©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 1 Chapter 3 Software Processes

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 2 Software Processes l Coherent sets of activities for specifying, designing, implementing and testing software systems.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 3 Objectives l To introduce software process models. l To describe a number of generic process models and when they may be used. l To outline lower-level process models for (1) requirements engineering, (2) software development, (3) testing, and (4) evolution. l To introduce CASE technology to support software process activities

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 4 Topics covered l Software process models l Process iteration l Software specification l Software design and implementation l Software verification & validation l Software evolution l Automated process support

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 5 The software process l A process is a structured set of activities required to develop a software system, e.g. Specification Design Validation / Verification Evolution l A process model is an abstract representation of a process. It presents a description of a process from some particular perspective l Models should be as simple as possible, but no simpler. – A. Einstein

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 6 Generic software process models l The Waterfall Model – separate and distinct phases of specification and development. Traditionally: not iterative. l Evolutionary Development – specification and development are interleaved. l Formal Systems Development – a mathematical system model is formally (or informally) transformed to an implementation. l Reuse-Based Development – the system is assembled from existing components. (And, at no additional cost: Incremental, eXtreme, and Spiral.)

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 7 Waterfall model (W. Royce)

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 8 Waterfall model problems l Inflexible partitioning of the project into distinct stages makes it difficult to respond to changing customer requirements. l Thus, this model is only appropriate when the requirements are well-understood (to begin with) l In general, the drawback of the waterfall model is the difficulty of accommodating change after the process is underway. Can we say anything good about the Waterfall model?

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 9 Evolutionary development l Exploratory Development * – objective is to work with customers and to evolve a final system from an initial outline specification. (Development starts with well-understood parts of system.) important theme in Agile Development l Throw-Away Prototyping – objective is to understand the system requirements. (Prototyping focuses on poorly understood requirements.) * also known as exploratory programming, or evolutionary prototyping

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 10 Evolutionary development customer trash

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 11 Evolutionary development l Potential problems Lack of process visibility. (via documents: c.f. Waterfall model) Final version/prototype is often poorly structured. Special skills (e.g., in languages for rapid prototyping) may be required. -- working effectively with people l Applicability For small or medium-size interactive systems. For parts of large systems (e.g., the user interface). For short-lifetime systems. (In the case of exploratory development – why?)

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 12 Formal systems development l Based on the transformation of a mathematical specification (through possibly several different representations) to an executable program. l Transformations are “correctness-preserving” so it is possible to show that the program conforms to its specification. l Embodied in Mills’ “Cleanroom” approach to software development.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 13 Formal systems development Code Generator? Not realistic, unless “Formal specification” is source code…

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 14 Formal transformations (futuristic…)

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 15 Formal systems development l Problems Need for specialized skills and training to apply the technique. Difficult to formally specify some aspects of the system such as the user interface (thus, focus is usually limited to functional requirements). l Applicability Critical systems, especially those where a safety or security case must be made before the system is put into operation. Critical parts of large systems.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 16 Reuse-oriented development l Based on systematic (as opposed to serendipitous) reuse of existing software units. l Units may be: Procedures or functions (common for past 40 years) Components (“component-based development”) Core elements of an application (“application family”) Entire applications -- COTS (Commercial-off-the-shelf) systems l May also be based on use of design patterns.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 17 Reuse-oriented development Process stages: Reusable software analysis (what’s available?) Requirements modification – why? System design with reuse Development and integration l This approach is becoming more important, but experience is still limited. “Software Repositories” research was a major DoD thrust in the late 80’s.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 18 Reuse-oriented development

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 19 Process iteration l For large systems, requirements ALWAYS evolve in the course of a project. l Thus, process iteration is ALWAYS part of the process. l Iteration can be incorporated in any of the generic process models. (but not in spirit of Waterfall…) l Two other approaches that explicitly incorporate iteration: Incremental development (Mills) Spiral development (Boehm)

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 20 Mill’s Incremental development l Rather than deliver the system as a single unit, the development and delivery is broken down into increments, each of which incorporates part of the required functionality. l User requirements are prioritised and the highest priority requirements are included in early increments. l Once the development of an increment is started, its requirements are “frozen” while requirements for later increments can continue to evolve. (Compromise between Waterfall & Evolutionary development)

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 21 Mill’s Incremental development

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 22 Incremental development advantages l Useful functionality is delivered with each increment, so customers derive value early. l Early increments act as a prototype to help elicit requirements for later increments. l Lower risk of overall project failure. l The highest priority system services tend to receive the most testing. (subject to more “validation” steps)

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 23 Potential problem with incremental development l Requirements may NOT be partitionable into stand- alone increments. (e.g., a compiler)

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 24 Extreme programming (Beck ’99) l Recent evolution of incremental approach based on Development and delivery of very small increments of functionality Significant customer involvement in process Constant code improvement Egoless, pair-wise programming l NOT document-oriented l Gaining acceptance in some small (and now medium sized) organizations. l Representative of the “Agile” development paradigm.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 25 Boehm’s spiral development l Process is represented as a spiral rather than a sequence of activities. l Each loop in the spiral represents a phase in the process. l No fixed phases such as specification or design – loops in the spiral are chosen depending on what is required. l Explicitly incorporates risk assessment and resolution throughout the process.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 26 Spiral model of the software process

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 27 Spiral model quadrants l Objective Setting – specific objectives for the phase are identified. l Risk Assessment and Reduction – risks are assessed and activities put in place to reduce the key risks. l Development and Validation – a development model for the system is chosen which can be any of the generic models. l Planning – the project is reviewed and the next phase of the spiral is planned.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 28 Models for (lower level) fundamental process activities l Software specification/requirements engineering (RE) l Software development (design and implementation) l Software verification and validation l Software evolution

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 29 Software specification / RE l The process of establishing what services are required and the constraints on the system’s operation and development. l Requirements Engineering (RE) process: Feasibility (technical and otherwise) study Requirements elicitation and analysis Requirements specification (documentation) Requirements validation

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 30 The requirements engineering process

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 31 Software design and implementation l The process of producing an executable system based on the specification Software design – design a software structure that realizes the specification. Implementation – translate this structure into an executable program. l The activities of specification, design, and implementation are closely related and may be interleaved.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 32 Design process activities l “High-Level” design activities Architectural design – subsystems and their relationships are identified Abstract specification – of each sub-system’s services Interface design – among sub-systems l “Low-Level” design activities Component design – services allocated to different components and their interfaces are designed Data structure design Algorithm design

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 33 The software design process

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 34 Design methods l Systematic (canned) approaches to developing a software design. the cookbook approach… l The design is usually documented as a set of graphical models. l Possible models: Data-flow model Entity-relation-attribute model Structural model Object models

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 35 Programming and debugging l Translating a design into a program and removing errors from that program. l Programming is a “personal activity” – there is no generic programming process. l Programmers carry out some program testing to discover faults (“unit testing”), and remove faults in the debugging process. (Compare this model with Cleanroom SE.)

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 36 The debugging process

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 37 Software verification & validation l Verification and validation (V&V) determines whether or not a system (1) conforms to its specification and (2) meets the needs of the customer. l Involves inspection / review processes and (machine- based) testing. l Testing involves executing system elements with test cases that are derived from specifications and/or program logic.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 38 Testing stages (topic 15) l Unit/Module testing - individual function/procedures are tested l (unit/module) Integration testing l Component testing - functionally related units/modules are tested together l (component) Integration testing l Sub-system/product testing - sub-systems or products are tested l (product/sub-system) Integration testing l System testing - testing of the system as a whole, including user acceptance test cf “traditional” (i.e., waterfall) model of testing

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 39 Software evolution l Software is inherently flexible and subject to change. l As requirements change through changing business circumstances, the software that supports the business must also evolve and change. l The distinction between development and evolution is increasingly irrelevant as fewer and fewer systems are completely new.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 40 System evolution e.g., change requests

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 41 Automated process support (CASE) l Computer-aided software engineering (CASE) is software to support software development and evolution processes. l Activity automation (examples): Graphical editors for system model development Data dictionaries for name management GUI builders for user interface construction Debuggers to support program fault finding Automated translators to generate new versions of a program (e.g., restructuring tools)

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 42 CASE technology l CASE technology has led to significant improvements in the software process, but not the order of magnitude improvements that were once predicted. Software engineering involves design activity requiring creative thought – this is not readily automatable. Software engineering is a team activity and, for large projects, much time is spent in team interactions. CASE technology does not support this well.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 43 CASE classification l Classification helps us understand the different types of CASE tools / systems and their support for process activities l Functional perspective – tools are classified according to their specific function. l Process perspective – tools are classified according to process activities that are supported. l Integration perspective – CASE systems are classified according to their breadth of support for the software process.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 44 Functional tool classification MS Project RCS, Make LISP, 4GL’s coverage tools

Activity-based classification Actually far more lower CASE tools than upper CASE

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 46 CASE integration l Tools – support individual process tasks such as design consistency checking, text editing, etc. l Workbenches – support a process phase such as specification or design, Normally include a number of integrated tools. l Environments – support all or a substantial part of an entire software process. Normally include several integrated workbenches.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 47 Tools, workbenches, environments

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 48 Key points l Software processes are the activities involved in producing and evolving a software system. They are represented in a software process model. l Fundamental (lower level) activities are specification, design and implementation, validation & verification, and evolution. l Generic models are very general process models representing different approaches to development. l Iterative process models describe the software process as a cycle of activities.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 3 Slide 49 Key points l Requirements engineering is the process of establishing what services are required and the constraints on the system’s operation and development. l Design and implementation processes produce an executable system based on the specification. l V&V involves checking that the system meets its specification and satisfies user needs. l Evolution is concerned with modifying the system after it is placed in use. l CASE technology supports software process activities.