Royce 1970 Dr. Winston W. Royce, “Managing the development of large software systems”. Proc. IEEE WESCON, Aug 1970. Reprinted 9 th Intl. Conf. Softw. Eng.,

Royce 1970

Dr. Winston W. Royce, “Managing the development of large software systems”.

Proc. IEEE WESCON, Aug 1970.

Reprinted 9th Intl. Conf. Softw. Eng., 1987. Universally cited as the reference for the

waterfall model But, the word “waterfall” is not mentioned And the model looks more like a cascade

Moreover, the paper is actually against the waterfall model

The Basic Waterfall Model

Engineering at its best!

With Fallback

Problems

Doing everything in a single sequence is unrealistic A better model involves iteration between

successive steps Testing comes too late and may uncover

problems in the initial design The solution: do it twice

(Same advice as Fred Brooks in The Mythical Man-Month, but referring to a full-scale system)

Using a Prototype

Note that prototypeis actually used!

New stage ofpreliminary design

Learning from Prototypes

Verify feasibility of approach and design Temporary executable system used to gain

experience and identify problems in performance, resource requirements, etc.

Study and compare alternative designs or implementations, and foster creativity

Try out the user interface, to show clients/users, get feedback, and understand requirements

May be used in real application domain and actually evolve into full system

Exercise:

The ministry of interior wants to create a computerized system for filing forms.

There are 31 different forms that need to be supported, with various fields.

Need to be able to save partially filled forms, return to form started earlier, print a completed form, and submit it electronically.

What would you implement in a prototype?

Additional Emphases Need to plan and control the testing Need to involve the client in key points

Additional Emphases Need to plan and control the testing Need to involve the client in key points Create multiple documents (requirements,

specification, design, test plan, manual) and keep them up to date “Write an overview document that is

understandable, informative, and current. Each and every worker must have an elemental understanding of the system.”

“If the documentation is in serious default my first recommendation is simple: replace project management.”

Summary

Frederick P. Brooks, Jr.

The Design of Design

Addison Wesley, 2010

Project manager for IBM’s System/360 and OS/360Founded CS department at University of North CarolinaReceived National Medal of Technology 1985Received Turing Award 1999Many other awards and honorsFamous for The Mythical Man-Month, Brooks’s Law, No Silver

Bullet

Waterfall Doesn’t Work

• Clients don’t know what they want

• We can’t map out all the alternatives in advance

• The design cannot be decomposed into a sequence of decisions

• The goodness function cannot be computed incrementally

• Desiderata and constraints keep changing

• Designers just don’t work that way

Root Causes I

• Requirements are done once up front

Need to get everything you might need in

• Sometimes done by committee

I’ll support yours if you support mine

Outcome: requirements bloat

Root Causes II

• we are not perfect

• Greedy clients try to pay less

• Builders tray to maximize income and not value/cost

• Misunderstandings abound

• So we need contracts

• Contracts must spell out the details before the project is started

Alternative Contracting Scheme

1. Develop programatic idea

2. Contract with architect for services

• Complete requirements

• Conceptual design

• Detailed design

• Pay for hours worked

3. Contract with constructor

• Use design document produced above

• Fixed price contract for production

The Frustration

Royce’s paper is very insightful and foreshadows several modern ideas.

On page 2 it says (of the waterfall) “I believe in this concept, but the implementation described above is risky and invites failure.”

Others have identified many additional problems.

So why is the waterfall model still being used?

And why is it a standard?