Software Engineering

Acknowledgement Charles Moen Sharon White Bun Yue

Software Development With the rapid increase in technology, the complexity and expectation of computer capabilities also increased. Requirements have increased rapidly. However, while users expect and demand a many features, customers generally want to minimize the cost for the software and they want it developed very fast. This becomes a recipe for disaster!

Software Disasters Software has played a role in many high-profile disasters. Mars probe - data calculated on the ground in Imperial units and reported that way to the navigation team who were expecting the data in metric units caused it to be destroyed. Therac-25 - A radiation therapy machine responsible for six overdoses due to faulty software. Airbus A320 - In the Airbus flight control systems, the computer has the final say on all decisions, meaning the safety of passengers depends upon the accuracy of the software specification, and the competence of the engineering teams producing the (multiple, independent) software stacks. The Strasbourg A320 crash of Jan 21, 1992 is partially related to software in that poor user interface design was a contributing factor.

Cause of the Software ‘Crisis’ Causes of the software crisis were linked to the overall complexity of the process and the relative immaturity of software engineering as a profession. The crisis manifested itself in several ways: – Projects running over-budget. (DIA – 2 billion over ) – Projects running over-time. (DIA – 16 months behind) – Software was of low quality. – Software often did not meet requirements. – Projects were unmanageable and code difficult to maintain.

Software Engineering This discipline deals with identifying, defining, and realizing the required performance characteristics of the resulting software. These performance characteristics include: reliability, maintainability, availability, testability, ease-of-use, portability, etc. Software engineering addresses these performance characteristics by preparing design and technology specifications, that if implemented properly, will enable the resulting software to meet these requirements.

Software Development Process What is the best way to make more and better software? Software engineers advocate many different technologies and practices, and the debate on how to accomplish this has gone on for many years and may continue forever.

Software Life Cycle Models With large numbers of software projects not meeting their expectations in terms of functionality, cost, or delivery schedule, effective project management is proving difficult. This has resulted in people apply project management techniques to writing software.

Waterfall Model In Royce's original waterfall model, the following phases are followed perfectly in order: – Requirements specification – Design – Construction (aka: implementation or coding) – Integration – Testing and debugging (aka: verification) – Installation – Maintenance

Key Software Activities 5 Phases of Software Life Cycle: Phase 1: Problem Analysis and Specification Phase 2: Design Phase 3: Implementation (Coding) Phase 4: Testing, Execution and Debugging Phase 5: Maintenance

Phase 1: Problem Analysis and Specification Computer Science programming assignment - specific statement of problem quantitative description clearly defined requirements: input, output, calculations, test data Computer Science programming assignment - specific statement of problem quantitative description clearly defined requirements: input, output, calculations, test data Easy in CS courses, not always in the real world.

“Real World” request - general statement of problem qualitative not quantitative precision missing for input, output, processing “Real World” request - general statement of problem qualitative not quantitative precision missing for input, output, processing Phase 1: Problem Analysis and Specification

Phase 2: Design CS courses – small systems – few hundred lines of code – simple, straightforward – self-contained “Real” world – large systems – Tens of thousands of lines of code – complex – many components

OOD: Object-Oriented Design 1. Identify the objects in the problem's specification and their types. 2. Identify the operations of the objects (methods) needed to solve the problem. 3. Arrange the operations in a sequence of steps, called an algorithm, which, when applied to the objects, will solve the problem.

OOD Goals Robust – “Capable of handling unexpected inputs” Adaptable – Able to evolve over time in response to changes Reusable – Code should be reusable in other applications – Components – e.g., code libraries like the Java libraries

Phase 3: Implementation (Coding) Select language of implementation Encode the design Verify integration Combining program units into a complete software system. Insure quality – programs must be correct, readable, and understandable, that is, well-structured, documented, and stylistic.

Phase 4: Testing, Execution, and Debugging Validation: "Are we building the right product?" – The software should do what the user really requires check that documents, program modules, etc. match the customer's requirements. Verification: "Are we building the product right?" – The software should conform to its specification check that products are correct, complete, consistent with each other and with those of the preceding phases.

Errors can occur anytime – Specifications don't accurately reflect given information or the user's needs/requests – Logic errors in algorithms – Incorrect coding or integration – Failure to handle boundary data or test values

Different kinds of testing – Unit tests: Each individual program unit works? – Program components tested in isolation – Integration tests : Units combined correctly? – Component interface and information flow tested – System tests: Overall system works correctly?

Phase 5: Maintenance Large % of computer center budgets – Large % of programmer's time – Largest % of software development cost Why? – Includes modifications and enhancements – Due to poor structure, poor documentation, poor style less likely to catch bugs before release make fixing of bugs difficult and time-consuming impede implementation of enhancements

Iterative Processes Iterative development prescribes the construction of initially small but ever larger portions of a software project to help all those involved to uncover important issues early before problems or faulty assumptions can lead to disaster. Iterative processes are preferred by commercial developers because it allows a potential of reaching the design goals of a customer who does not know how to define what he wants.

Rational Unified Process An example of an iterative process