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©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11

ECE 452- Introduction to Software

Engineering

Lecture 7: Software

Architectures (I. Sommerville)

Manish Parashar

[email protected]

Department of Electrical & Computer

Engineering

Rutgers University

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Architectural Design ● Establishing the overallstructure of a software system

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Objectives ● To introduce architectural design and to discussits importance ● To explain why multiple models are required todocument a software architecture ● To describe types of architectural model that maybe used ● To discuss how domain-specific reference modelsmay be used as a basis for product-lines and tocompare software architectures

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Topics covered ● System structuring ● Control models ● Modular decomposition ● Domain-specific architectures

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Software architecture ● The design process for identifying the sub-systems making up a system and the frameworkfor sub-system control and communication is architectural design ● The output of this design process is a descriptionof the software architecture

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Architectural design ● An early stage of the system design process ● Represents the link between specification anddesign processes ● Often carried out in parallel with somespecification activities ● It involves identifying major system componentsand their communications

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Advantages of explicit architecture ● Stakeholder communication

Architecture may be used as a focus of discussion by systemstakeholders ● 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

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Architectural design process ● System structuring

The system is decomposed into several principal sub-systemsand communications between these sub-systems are identified ● Control modelling - A model of the control relationships between the different partsof the system is established ● Modular decomposition - The identified sub-systems are decomposed into modules

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Sub-systems and modules ● A sub-system is a system in its own right whoseoperation is independent of the services providedby other sub-systems. ● A module is a system component that providesservices to other components but would notnormally be considered as a separate system

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Architectural models ● Different architectural models may be producedduring the design process ● Each model presents different perspectives on thearchitecture

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Architectural models ● Static structural model that shows the majorsystem components ● Dynamic process model that shows the processstructure of the system ● Interface model that defines sub-system interfaces ● Relationships model such as a data-flow model

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Architectural styles ● The architectural model of a system may conformto a generic architectural model or style ● An awareness of these styles can simplify theproblem of defining system architectures ● However, most large systems are heterogeneousand do not follow a single architectural style

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Architecture attributes ● Performance

Localise operations to minimise sub-system communication ● Security - Use a layered architecture with critical assets in inner layers ● Safety - Isolate safety-critical components ● Availability - Include redundant components in the architecture ● Maintainability - Use fine-grain, self-contained components

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 System structuring ● Concerned with decomposing the system intointeracting sub-systems ● The architectural design is normally expressed asa block diagram presenting an overview of thesystem structure ● More specific models showing how sub-systemsshare data, are distributed and interface with eachother may also be developed

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Packing robot control system Visionsystem Object identification system Arm controller Grippercontroller Packagingselection system Packingsystem Conveyorcontroller

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 The repository model ● Sub-systems must exchange data. This may bedone in two ways:

Shared data is held in a central database or repository and maybe accessed by all sub-systems - Each sub-system maintains its own database and passes dataexplicitly to other sub-systems ● When large amounts of data are to be shared, therepository model of sharing is most commonlyused

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 CASE toolset architecture Project repository Design translator Program editor Designeditor Code generator Designanalyser Report generator

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Repository model characteristics ● Advantages

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

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Client-server architecture ● Distributed system model which shows how dataand processing is distributed across a range ofcomponents ● Set of stand-alone servers which provide specificservices such as printing, data management, etc. ● Set of clients which call on these services ● Network which allows clients to access servers

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Film and picture library Catalogue server Catalogue Videoserver Film clip files Pictureserver Digitized photographs Hypertext server Hypertext web Client 1 Client 2 Client 3 Client 4 Wide-bandwidth network

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Client-server characteristics ● Advantages

Distribution of data is straightforward - Makes effective use of networked systems. May require cheaperhardware - Easy to add new servers or upgrade existing servers ● Disadvantages - No shared data model so sub-systems use different dataorganisation. data interchange may be inefficient - Redundant management in each server - No central register of names and services - it may be hard to findout what servers and services are available

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Abstract machine model ● Used to model the interfacing of sub-systems ● Organises the system into a set of layers (orabstract machines) each of which provide a set ofservices ● Supports the incremental development of sub-systems in different layers. When a layer interfacechanges, only the adjacent layer is affected ● However, often difficult to structure systems inthis way

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Version management system Operating system Database system Version managementObject management

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Control models ● Are concerned with the control flow between sub-systems. Distinct from the system decompositionmodel ● Centralised control

One sub-system has overall responsibility for control and startsand stops other sub-systems ● Event-based control - Each sub-system can respond to externally generated eventsfrom other sub-systems or the system’s environment

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 11 Centralised control ● A control sub-system takes responsibility formanaging the execution of other sub-systems ● Call-return model

Top-down subroutine model where control starts at the top of asubroutine hierarchy and moves downwards. Applicable tosequential systems ● Manager model - Applicable to concurrent systems. One system componentcontrols the stopping, starting and coordination of other systemprocesses. Can be implemented in sequential systems as a casestatement