Software Engineering The Software Process
Why Software Engineering? Can you approach building software as building a bridge? Why? Why not? How is software engineering like other engineering disciplines?
Software Process A structured set of activities required to develop a software system. Fundamental Assumption: Good processes lead to good software Good processes reduce risk
What can go wrong in a software project? How can the risk be reduced? Risk Management What can go wrong in a software project? How can the risk be reduced?
The software process Many different software processes but all involve: Specification: defining what the system should do; Design and implementation: defining the organization of the system and implementing it Validation – checking that it does what the customer wants; Evolution – changing the system in response to changing customer needs.
Software Process Models A software process model is an abstract representation of a process. The waterfall model Separate and distinct phases of specification and development (product focused) Evolutionary development Specification, development and validation are interleaved e.g. XP (Beck), increment driven Component-based software engineering The system is assembled from existing components. Spiral (Boehm) driven by risk analysis and mitigation
Background on software process models 1950 ：Code-and-fix model 1956 ： Stagewise model (Bengington ) 1970 ： Waterfall model (Royce) 1971 ： Incremental model(Mills) 1977 ： Prototyping model(Bally and others) 1988 ： Spiral model(Boehm)
Use of the Models in Practice
The Waterfall Model Requirements Definition System and Software design Programming and Unit Testing Integration and System Testing Operation and Maintenance
Waterfall Model material adapted from Steve Easterbrook requirements design code integrate test perceived need View of development: A process of stepwise refinement High-level management view Problems: Static view of requirements (ignores volatility) Lack of user involvement once spec is written Unrealistic separation of spec from design Doesn’t accommodate prototyping, reuse
The Waterfall Model Requirements Definition System and Software design Programming and Unit Testing Integration and System Testing Operation and Maintenance
[1] Requirements Analysis and Definition The system's services, constraints and goals are established by consultation with system users. They are then defined in a manner that is understandable by both users and development staff. This phase can be divided into: Feasibility study (often carried out separately) Requirements analysis Requirements definition Requirements specification
Terminology A requirement is a technical objective which is imposed upon the software, i.e., anything that might affect the kind of software that is produced A requirement may be imposed by the customer the developer the operating environment The requirement must be documented Requirements fall into two broad categories functional non-functional
Functional Requirements Functional requirements are concerned with what the software must do capabilities, services, or operations Functional requirements are not concerned with how the software does things, i.e., they must be free of design considerations
Non-Functional Requirements Non-functional requirements place restrictions on the range of acceptable solutions Non-functional requirements cover a broad range of issues interface constraints performance constraints operating constraints life-cycle constraints economic constraints political constraints manufacturing
Case Study We participated in a group project where we designed and developed a cell phone game suitable for a cell phone using the programming language Java 2 Micro Edition. We designed everything from the beginning with no game requirements given to us. The process consists of defining system requirements, building the system, and testing the system. The goal of the project was to have a working game. We were restricted by time since this was a summer program
Define System Requirements The system is designed to make the primary persona happy but the other personas should not be unhappy Define user scenarios User scenarios describe how someone will interact with the system. This includes actual scenarios that could happen to a person while playing through the game.
Define System Requirements Describe the user interface Describe main interface that the user will interact with. This includes screen interactions and all options the user will have. Detailed requirements Includes all functional, nonfunctional, and constraint requirements that the system must satisfy
The Waterfall Model Requirements Definition System and Software design Programming and Unit Testing Integration and System Testing Operation and Maintenance
[2] System and Software Design System design: Partition the requirements to hardware or software systems. Establishes an overall system architecture Software design: Represent the software system functions in a form that can be transformed into one or more executable programs Unified Modeling Language (UML)
Case Study We participated in a group project where we designed and developed a cell phone game suitable for a cell phone using the programming language Java 2 Micro Edition. We designed everything from the beginning with no game requirements given to us. The process consists of defining system requirements, building the system, and testing the system. The goal of the project was to have a working game. We were restricted by time since this was a summer program
Build System Define the system classes Classes are derived from the nouns in the user scenarios. Every system activity or primitive function should reside in a class Define system sequence diagrams Sequence diagrams show how the classes work together to execute each use-case function
Build System
[3] Programming and Unit Testing The software design is realized as a set of programs or program units. (Written specifically, acquired from elsewhere, or modified.) Individual components are tested against specifications.
All coding takes place here. Build System Implement the system The class skeletons are coded based on the code generated from the class diagrams. All coding takes place here.
The Waterfall Model Requirements Definition System and Software design Programming and Unit Testing Integration and System Testing Operation and Maintenance
[4] Integration and System Testing The individual program units are: integrated and tested as a complete system tested against the requirements as specified delivered to the client
The Waterfall Model Requirements Definition System and Software design Programming and Unit Testing Integration and System Testing Operation and Maintenance
[5] Operation and Maintenance Operation: The system is put into practical use. Maintenance: Errors and problems are identified and fixed. Evolution: The system evolves over time as requirements change, to add new functions or adapt the technical environment. Phase out: The system is withdrawn from service.
Discussion of the Waterfall Model Advantages: Process visibility Dependence on individuals Quality control Cost control
The Classical Software Lifecycle The classical software lifecycle models the software development as a step-by-step “waterfall” between the various development phases. Requirements Collection Analysis Design Implementation Testing Maintenance The waterfall model is unrealistic for many reasons: requirements must be frozen too early in the life-cycle requirements are validated too late
Problems with the Waterfall Lifecycle “Real projects rarely follow the sequential flow that the model proposes. Iteration always occurs and creates problems in the application of the paradigm” “It is often difficult for the customer to state all requirements explicitly. The classic life cycle requires this and has difficulty accommodating the natural uncertainty that exists at the beginning of many projects.” Each stage in the process reveals new understanding of the previous stages, that requires the earlier stages to be revised. NB: Early prototyping helps to alleviate the 2nd and 3d problems.
Cons Not a good model for complex and object-oriented projects. High amounts of risk and uncertainty. Not suitable for the projects where requirements are at a moderate to high risk of changing. So risk and uncertainty is high with this process model.
Problem Case Study Publisher of books want to develop a product that will carry out all the accounting functions of the company and provide online information to the head office staff regarding orders and inventory Terminals are required for 15 accounting clerks, 32 order clerks, and 15 managers need access to the data. The publisher will pay 30000 and wants the complete product in 4 weeks What do you tell him?
Case Study: Sensor Display Consider a small system that displays the status of several sensors (e.g., pressure, temperature, rate of change, etc.) on a limited-size screen What are some of its functional requirements? What are some of its non-functional requirements? Examine the issue of system boundaries. - two pressure sensors - two temperature sensors - one six character display Investigate the requirements/design separation. - display layout - how often do we read the sensors - how do we debounce values Contrast several levels of abstraction for the requirements. - display all important parameters in a meaningful way » min/max pressure and temperature over the last minute » average the readings for each sensor pair (before or after computing the min/max?)) » discard unreasonable readings (what criterion?) » compute rate of change (over what interval?) - display all parameters in order for 2 seconds each using specific formats Investigate the issue of models and analysis - new requirement: display out of range values » instantaneous (corrected) average - error conditions need to be handled differently » new mode of operation (attention) is needed to show the error condition » there may be more than one (table of priorities) » the condition may go away (hold it anyway) » how can one see lower priority error conditions or current status (add reset button—interface change) - investigate mode transitions (normal and attention) » being in attention mode does not mean that an error condition is present Examine the potential implications of a variety of possible constraints. Size, cost, power, environmental conditions. Political considerations (e.g., export restrictions). Human factors (e.g., blind employees). Sensor capabilities (e.g., reading frequency, accuracy, reliability).
Iterative Development In practice, development is always iterative, and all activities progress in parallel. Requirements Testing based on requirements Collection Testing Maintenance through iteration Analysis Testing throughout implementation Validation through prototyping If the waterfall model is pure fiction, why is it still the dominant software process? Implementation Design Design through refactoring
Iterative Refinement Requirements Evaluation Implementation (prototype) Design
Phased Models material adapted from Steve Easterbrook design code test integrate O&M reqts Requirements version 1 version 2 version 3 Release 1 release 2 release 3 release 4 lessons learnt Incremental development (each release adds more functionality) Evolutionary development (each version incorporates new requirements)
Incremental Development Plan to incrementally develop (i.e., prototype) the system. If possible, always have a running version of the system, even if most functionality is yet to be implemented. Integrate new functionality as soon as possible. Validate incremental versions against user requirements.
Incremental development
Incremental delivery Rather than deliver the system as a single delivery, the development and delivery is broken down into increments with each increment delivering part of the required functionality. User requirements are prioritised and the highest priority requirements are included in early increments. It is easier to get customer feedback on the development work that has been done.
Incremental development advantages Customer value can be delivered with each increment so system functionality is available earlier. Early increments act as a prototype to help elicit requirements for later increments. Lower risk of overall project failure. The highest priority system services tend to receive the most testing. The cost of accommodating changing customer requirements is reduced.
When to use the Incremental model This model can be used when the requirements of the complete system are clearly defined and understood. Major requirements must be defined; however, some details can evolve with time. There is a need to get a product to the market early. A new technology is being used Resources with needed skill set are not available There are some high risk features and goals.
Evolutionary Process Model
Evolutionary New versions implement new and evolving requirements Design Coding Test Deployment Requirements Version 2 Design Coding Test Deployment Requirements Version 3 Feedback Design Coding Test Deployment Requirements New versions implement new and evolving requirements (Some call it iterative)
Evolutionary development Problems Lack of process visibility; Systems are often poorly structured; Special skills (e.g. in languages for rapid prototyping) may be required. Applicability For small or medium-size interactive systems; For parts of large systems (e.g. the user interface); For short-lifetime systems.
Spiral development Process is represented as a spiral rather than as a sequence of activities with backtracking. Each loop in the spiral represents a phase in the process. No fixed phases such as specification or design - loops in the spiral are chosen depending on what is required. Risks are explicitly assessed and resolved throughout the process.
Spiral Model material adapted from Steve Easterbrook Determine goals, alternatives, constraints Evaluate alternatives and risks Plan Develop and test budget1 budget2 budget3 budget4 prototype1 prototype2 prototype3 prototype4 alternatives4 alternatives3 Altern- atives2 constraints4 constraints3 Constr- aints2 risk analysis4 risk analysis3 risk analysis2 analysis1 concept of operation software requirements validated design validated, verified design detailed code unit system acceptance requirements, lifecycle plan development plan integration and test plan implementation plan
Details of the Spiral
Boehm’s Spiral Lifecycle Planning = determination Risk Analysis = Analysis of of objectives, alternatives alternatives and identification/ and constraints resolution of risks Risk = something that will delay project or initial requirements increase its cost go, no-go decision completion first prototype alpha demo Customer Evaluation = Engineering = Assessment of the Development of the results of engineering evolving system next level product
Spiral model sectors Objective setting Specific objectives for the phase are identified. Risk assessment and reduction Risks are assessed and activities put in place to reduce the key risks. Development and validation A development model for the system is chosen which can be any of the generic models. Planning The project is reviewed and the next phase of the spiral is planned.
The spiral model The Risk Management Plan Identify the project’s top 10 risk items. Present a plan for resolving each risk item. Update list of top risk items, plan, and results monthly. Highlight risk-item status in monthly project reviews. Compare with previous month’s rankings, status. Initiate appropriate corrective actions.
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.
When to use Spiral model When costs and risk evaluation is important For medium to high-risk projects Long-term project commitment unwise because of potential changes to economic priorities Users are unsure of their needs Requirements are complex New product line
V Model material adapted from Steve Easterbrook system requirements software preliminary design detailed code and debug unit test component integration acceptance “analyze and design” “test and integrate” time Level of abstraction
V Model V- model means Verification and Validation model. Just like the waterfall model, the V-Shaped life cycle is a sequential path of execution of processes. Each phase must be completed before the next phase begins. Testing of the product is planned in parallel with a corresponding phase of development.
V Model Requirements begin the life cycle model just like the waterfall model. But, in this model before development is started, a system test plan is created. The test plan focuses on meeting the functionality specified in the requirements gathering.
Build and Fix Model Advantage Disadvantage Appropriate for small programs written by one person Disadvantage Understandability and maintainability decrease rapidly with increasing program size Totally unsatisfactory Need a life-cycle model “Game plan” Phases Milestones
Case Study Test System The final phase in the process was to test the system. We ran visual acceptance tests by running the program and playing through the game. This proved helpful in ensuring that the game is error free, not necessarily that the system is error free.
Children hospital Case study Care for children in the Alexandria city and some Arab countries The average length of stay of inpatients is 3.6 days All patients are accompanied by a parent for the entire duration of their stay at hospital
Case Study We participated in a group project where we designed and developed a cell phone game suitable for a cell phone using the programming language Java 2 Micro Edition. We designed everything from the beginning with no game requirements given to us. The process consists of defining system requirements, building the system, and testing the system. The goal of the project was to have a working game. We were restricted by time since this was a summer program
Bank example Requirements Open an account for a customer (savings or chequing) Deposit Withdraw Display details of an account Change LOC Produce monthly statements Print a list of customers Ambiguities What is the difference between savings and chequing?
Design of Bank – Identify Classes TELLER CUSTOMER SAVINGS_ACCOUNT CHEQUING_ACCOUNT MAIN_MENU BALANCE_INQUIRY INTEREST_RATE