CS560 Lecture: Software Architecture Includes slides by I. Sommerville

Transcription

1 CS560 Lecture: Software Architecture 2009 Includes slides by I. Sommerville

2 Architectural Design Design process for identifying the sub-systems making up a system and the framework for sub-system control and communication Output of this design process is a description of the software architecture Early stage of the system design Often carried out in parallel with some specification activities Involves identifying major system components and their communication 2

3 Definition of Software Architecture "a set of significant decisions about the organisation of a software system, the selection of the structural elements and the interfaces by which the system is composed, together with their behaviour as specified in the collaboration among these elements... composition into progressively larger subsystems" UML User Guide, G. Booch et al. "The structure of the components of a program/system, their interrelationships, and the principles governing their design and evolution over time." D. Garlan and D.E. Perry, IEEE Trans. on Soft. Eng., 21(4): ,

4 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 nonfunctional requirements is possible. Large-scale reuse The architecture may be reusable across a range of systems. 4

5 System Structuring Concerned with decomposing the system into interacting subsystems. The architectural design is normally expressed as a block diagram presenting an overview of the system structure. Box and line diagrams - do not show the nature of component relationships nor the externally visible properties of the subsystems More specific models showing how sub-systems share data, are distributed and interface with each other may also be developed. 5

6 Architectural Design Decisions 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? 6

7 Architectural Styles The architectural model of a system may conform to a generic architectural model or style. An awareness of these styles can simplify the problem of defining system architectures. However, most large systems are heterogeneous and do not follow a single architectural style. 7

8 Some Styles Data-centred Architectures - a data store, e.g. a file system of database, resides at the centre of this architecture. client-server blackboard - Used in AI applications such as expert systems. Shared blackboard for reading and writing. Sends notification to clients when data of interest to the clients changes. Data Flow Architecture - input data are to be transformed through a series of components called processes. Pipes and filters - pipes transmit data from one filter to the next. Call and return architecture - classic program structure with a main() and subprograms. Also called procedures or functions. remote procedure call - function invocation across multiple computers. 8

9 More Styles Object-oriented Architecture - objects communicate via message passing. Objects can contain data and program logic. Variation - software framework - inverted library for application development. Layered Architecture - different layers defined at different levels of abstraction. Enables hiding of low-level detail from the upper layers. Used for protocol definition such as in computer networking. Virtual Machine - Simulate a system in software for the purposes of security or emulation. Example: Java virtual machine. Independent Components - uses events or signals for communication and coordination, e.g. Communication Sequential Processes (CSPs), and event-based systems such as interrupts. 9

10 Examples: The following slides give examples from Sommerville textbook. 10

11 Data-centred: CASE toolset architecture Design editor Code gener ator Design transla tor Project repository Pr ogram editor Design anal yser Repor t gener ator 11

12 Data-centred model Issues Advantages Efficient way to share large amounts of data; Sub-systems need not be concerned with how data is produced Centralised 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 a compromise; No scope for specific management policies; Difficult to distribute efficiently. 12

13 Client-Server: Film and picture library Client 1 Client 2 Client 3 Client 4 Internet Catalo gue server Video server Picture server Web serv er Library catalo gue Film clip files Digitised photo g raphs Film and photo info. 13

14 Data Flow: Invoice processing system Issue receipts Receipts Read issued invoices Identify payments Find pa yments due Issue pa yment reminder Reminders Invoices Payments 14

15 Layered: Version management system Configuration management system layer Object management system layer Database system layer Operating system layer 15

16 Layered Model 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. However, often artificial to structure systems in this way. 16

17 Call-return model Main pro gram Routine 1 Routine 2 Routine 3 Routine 1.1 Routine 1.2 Routine 3.1 Routine

18 Interrupt-driven control Interrupts Interrupt vector Handler 1 Handler 2 Handler 3 Handler 4 Process 1 Process 2 Process 3 Process 4 18

19 Architectural Specification Architecture Style = {Component/Connector Vocabulary, Topology, Semantic Constraints} Questions to Consider: What is the underlying structural model? What is the underlying computational model? What are the properties of styles? What functions do they serve? 19

20 Structure Components (locus of computation), e.g. filter data store object process server Connectors (interactions between components), e.g. procedure call RPC pipe TCP/IP 20

21 Reading Software Engineering Textbooks Chapter 31 and 32 in Larman (2 nd ed.) Chapter 10 in Pressman (6 th ed.) Chapter 11 in Sommerville (8 th ed.) Chapter 9 in Maciaszek Garlan, D. and Shaw, M. An Introduction to Software Architecture, Advances in Software Engineering and Knowledge Engineering, Volume 1, World Scientific Publishing Co., ftarch.html introduction to the field of Software Architecture -the current state of the discipline and examines the ways in which architectural design can impact software design. outlines number of common architectural styles based onl experience 21

22 Books on Software Architecture Software Architecture in Practice 2nd ed., Len Bass, Paul Clements, Rick Kazman, Addison Wesley, Documenting Software Architectures: Views and Beyond, Paul Clements, Felix Bachmann, Len Bass, David Garlan, James Ivers, Reed Little, Robert Nord, Judith Stafford, Addison Wesley, The Art of Software Architecture: Design Methods and Techniques, Stephen T. Albin, Wiley, Large-Scale Software Architecture: A Practical Guide using UML, Jeff Garland, Wiley, Software Systems Architecture: Working With Stakeholders Using Viewpoints and Perspectives, Nick Rozansk, Eóin Woods, Addison- Wesley