenting software architectures.
• Block diagrams are a useful way for communication between stakeholders and for project planning.
• However, they 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.

Architectural Design Decisions

• Architectural design is a creative process that differs depending on the type of system being developed, the background and experience of the system architect, and the specific requirements for the system.
• It is useful to think of architectural design as a series of decisions rather than a sequence of activities.
• System architects have to make a number of structural decisions that affect the system, its development process, and its non-functional characteristics.

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 or more architectural patterns or ‘styles’.

Architecture and System Characteristics

• Performance: Localize critical operations and minimize communications. Use large rather than fine-grain components.
• Security: Use a layered architecture with critical assets in the inner layers.
• Safety: Localize safety-critical features in a small number of sub-systems to reduce the costs and problems of safety validation.
• Availability: Include redundant components and mechanisms for fault tolerance.
• Maintainability: Use replaceable components to be readily changed and separate producers of data from consumers.

Architectural Views

• A logical view shows the key abstractions in the system as objects or object classes.
• A process view shows how, at run-time, the system is composed of interacting processes.
• A development view shows how the software is decomposed for development.
• A physical view shows the system hardware and how software components are distributed across the processors in the system.
• A conceptual view (+1) is an abstract view of the system that decomposes high-level requirements into more detailed specifications.

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 they are not useful.
• Patterns may be represented using tabular and graphical descriptions.

The Model-View-Controller (MVC) Pattern

• The MVC pattern separates presentation and interaction from the system data.
• The system is structured into three logical components that interact with each other: the Model, the View, and the Controller.
• The Model component manages the system data and associated operations on that data.
• The View component defines and manages how the data is presented to the user.
• The Controller component manages user interaction and passes these interactions to the View and the Model.

The Layered Architecture Pattern

• The Layered architecture pattern organizes the system into a set of layers each of which provides a set of services.
• 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.
• Supports the incremental development of sub-systems in different layers.
• When a layer interface changes, only the adjacent layer is affected.

The Repository Pattern

• 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.
• Used when you have a system in which large volumes of information are generated that has to be stored for a long time.

The Client-Server Pattern

• 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.
• Used when data in a shared database has to be accessed from a range of locations.
• Because servers can be replicated, may also be used when the load on a system is variable.