The Mythical Man Month Essays on Software Engineering

Presented by

Prashant KashyapBtech 2000

FREDERICK P. BROOKS, JR

An Overview

The tar pit

The Good Joys of programming

The Bad Woes of programming

The Ugly Systems integration is a

punch in the face

System programming has been such a tar pit – very Sticky !

The Mythical Man Month

What is the man-month? Cost varies as the product of the number

men and the number of months Progress does not Hence the man-month as a unit for

measuring the size of a job is a dangerous and deceptive myth It implies that men and months are

interchangeable

The Mythical Man Month

“The bearing of a child takes nine months, no matter how many women are assigned” Software projects are sequential in

nature. They generally consist of several steps that

must be completed one after another These steps cannot be worked on at the

same time. (un-partitionable)

The Mythical Man Month Throwing more people at a software project has no real effect upon the

date of project completion. In fact, the overheads, training preserving “conceptual integrity” communication pathsmay delay the project further.

“Adding manpower to a late project makes it later.”

Why? The work and disruption or repartitioning jobs The time consumed training new people The added intercommunication between people

There is a [n(n-1) / 2] increase in effort

The Mythical Man Month

20 PEOPLE, 190 CHANNELS!

2 people, 1 channel

3 people, 3 channels

4 people, 6 channels 5 people, 10 channels

N=n(n-1) 2

Communication Paths

The Mythical Man MonthPartitionable Task

Men

Mon

ths

Unpartitionable Task

Men

Mon

ths

Communication Required

Men

Mon

ths

Complex Interrelationships

Men

Mon

ths

Surgical Team

There is an order of magnitude difference between good programmers and bad programmers.

More people working on a project = more miscommunication between those people Smaller groups are better, but a small group is too

slow to create a large system in a reasonable amount of time.

Problem:Problem:

Surgical Team

Miller’s Proposal: Break up the programming teams into

smaller sub-groups, with only one or two people doing actual coding. Everyone else in the team acts as support for these people. Thus, only a limited number of minds need to coordinate ideas, which reduces communications problems.

Solution:Solution:

Surgical Team

Surgeon

Administrator Editor Copilot

Secretary Secretary Programming Clerk

Tester

Tool-smith

Language Lawyer

supervisorthe pilot

documentation

the know-it-all guy

Aristocracy, Democracy, and System Design

Conceptual Integrity is the most important consideration in system design “It is better to have a system omit certain

anomalous features and improvements, but to reflect one set of design ideas, than to have one that contains many good but independent and uncoordinated ideas.

Features Vs. Simplicity Where they balance is when ease of use is

optimum.

Aristocracy, Democracy, and System Design

“If a system is to have conceptual integrity, someone has to control the concepts.” The architects are to become the

aristocrats from whom all design decisions originate.

The implementers are then the peasants, soldiers, builders, and thinkers that support the aristocrat’s ideas.

Where architecture tells what is to happen, while implementation tells how it is to happen.

Architect

Implementers

The Second-system Effect When a programmer works on a large system for

the first time, he generally keeps things simple and to the point.

Radical ideas, features, or innovative implementations he would like to try are filed away for later use.

When a programmer works on his second large system, these files come out of storage with a vengeance.

Great care must be taken to insure that the system design is adhered to by all team members.

Passing the Word

Manuals External specifications of the product (Product Manual)

Describes everything the user sees, and nothing that he doesn’t see

Internal specifications of the product (System Manual) Describes every technical detail about the system that may

be used by the implementers.

Meetings Weekly half-day conferences Annual or semi-annual “Supreme Court” meetings

Email logs

The Fall of Babel

… “…let us build ourselves a city with a tower whose top shall reach the heavens…”… then

the Lord came down... said “Come, let us go down, and there

make such a babble of their language that

they will not understand one

another’s speech.” And thus the Tower of Babel was

left incomplete.

The Fall of Babel

When people lost the ability to communicate with one another, they could no longer work together to complete the Tower of Babel.

Likewise, when communication is lost to a software development team, the project they are working is doomed as well.

Combative measures Informal email / telephone service Regularly scheduled meetings Project Workbook

Calling the Shot

Programming effort is a function of program size. Naturally, the larger and more complex the

program, the longer it takes to complete However, this relationship is not linear. It’s

an exponential growth function. Effort = (constant) x (number of

instructions)

Calling the Shot

0

1000

2000

3000

4000

5000

6000

7000

8000

0 100 200 300 400 500 600 700

extrapolated

Actual data

LinearlyextrapolatedActual workcurve

Thousands of machine instructions

Man

-mon

ths

incomplete

Ten Pounds in a Five Pound Sack

Program space (memory) is a cost Thus size control is important Two methods to control size: space control

and data representation Space techniques

Trading function for size Space - time tradeoffs (CPU power)

Representation techniques Efficient data representation (objects, data

structures)

The Documentary Hypothesis

Reasons for having formal documents Recording decisions Communicating those decisions Data base / checklist (programmer

directives)

Plan to Throw One Away "The throwaway is the

acceptance of the fact that as one learns, he changes the design."

Pilot plants - upscale from small to large Modern approach: 'alpha' and 'beta'

testing Plan your system for change

Modern approaches: pre release: code base /

configuration managers (e.g.: SourceSafe)

post release: patching systems

Collapse of the aerodynamically misdesigned Tacoma Narrows Bridge1940

Sharp Tools

“A good craftsman is known by his tools.” The tools of a programmer are:

Target machines The final testing environment

Vehicle machines The programmer’s work environment

Data services Compiler, assembler, libraries, debugging tools

HLL & interactive programming Language and programming tool of choice (e.g.: Java

and JBuilder)

Hatching a Catastrophe

Schedules are important to keep Give the team 100% certifiable milestones

Clearly define what needs to be done Pay attention to schedule slipping

Small slips can quickly compound into major project tardiness

Under the employee rug There are always manager-boss conflicts Minimize the conflict between you and your managers But occasionally, yank the rug out from under them

The Other Face

Documentation is important to the user, as well What kinds of documentation do they need?

How to use the product How the product works How to adapt the product to their needs

Flowcharts are flawed: they can be easily represented with code comments

Self-documenting programs Merge source code and documentation (well-

commented code)

The Whole and the Parts

Design the bugs out of the system Bug-proof the definition Use structured programming

Component debugging Test cases System debugging Use debugged components Scaffolding Control changes Add one component at a time

Exhaustive Testing. Really?

Simple loop, executing ≤ 20 times 1014 possible paths at 1 ms per test,

would take 3170 years

loop Š 20 times

No Silver Bullet- Essence and Accident in Software Engineering

“There is no single development, in either technology or management technique, which by itself promises even one order of magnitude improvement in productivity in reliability, in simplicity.” - Fred Brooks

“... building software will always be hard” due to difficulties in its intrinsic nature (essence);

however, improved process can solve difficulties which surround software production and are not part of its nature (accidents)

No Silver Bullet:Difficulties in Essence of Software

complexity: due to interaction of components, number of possible states grows much faster than lines of code.

No Silver Bullet:Difficulties in Essence of Software

changeability: must model changing real world, increase functionality, run on new hardware.

No Silver Bullet:Difficulties in Essence of Software

conformity: must interface with existing systems.

No Silver Bullet:Difficulties in Essence of Software

invisibility: cannot visualize all aspects at once.

No Silver Bullet: Breakthroughs for improving the Accidents

software development environments

use of “off the shelf” software and components

rapid prototyping incremental development training, encouraging

great designers aspects at once.

Some Methods for Dealing with Essential Difficulties

Complexity Abstract models Breakdown of system into

manageable modules Breakdown of development

process into phases, tasks, and subtasks

Team programming Changeability and Conformity

Change management Configuration management

Invisibility use of multiple models, for many

views of system

No Silver Bullet: Refired

What is accidental? Intent

accidental: incidental or appurtenant Misunderstanding

accidental: occurring by chance,

“Shrinking the accidental part to zero will not give an order of magnitude productivity improvement.”

No Silver Bullet: Refired

Complexity Not all complexity is inevitable “… Yesterday’s complexity is tomorrow’s order.” There are no general rules to avoid complexity Complexity is by levels

Complexity in software construct is due to implementation rather than conformity to the external world.

Solution Hierarchically, by layered modules or objects Incrementally, so that the system always works

No Silver Bullet: Refired

Harel’s “Gloom” themes “Sharp separation into essence and accident”

“Treatment of each silver bullet candidate in isolation”

“Predicting for only 10 years, instead of a long enough time in which ‘to expect any significant improvement’”

No Silver Bullet: Refired Productivity Follows Quality

“Focus on quality, and productivity will follow.” - Jones’s view “… productivity drops again as one pursues extreme

quality…” - Boehm’s view “… systematic software development disciplines were

developed in response to quality concerns … rather than productivity concerns.” - Coqui’s view

What has happened to productivity? Measuring productivity is difficult Shrink-wrapped software - Buy; don’t build Tools improve productivity

cheap, powerful tools that are easy to use

No Silver Bullet: Refired

What about REUSE? Don’t build software. Aims

Programmer level - 30% Corporate level - 75%

How does corporate-level reuse fare today?- Jones

Large companies have reuse research Small companies don’t

“Reuse is something that is far easier to say than to do.” - Parnas

No Silver Bullet: Refired

CONCLUSION Complexity

Cannot do away with completely Productivity

Focus on Quality and it will lead to productivity

Reuse Don’t reinvent the wheel

The Mythical Man-Month after 20 Years

We, the programmers, pledge not to ask the presenter of this presentation about this! We UNDERSTAND where it stands.

Game for a Discussion? (not me )

References

1. The Mythical Man Month – Fredrick P. Brook’s Jr., Pearson Education Asia Publishers.

2. Software Engineering – A practitioner’s Approach by Roger S. Pressman

3. Software Project Management (Lecture)- Peking University, Fall Semester, 2001

4. No Silver Bullet : Essence and Accident in Software Engineering by Prof. Fred Brooks, IEEE Computer, April 1987

5. Frederick P. Brooks Jr. The Mythical Man-Month. Addison-Wesley, 1995

6. Brad Cox, No Silver Bullet Revisited, http://www.virtualschool.edu/cox/AmProTTEF.html

7. Information on Fred Brooks, http://www.cs.unc.edu/Events/News/TuringAward.html

8. Ed Yourdon, Managing Projects to Produce "Good Enough" Software, http://www.yourdon.com/articles/9503ieee.html