Proc. of 1994 ICSE By David Parnas

Software Aging

Proc of 1994 ICSE – David Parnas

Presented by Preethi Mahadev Date 03/07/2003

Who is Parnas?

• David Lorge Parnas received his B.S., M.S. and Ph.D. in Electrical Engineering - Systems and Communications Sciences from Carnegie Mellon University.

• Professor Parnas is the author of more than 200 papers and reports.

• He won an ACM "Best Paper" Award in 1979, and two "Most Influential Paper" awards from the International Conference on Software Engineering.

• He was the 1998 winner of ACM SIGSOFT's "Outstanding Research Award.“

• He received honorary doctorates from the ETH in Zurich and the Catholic University of Louvain in Belgium.

What is software aging?

Closely resembles the phenomenon of human aging

i. Performance of the software system degrades with time

ii. Software Aging cannot be preventediii. Software Aging can be slowed down

Purpose of this paper

i. To explain how an abstract mathematical product like software aging can age

ii. To review approaches to deal with it• Treating aged software• Slowing down the deterioration

Reaction of some computer scientists

Software aging doesn’t make sense

“Software is a mathematical product and mathematics don’t decay with time.”

Parnas argues “True but not relevant” Why?i. Old software has begun to cripple

its once proud ownersii. Many products are now seen as a

burdensome legacy from the pastiii. Old softwares have become

essential cogs in the machinery of our society

Why is Software aging significant?

i. Growing economic importance of software

ii. Software is the major “capital” of many high tech firms

iii. Software aging is an impediment for further development of the systems

Causes of software aging

i. Lack of movement• Failure to modify software to meet its

changing needs• Unless updated, software is considered as

old and outdated

ii. Ignorant surgery• Changes made to the current system is harder to maintain• Nobody understands the modified

products

leads to rapid decline in value of the software

Software aging Vs System slow down Causes of System slow down

i. Failure to release allocated memoryii. Files grow and require pruningiii. Swap space decreases and performance

degrades

Analogy: Kidney failure and Dialysis type treatment as solution

Causes of Software aging i. Existing software no longer satisfies the ownerii. Changes made to the software makes it harder

to maintain

System degrading in performance can be easily cured than aging

The costs of software aging

i. Inability to keep up: Lose customers because it is difficult to keep up with the market

ii. Reduced performance: Software Aging degrades performance of the system on the whole

iii. Decrease in reliability: Too many errors due to inconsistent changes made

Reducing the costs of Software aging

Inexperienced programmers have short term goals

We should look far beyond the first release to the time when the product is old

Preventive medicine

“ delay the decay” Ways of slowing deterioration:

i. Design for successii. Documentationiii.Second opinions-reviews

Design for success is Design for change

i. Principle to be applied: Object Orientation

ii. We cannot predict actual changes, predictions will be about classes of changes

iii.Confine the probable section of code

iv.Estimate the probability of types of changes

Design for success ..contd… The barriers of ” design for change”

i. Impatient programmers and managers are more worried about meeting deadlines and starting a new one

ii. Programmers tend to confuse design principles with languages

iii. Code is rarely designed to be easily changed

Documentation

i. Important aspect of software engineeringii. Inconsistent or inadequate

documentation are developed in most cases

iii. Programmers and managers are driven by imminent deadlines

iv. For some, documentation is not interesting

If not documented, we save a little and pay much more in future

Second opinions-reviewsi. Often Commercial programs don’t have adequate

reviewii. Many programmers have no professional training,

so they neglect documentation and reviewsiii. Much software is produced as cottage industryiv. Often produced under time pressurev. Programmers resent the idea of being reviewed

Every design must be approved by someone whose responsibilities are for long term future of the product

Aging is inevitable …Why?i. Changes may violate original assumptions

ii. Documentation will never be perfectiii. There will be Issues that reviewers

overlook iv. The idea of eliminating software aging is

not practical

Software geriatrics

Prevention is better than cure .But..

How to deal with already old software?

i. Stopping deteriorationii. Retroactive documentationiii. Retroactive incremental modularizationiv. Amputationv. Major surgery-restructuring

Stopping deterioration

i. Reviews must insure consistent changes

ii. New documents must be created and

reviewed

iii. Nipping the growth in the bud is by far preferable than retrenchment

Retroactive documentation

i. Often documentation is neglected• Either because of haste to correct

problems in software• Or to introduce new features into the

software

ii. Side effect of documentation is improvement of software because it reveals bugs, redundancy etc.

Retroactive incremental modularization

i. Each module must comprise of all the programs that are based on a particular design decision

ii. Confining knowledge of a design decision that is likely to change to one module

Amputation

i. Code that is modified so often needs to be cut off from the main code

ii. It can then be replaced by artificial stumps

iii. Amputation is controversial

Major surgery -restructuring

i. Restructure , analyze and get rid of redundant components

ii. Separate versions can be combined by using switches which reduces maintenance costs

Planning ahead

i. A new life style• Imposing standards

ii. Planning for change • Analyze the future changes• Designate a distinct department

iii. No document ? nothing done• Documentation done after shipping the

product is usually inaccurat

iv. Retirement savings plan• Make sure that funds and people are available at the right time

Barriers to progress The assumptions and attitudes

i. A 25 year crisis?• Quick and easy solution doesn’t work out, need

long term solutions

ii. “Our industry is different”• Very little cross communication between

industries• Intellectual isolation is inappropriate and costly

iii. Where are the professionals• The idea that anyone can code is not

professional iv. Talking to ourselves

• Researchers should broaden the audience beyond their colleagues

Conclusions for our profession

i. We cannot assume that the old stuff is known and didn’t work

ii. We cannot assume that the old stuff will work

iii.We cannot ignore the splinter software engineering groups

iv.Model products are a must for others to imitate

v. We need professional standards and identity

Patriot missile failure Failed to intercept an incoming Iraqi

scud missile An inaccurate calculation of the time

since boot due to computer arithmetic errors

Calculation was performed using 24 bit fixed point register

Bad time calculation had been improved only in some parts of the code

Software rejuvenation method: Switch the system on and off every 8 hours to regain clean internal states

Discussions..comments…