Architectural Design Architectural Design and Software Architecture The

Architectural Design

Architectural Design and Software Architecture Ø The design process for identifying the sub-systems making up a system and the framework for sub-system control and communication is architectural design. Ø The output of this design process is a description of the software architecture.

Architectural Design Ø An early stage of the system design process. Ø Represents the link between specification and design processes. Ø Often carried out in parallel with some specification activities. Ø It involves identifying major system components and their communications.

Software Design Model Functional model Behavioral model Information model Data design Design Architectural design Code Other requirements Procedural design Program modules Test Integrated & validated software

Steps Architectural Design Begins with Data Design Proceeds to Architectural structure of the system

Steps (contd. ) Analysis of alternative architectural styles or patterns Selection of Alternative Elaboration of Architecture

Advantages of Explicit Architecture Ø Stakeholder communication ØArchitecture may be used as a focus of discussion by system stakeholders. Ø System analysis ØMeans that analysis of whether the system can meet its non-functional requirements is possible. Ø Large-scale reuse ØThe architecture may be reusable across a range of systems.

Architecture and System Characteristics Ø Performance Ø Localize critical operations and minimize communications. Use large rather than fine-grain components. Ø Security Ø Use a layered architecture with critical assets in the inner layers. Ø Safety Ø Localize safety-critical features in a small number of subsystems. Ø Availability Ø Include redundant components and mechanisms for fault tolerance. Ø Maintainability Ø Use fine-grain, replaceable components.

Architectural Conflicts Ø Using large-grain components improves performance but reduces maintainability. Ø Introducing redundant data improves availability but makes security more difficult. Ø Localizing safety-related features usually means more communication so degraded performance.

System Structuring Ø Concerned with decomposing the system into interacting subsystems. Ø The architectural design is normally expressed as a block diagram presenting an overview of the system structure. Ø More specific models showing how sub-systems share data, are distributed and interface with each other may also be developed.

Packing Robot Control System Vision system Object identifica tion system Ar m contr oller Gripper contr oller P acka g ing selection system Packing system Con v ey or contr oller

Box and Line Diagrams Ø Very abstract - they do not show the nature of component relationships nor the externally visible properties of the subsystems. Ø However, useful for communication with stakeholders and for project planning.

Architectural Design Decisions Ø Architectural design is a creative process so the process differs depending on the type of system being developed. Ø However, a number of common decisions span all design processes.

Architectural Design Decisions Ø Ø Ø Ø 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?

Architecture Reuse Ø Systems in the same domain often have similar architectures that reflect domain concepts. Ø Application product lines are built around a core architecture with variants that satisfy particular customer requirements.

Architectural Models Ø Used to document an architectural design. Ø Static structural model that shows the major system components. Ø Dynamic process model that shows the process structure of the system. Ø Interface model that defines sub-system interfaces. Ø Relationships model such as a data-flow model that shows sub-system relationships. Ø Distribution model that shows how sub-systems are distributed across computers.

Architectural Styles Ø The architectural model of a system may conform to a generic architectural model or style. Ø An awareness of these styles can simplify the problem of defining system architectures. Ø However, most large systems are heterogeneous and do not follow a single architectural style.

Architectural Styles Each style describes a system category that encompasses: (1) a set of components (e. g. , a database, computational modules) that perform a function required by a system, (2) a set of connectors that enable “communication, coordination and cooperation” among components, (3) constraints that define how components can be integrated to form the system, and (4) semantic models that enable a designer to understand the overall properties of a system by analyzing the known properties of its constituent parts. l l l Data-centered architectures Data flow architectures Call and return architectures Object-oriented architectures Layered architectures

Data Centered or Repository Model Ø Sub-systems must exchange data. This may be done in two ways: ØShared data is held in a central database or repository and may be accessed by all subsystems; ØEach sub-system maintains its own database and passes data explicitly to other sub-systems. Ø When large amounts of data are to be shared, the repository model of sharing is most commonly used.

Data-Centered Architecture

Repository Model Characteristics Ø Advantages Ø Efficient way to share large amounts of data; Ø Sub-systems need not be concerned with how data is produced Centralised management e. g. backup, security, etc. Ø Sharing model is published as the repository schema. Ø Disadvantages Ø Sub-systems must agree on a repository data model. Inevitably a compromise; Ø Data evolution is difficult and expensive; Ø No scope for specific management policies; Ø Difficult to distribute efficiently.

Data Flow Architecture

Call and Return Architecture

Abstract Machine (Layered) Model Ø Used to model the interfacing of sub-systems. Ø Organises the system into a set of layers (or abstract machines) each of which provide a set of services. Ø Supports the incremental development of sub-systems in different layers. When a layer interface changes, only the adjacent layer is affected. Ø However, often artificial to structure systems in this way.

Layered Architecture