Proc. of 1994 ICSE By David Parnas

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Proc. of 1994 ICSE
By David Parnas

• Software Aging
• Proc of 1994 ICSE David Parnas
• Presented by
• Preethi Mahadev
• Date 03/07/2003

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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.

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What is software aging?

• Closely resembles the phenomenon of human aging
• Performance of the software system degrades with
  time
• Software Aging cannot be prevented
• Software Aging can be slowed down

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Purpose of this paper

• To explain how an abstract mathematical product
  like software aging can age
• To review approaches to deal with it
• Treating aged software
• Slowing down the deterioration

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Reaction of some
computer scientists

• Software aging doesnt make sense
• Software is a mathematical product and
  mathematics dont decay with time.

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Parnas argues

• True but not relevant
• Why?
• Old software has begun to cripple its once proud
  owners
• Many products are now seen as a burdensome legacy
  from the past
• Old softwares have become essential cogs in the
  machinery of our society

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Why is Software aging significant?

1. Growing economic importance of software
2. Software is the major capital of many high tech
   firms
3. Software aging is an impediment for further
   development of the systems

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Causes of software aging

• Lack of movement
• Failure to modify software to meet its changing
  needs
• Unless updated, software is considered as old and
  outdated
• 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

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Software aging Vs System slow down

• Causes of System slow down
• Failure to release allocated memory
• Files grow and require pruning
• Swap space decreases and performance degrades
• Analogy Kidney failure and Dialysis type
  treatment as solution
• Causes of Software aging
• Existing software no longer satisfies the owner
• Changes made to the software makes it harder to
  maintain
• System degrading in performance
• can be easily cured than aging

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The costs of software aging

1. Inability to keep up Lose customers because it
   is difficult to keep up with the market
2. Reduced performance Software Aging degrades
   performance of the system on the whole
3. Decrease in reliability Too many errors due to
   inconsistent changes made

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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

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Preventive medicine

• ? delay the decay
• Ways of slowing deterioration
• Design for success
• Documentation
• Second opinions-reviews

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Design for success is Design for change

• Principle to be applied Object Orientation
• We cannot predict actual changes, predictions
  will be about classes of changes
• Confine the probable section of code
• Estimate the probability of types of changes

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Design for success ..contd

• The barriers of design for change
• Impatient programmers and managers are more
  worried about meeting deadlines and starting a
  new one
• Programmers tend to confuse design principles
  with languages
• Code is rarely designed to be easily changed

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Documentation

• Important aspect of software engineering
• Inconsistent or inadequate documentation are
  developed in most cases
• Programmers and managers are driven by imminent
  deadlines
• For some, documentation is not interesting
• ? If not documented, we save a little and
• pay much more in future

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Second opinions-reviews

• Often Commercial programs dont have adequate
  review
• Many programmers have no professional training,
  so they neglect documentation and reviews
• Much software is produced as cottage industry
• Often produced under time pressure
• Programmers resent the idea of being reviewed
• ? Every design must be approved by someone whose
  responsibilities are for long term future of the
  product

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Aging is inevitable Why?

1. Changes may violate original assumptions
2. Documentation will never be perfect
3. There will be Issues that reviewers overlook
4. The idea of eliminating software aging is not
   practical

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Software geriatrics

• Prevention is better than cure .But..
• How to deal with already old software?
• Stopping deterioration
• Retroactive documentation
• Retroactive incremental modularization
• Amputation
• Major surgery-restructuring

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Stopping deterioration

• Reviews must insure consistent changes
• New documents must be created and reviewed
• Nipping the growth in the bud is by far
  preferable than retrenchment

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Retroactive documentation

• Often documentation is neglected
• Either because of haste to correct problems in
  software
• Or to introduce new features into the
• software
• Side effect of documentation is improvement of
  software because it reveals bugs,
  redundancy etc.

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Retroactive incremental modularization

• Each module must comprise of all the programs
  that are based on a particular design decision
• Confining knowledge of a design decision that is
  likely to change to one module

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Amputation

• Code that is modified so often needs to be cut
  off from the main code
• It can then be replaced by artificial stumps
• Amputation is controversial

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Major surgery -restructuring

• Restructure , analyze and get rid of redundant
  components
• Separate versions can be combined by using
  switches which reduces maintenance costs

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• Planning ahead
• A new life style
• Imposing standards
• Planning for change
• Analyze the future changes
• Designate a distinct department
• No document ? nothing done
• Documentation done after shipping the
• product is usually inaccurat
• Retirement savings plan
• Make sure that funds and people are
• available at the right time

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Barriers to progress

• The assumptions and attitudes
• A 25 year crisis?
• Quick and easy solution doesnt work out, need
  
• long term solutions
• Our industry is different
• Very little cross communication between
  industries
• Intellectual isolation is inappropriate and
  costly
• Where are the professionals
• The idea that anyone can code is not professional
  
• Talking to ourselves
• Researchers should broaden the audience
• beyond their colleagues

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Conclusions for our profession

1. We cannot assume that the old stuff is known and
   didnt work
2. We cannot assume that the old stuff will work
3. We cannot ignore the splinter software
   engineering groups
4. Model products are a must for others to imitate
5. We need professional standards and identity

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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

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Discussions..comments
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