1. Lecture 3

2. Scope and necessity of software engineering
Engineering is an engineering approach for software development.
We can alternatively view it as a systematic collection of past experience. The experience is arranged in the form of methodologies and guidelines.
A small program can be written without using software engineering principles. But if one wants to develop a large software product, then software engineering principles are MUST to achieve a good quality software cost effectively.

3. Scope and necessity of software engineering
Without using software engineering principles it would be difficult to develop large programs.
A program of size 1,000 lines of code has some complexity. But a program with 10,000 LOC is not just 10 times more difficult to develop, but may as well turn out to be 100 times more difficult unless software engineering principles are used.
In such situations software engineering techniques come to rescue.
Software engineering helps to reduce the programming complexity.
Software engineering principles use two important techniques to reduce problem complexity.
Abstraction and Decomposition

4. Abstraction
The principle of abstraction implies that a problem can be simplified by omitting irrelevant details.
In other words, the main purpose of abstraction is to consider only those aspects of the problem that are relevant for certain purpose and suppress other aspects that are not relevant for the given purpose.
Once the simpler problem is solved, then the omitted details can be taken into consideration to solve the next lower level abstraction, and so on.
Abstraction is a powerful way of reducing the complexity of the problem.

5. Abstraction

6. Decomposition
In this technique, a complex problem is divided into several smaller problems and then the smaller problems are solved one by one.
However, in this technique any random decomposition of a problem into smaller parts will not help.
The problem has to be decomposed such that each component of the decomposed problem can be solved independently and then the solution of the different components can be combined to get the full solution.
A good decomposition of a problem should minimize interactions among various components.
If the different subcomponents are interrelated, then the different components cannot be solved separately and the desired reduction in complexity will not be realized.

7. Decomposition

8. Program vs. Software product
Programs are developed by individuals for their personal use. They are therefore, small in size and have limited functionality but software products are extremely large such as Windows 7.
In case of a program, the programmer himself may or may not be sole user but on the other hand, in case of a software product, most users are not involved with the development.
For a program, the user interface may not be very important, because the programmer is the sole user. On the other hand, for a software product, user interface must be carefully designed and implemented because developers of that product and users of that product are totally different.
In case of a program, very little documentation is expected, but a software product must be well documented.
A program can be developed according to the programmer’s individual style of development, but a software product must be developed using the accepted software engineering principles.

9. Future of Software Engineering
Microsoft, IBM
Any Commercial Software
App Development
Apple Store
Android
Two Tier
Software
Engineering

10. Software
Q : If you have to write a 10,000 line program in C to solve a problem, how long will it take?
Answers: generally range from 2-4 months
Let us analyze the productivity
Productivity = output/input resources
In SW output is considered as LOC
Input resources is effort - person months; overhead cost modeled in rate for person month
Though not perfect, some productivity measure is needed, as project has to keep it high
SOFTWARE ENGINEERING

11. Software … The productivity is 2.5-5 KLOC/PM
Q: What is the productivity in a typical commercial SW organization ?
A: Between 100 to 1000 LOC/PM
Q: Why is it low, when your productivity is so high? (people like you work in the industry)
A: What the student is building and what the industry builds are two different things
SOFTWARE ENGINEERING

12. Software…
In a univ a student system is built while the commercial org builds industrial strength sw
What is the difference between a student program and industrial strength sw for the same problem?
Software (IEEE): collection of programs, procedures, rules, and associated documentation and data
SOFTWARE ENGINEERING

13. Software… Industrial Strength Student Others are the users
bugs not tolerated
UI v. imp. issue
Documents needed for the user as well as for the organization and the project
Student
Developer is the user
bugs are tolerable
UI not important
No documentation
SOFTWARE ENGINEERING

14. Software… Student SW not in critical use
Reliability, robustness not important
No investment
Don’t care about portability
Industrial Strength
Supports important functions / business
Reliability , robustness are very important
Heavy investment
Portability is a key issue here
SOFTWARE ENGINEERING

15. Industrial strength software
Student programs for a problem & industrial strength software are two different things
Key difference is in quality (including usability, reliability, portability, etc.)
Brooks thumb-rule: Industrial strength sw costs 10 time more than student sw
In this course, software means industrial strength software
This software has some characteristics
SOFTWARE ENGINEERING

16. Is Expensive Let us look at costs involved
Productivity = Appx 1000 LOC/PM
Cost = $3K to $10K/PM
Cost per LOC = $5 to $15
I.e, each line of delivered code costs many $s
A simple application for a business may have 20KLOC to 50KLOC
Cost = $100K to $2.25Million
Can easily run on $10K-$20K hardware
So HW costs in an IT solution are small as compared to SW costs.
SOFTWARE ENGINEERING

17. Requires tight Schedule
Business requirements today demand short delivery times for software
In the past, software products have often failed to be completed in time
Along with cost, cycle time is a fundamental driving force

18. Cost, Schedule and Quality
That quality, cost, and schedule are the main forces that drive a (industrial strength)
software project.
How cost and productivity are defined and measured for such a project, and how quality of software is characterized and measured.
That large scale and change are important attributes of the problem domain and solution approaches have to handle them.

19. Productivity – for cost and schedule
An industrial strength software project is driven by cost and schedule
Both can be modeled by productivity, measured in terms of output per unit effort (e.g. LOC per person month)
Higher productivity leads to lower cost
Higher productivity leads to lower cycle time
Hence, for projects (to deliver software), quality and productivity are basic drivers
SOFTWARE ENGINEERING

20. Quality
Along with productivity, quality is the other major driving factor
Developing high Q sw is a basic goal
Quality of sw is harder to define
SOFTWARE ENGINEERING

21. Quality – ISO standard
SOFTWARE ENGINEERING

22. Quality – ISO std… ISO std has six attributes Functionality
The capability to provide functions which meet stated and implied needs when the software is used.
Reliability
The capability to provide failure-free service.
Usability
The capability to be understood, learned, and used.
Efficiency
The capability to provide appropriate performance relative to the amount of resources used.
SOFTWARE ENGINEERING

23. Quality – ISO std… ISO std has six attributes Maintainability
The capability to be modified for purposes of making corrections, improvements, or adaptation.
Portability
The capability to be adapted for different specified environments without applying actions or means other than those provided for this purpose in the product.
SOFTWARE ENGINEERING

24. Quality…
Multiple dimensions mean that not easy to reduce Q to a single number
Concept of Q is project specific
For some reliability is most important
For others usability may be more important
Reliability is generally considered the main Q criterion
SOFTWARE ENGINEERING

25. Quality… Reliability => Probability of failure
hard to measure
approximated by no. of defects in software
To normalize, Quality = Defect density
Quality = No. of defects delivered / Size
Defects delivered - approximated with no. of defects found in operation
Current practices: less than 1 def/KLOC
What is a defect? Project specific!
SOFTWARE ENGINEERING

26. Quality – Maintainability
Once sw delivered, it enters the maintenance phase, in which
Residual errors are fixed – this is corrective maintenance
Upgrades and environment changes are done – this is adaptive maintenance
Maintenance can consume more effort than development over the life of the software (can even be 20:80 ratio!)
Hence maintainability is another quality attribute of great interest
SOFTWARE ENGINEERING

27. Quality and Productivity
Hence, quality and productivity (Q&P) are the basic drivers in a sw project
The aim of most methodologies is to deliver software with a high Q&P
Besides the need to achieve high Q&P there are some other needs
SOFTWARE ENGINEERING

28. Change Only constant in business is change!
Requirements change, even while the project is in progress
In a project, up to 40% of development effort may go in implementing changes
Practices used for developing software must accommodate change
SOFTWARE ENGINEERING

29. Scale Most industrial strength software tend to be large and complex
Methods for solving small problems do not often scale up for large problems
Two clear dimensions in a project
engineering
project management
For small, both can be done informally, for large both have to be formalized
SOFTWARE ENGINEERING

30. Scale…
SOFTWARE ENGINEERING

31. Scale…
An illustration of issue of scale is counting the number of people in a room vs taking a census
Both are counting problems
Methods used in first not useful for census
For large scale counting problem, must use different techniques and models
Management will become critical
SOFTWARE ENGINEERING

32. Scale: Examples Gcc 980KLOC C, C++, yacc Perl 320 KLOC C, perl, sh
Appache
100 KLOC
C, sh
Linux
30,000 KLOC
C, c++
Windows XP
40,000 KLOC
C, C++
The computer program Yacc is a parser generator developed by Stephen C. Johnson at AT&T for the Unix operating system in The name is an acronym for "Yet Another Compiler Compiler." It generates a parser (the part of a compiler that tries to make syntactic sense of the source code) based on an analytic grammar written in a notation similar to BNF
The GNU Compiler Collection includes front ends forC,C++, Objective-C, Fortran,Java, Ada, and Go, as well as libraries for these languages (libstdc++, libgcj,...). GCC was originally written as the compiler for the GNU operating system.
Perl is a high-level, general-purpose, interpreted, dynamic programming language.
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of Linux is the Linux kernel, an operating system kernel first released 5 October 1991 by Linus Torvalds.[11][12]
SOFTWARE ENGINEERING

33. Scale…
As industry strength software tends to be large, hence methods used for building these must be able to scale up
For much of the discussion, we will high Q&P as the basic objective
SOFTWARE ENGINEERING

34. Summary The problem domain for SE is industrial strength software
SE aims to provide methods for systematically developing (industrial strength) software
Besides developing software the goal is to achieve high quality and productivity (Q&P)
Methods used must accommodate changes, and must be able to handle large problems
SOFTWARE ENGINEERING