Software Engineering. Software Engineering. Software Costs

Transcription

1 Software Engineering Software Engineering is the science and art of building significant software systems that are: 1) on time 2) on budget 3) with acceptable performance 4) with correct operation. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 1 Software Engineering The economies of all developed nations are dependent on software. More and more systems are software controlled. Software engineering is concerned with theories, methods and tools for professional software development. Software engineering expenditure represents a significant fraction of the GNP of developed countries. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 2 Software Costs Software costs often dominate system costs. The costs of software on a PC are often greater than the hardware cost. Software costs more to maintain than it does to develop. Software engineering is concerned with costeffective software development. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 3

2 Software Products Generic products: Stand-alone systems which are produced by a development organization and sold on the open market to any customer. Customized products: Systems which are commissioned by a specific customer and developed specially by some contractor. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 4 Software Product Attributes Maintainability Dependability Efficiency Usability Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 5 Importance of Product Characteristics The relative importance of these characteristics depends on the product and the environment in which it is to be used. In some cases, some attributes may dominate In safety-critical real-time systems, key attributes may be dependability and efficiency. Costs tend to rise exponentially if very high levels of any one attribute are required. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 6

3 Efficiency Costs Cost Efficiency Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 7 The Software Process Structured set of activities required to develop a software system Specification Design Validation Evolution Activities vary depending on the organization and the type of system being developed. Must be explicitly modeled if it is to be managed. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 8 Engineering Process Model Specification: Set out the requirements and constraints on the system. Design: Produce a model of the system. Manufacture: Build the system. Test: Check the system meets the required specifications. Install: Deliver the system to the customer and ensure it is operational. Maintain: Repair faults in the system as they are discovered. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 9

4 Software Engineering is Different Normally, specifications are incomplete. Very blurred distinction between specification, design and manufacture. No physical realization of the system for testing. Software does not wear out - maintenance does not mean component replacement. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 10 Generic Software Process Models Waterfall Separate and distinct phases of specification and development Evolutionary Specification and development are interleaved Formal Transformation A mathematical system model is formally transformed to an implementation Reuse-based The system is assembled from existing components Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 11 Waterfall Process Model Requirements definition System and software design Implementation and unit testing Integration and system testing Operation and maintenance Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 12

5 Evolutionary Process Model Concurrent activities Specification Initial version Outline description Development Intermediate versions Validation Final version Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 13 Process Model Problems Waterfall High risk for new systems because of specification and design problems. Low risk for well-understood developments using familiar technology. Prototyping Low risk for new applications because specification and program stay in step. High risk because of lack of process visibility. Transformational High risk because of need for advanced technology and staff skills. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 14 Hybrid Process Models Large systems are usually made up of several sub-systems. The same process model need not be used for all subsystems. Prototyping for high-risk specifications. Waterfall model for well-understood developments. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 15

6 Spiral Process Model Determine objectives alternatives and constraints Plan next phase REVIEW Requirements plan Life-cycle plan Development plan Integration and test plan Evaluate alternatives identify, resolve risks Risk analysis Risk analysis Risk analysis Operational Prototype 3 Prototype 2 protoype Risk analysis Prototype 1 Simulations, models, benchmarks Concept of Operation S/W requirements Product design Detailed Requirement design validation Code Design Unit test V&V Integration Acceptance test Service test Develop, verify next-level product Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 16 Spiral Model Advantages Focuses attention on reuse options. Focuses attention on early error elimination. Puts quality objectives up front. Integrates development and maintenance. Provides a framework for hardware/software development. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 17 Spiral Model Problems Contractual development often specifies process model and deliverables in advance. Requires risk assessment expertise. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 18

7 Process Visibility Software systems are intangible so managers need documents to assess progress. Waterfall model is still the most widely used model. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 19 Waterfall Model Documents Activity Requirements analysis Requirements definition System specification Architectural design Interface design Detailed design Coding Unit testing Module testing Integration testing System testing Acceptance testing Output documents Feasibility study, Outline requirements Requirements document Functional specification, Acceptance test plan Draft user manual Architectural specification, System test plan Interface specification, Integration test plan Design specification, Unit test plan Program code Unit test report Module test report Integration test report, Final user manual System test report Final system plus documentation Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 20 Process Model Visibility Process model Waterfall model Evolutionary development Formal transformations Reuse-oriented development Spiral model Process visibility Good visibility, each activity produces some deliverable Poor visibility, uneconomic to produce documents during rapid iteration Good visibility, documents must be produced from each phase for the process to continue Moderate visibility, it may be artificial to produce documents describing reuse and reusable components. Good visibility, each segment and each ring of the spiral should produce some document. Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 21

8 Professional Responsibility Software engineers should not just be concerned with technical considerations. They have wider ethical, social and professional responsibilities. No clear rights and wrongs about many of these issues: Development of military systems Whistle blowing Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 22 Ethical Issues Confidentiality Competence Intellectual property rights Computer misuse Ian Sommerville 1995/2000 (Modified by Spiros Mancoridis 1999) Software Engineering, 6th edition. Chapters 1,3 Slide 23