CS 4310 Software Requirements Engineering

Dr. Steve Roach

Dr. Tanja Magoc

University of Texas at El Paso

Department of Computer Science

Fall 2009

Introductions

• CS 4310, Software Requirements Engineering– Required course: – must pass with C or better to take 4311

• Text book: – Van Vliet, Software Engineering– Wiegers, Software Requirements– Required, bring it next time.

• Course Notes: Available online• TA: Evelyn Torres

Syllabus. Key points

• 0:3-0:5• GROUP: split into groups of 3• Groups introduce selves, select a team scribe who writes

down decisions of team

• objective: software requirements engineering• email• grading• attendance• office hours and policy

Quiz

• Three email addresses are listed on the syllabus. What are they?

• Write the pseudo code for the grading algorithm.

• What is the attendance policy?

Assignments

• Email

• Reading: Ch 1 Van Vliet; Ch 1, Wiegers (by Thurs)

• Quiz– Open notes, not open book

email

Plan on checking your mail daily.

Instructor notes

• 0:10-0:15

• GROUP: discuss and answer the questions:

What are the major steps needed to develop a reliable software system?

Discuss and answer the question

What are the major steps needed to develop a reliable software system?

Teams should record answers and team members.

Instructor Notes• 0:15-0:20 GROUP DISCUSSION:• ask each group to contribute to list. When done, we should

have– idea– feasibility study– requirements analysis– specification– design – coding and unit testing– integration and system testing– delivery– maintenance

Software Engineering

What is software engineering?

Software engineering is an attempt to formalize the construction of software. In particular, we are concerned with large, complex systems, systems that require multi-person effort and usually multiple versions.

Do we need software engineering?

Do we need software engineering?

• All great software is written by small teams: look at unix (2 guys), Apple (started in a garage), …

Do we need software engineering?

• All great software is written by small teams: look at unix (2 guys), Apple (started in a garage), …

• Writing software is a creative process, an “art” or a “craft”, and cannot be engineered

Ariane 5

• June 4, 1996

• European Space Agency lost newest rocket, the Ariane 5, successor to the Ariane 4

• $500 Million lost on first flight

Ariane 5• Problem was overflow in the conversion of a 64-bit

float to a 16-bit signed number in the SRI inertial reference system (why do you take architecture?)

• The broken data serves no purpose in the SRI after takeoff. But in the A-4, the software ran after takeoff so that if the launch sequence was interrupted, it could be restarted quickly. This was not necessary in the A-5.

• The A-5 accelerated much faster than the A-4. The A-4 never had an overflow. The A-5 did overflow.

• The backup system on the A-5 was identical to the original, and it failed for exactly the same reason. The rocket broke apart and was destroyed.

 

Therac 25

• Late 1980's.• Radiation therapy device with 3 modes:

– light: for alignment– electron: low power, magnets to spread beam– X-ray: high power, flattener plate to spread

beam

• Needed flattener plate in place in X-ray operation.

Therac 25• There was a race condition in the operator

interface. If the operator made a mistake, used the back space to correct it, the machine could run in X-ray mode without the flattener in place.

• Previous version of machine had hardware interlock to prevent X-ray mode without flattener.

• Therac-25 depended on software.• Difficult to reproduce. The physicist spent at least

two weeks practicing once he had seen the technician reproduce the problem.

• Several rather horrible deaths resulted.

Others:

• 1998 GM ABS recall• 2003 East Coast Power Failure: power cut to parts

of at least 5 states (cost, $7-$10 billion)• 2004: Air traffic control system shuts down voice

communication system in southwest US, losing contact with 400 aircraft in flight.

• 2009: Washington DC Metro crash kills 9 people: train under software control at the time

Assignment Due Before Next Class• A: find two more software failures that have resulted in

significant harm (loss of life, serious risk, or loss of money) in the past 3 years.

• B: Describe two approaches to eliminating the types of software failures discussed here.

• You must find no less than two independent citations for each. • Legitimate citations include IEEE or ACM publications, web

sites sponsored by Fortune 500 companies, or Prentice-Hall, Wiley, O’Rielly, Addison-Wesley, or Microsoft Press. Or anything by Parnas or McConnel.

• You must include your citations.• Email this list to me before the next class.

Department of Defense

• "About the only thing you can do with an F-22 without software is to take a picture of it."

– --Unidentified Air Force General

• The DoD estimates it will field systems utilizing 20 million LOC.

» CMMI Distilled, Ahern

NASA’s Need

“The continued growth in scope, complexity, and cost of software systems represents a threat to the success of NASA missions.

NASA must take proactive steps to strengthen its capability in software engineering and management:

• Minimize technical, cost, and schedule issues

• Increase safety, quality, and reliability

Orlando Figueroa

Former Deputy Chief Engineer for Systems Engineering

April, 2001

Instructor Notes

• 0:40-0:50– A key feature of 4310/11 is the project.– You will need to bring to bear all of the things

you know, not just what I say

Project course

• "The hardest single part of building a system is deciding what to build." Brooks.

• Why a project?– Experience is better than just spitting back what

I tell you.– Employers know this, and will ask at your

interviews.– It gives you the chance to think "outside the

box" out of the context of class

Requirements Engineering:

• requirements elicitation: developers and customers discover and articulate users' needs

• requirements analysis: analyze the customer and user needs to arrive at a definition

• requirements specification: develop a document • requirements verification: show the document is

correct • management of requirements: control the

process

Outline of course:

• Build software teams

• Interview customer, interview report

• Feasibility study

• Prototype

• Analysis: use cases, diagrams, modeling

• Specification (SRS)

• Tracing

GROUP:

• (15 minutes total)– Read the article, answer the questions

  Why is software difficult to construct so that it works correctly? (5 minutes)

 

Software error costs

• In your groups:– Read the article– Answer the questions:

• What are the costs associated with software errors?

• What are the sources of errors?

• Why is software different from other products?

• What is liability?

Why software projects fail (Software Runaways, Robert Glass, 1998, Prentice)

• runaway: fails to deliver or is 30% or more over budget

• faulty specification 51%• faulty planning and estimation 48%• based on unstable new technology 45%• inadequate management: 42%• lack of senior programming staff: 42%• problems with hardware/software from vendors: 42%

Trends with failures:

• overly ambitious

• result from multiple causes

• schedule overruns are more common than cost overruns

• most failures are spotted by development team before management

• most did not do risk management

Problems with Specifications

• too many specs: project is large

• unstable specs

• ambiguous specs

• incomplete specs

Do we need software engineering?

• What is a large software system?

• What is a complex software system?

• What is safety critical/high consequence?

Cost of SE vs Cost of not SE• Each stage of software development has overhead

costs– management– time reporting– testing– requirements analysis– design

• Imagine stripping everything except the parts that lead directly to the program itself: coding

• SE entails overhead. The amount of overhead should match the project.

History: How did we get here?

• 1930s-40s: we only had very highly skilled users: the builders, physicists, EE's.

• 50's and 60's: The advent of 3GLs and programmers– More general, but still highly skilled– They started generating output for users.

• Mid 60s: started having large projects like OS360: 5000 man-years. – Too big for a single person to understand. – Lots of scaling problems. – Projects are over budget and late.

History: How did we get here?

• 1970s & 80s: Mainframes running mostly batch programs and microcomputers running simple software

• 90s: windowing and multitasking OS on PCs, the rise of the internet– The platforms for software become more sophisticated, but

not enough for the software running on top of them– Lots of people writing software, not all are highly skilled.

• 2000: Software becomes ubiquitous. Software defines products. – User interfaces and domain expertise needed

Solutions:

• Better management

• Better tools and languages

• More life-cycle-centric view of development: software development isn’t just coding anymore