Chapter 11

Maintaining the System

System evolution Legacy systems Software rejuvenation

Software maintenance

Maintenance: any work done to change the system after it is in operation Software does not degrade or require

periodic maintenance like hardware However, software is continually evolving

Maintenance process can be difficult

Discussion question

If we build the system right in the first place, will it need to be maintained?

Maintenance is inevitable New requirements emerge when the

software is used The business environment changes Errors must be repaired New equipment must be accommodated The performance or reliability may have

to be improved

Characterizing systems

Lehman’s system types (1980) S-system: formally defined, derivable

from a specification. P-system: requirements based on

approximate solution to a problem, but real-world remains stable.

E-system: embedded in the real world and changes as the world does.

Maintenance effort Typical systems (1983)

Development time: 2 years Maintenance time: 5 to 6 years

Effort (1979) 39% of effort in development 61% of effort in maintenance

80-20 rule 20% of effort in development 80% of effort in maintenance

Software evolution vs. retirement

When is it time to let an aging system retire?

Maintenance versus retirement Is the cost of maintenance too high?

Is the system reliability unacceptable?

Can the system no longer adapt to further change, and within a reasonable amount of time?

Is system performance still beyond prescribed constraints?

Are system functions of limited usefulness?

Can other systems do the same job better, faster, or cheaper?

Is the cost of maintaining the hardware great enough to justify replacing it with cheaper, newer hardware?

Legacy system An application program which continues to be

used because of the cost of replacing or redesigning it.

The implication is that the system is large, monolithic and difficult to modify.

If legacy software only runs on antiquated hardware the cost of maintaining the hardware may eventually outweigh the cost of replacing both the software and hardware unless some form of emulation or backward compatibility allows the software to run on new hardware.

Types of maintenance Corrective: reactive modification of a software product

performed after delivery. Correcting problems

Adaptive: modifications to keep software usable in a changing environment (hardware, peripherals, etc. Adapting to new enviroments

Perfective: to better meet the needs and desires of customers. Adding new features, changing interfaces, etc. Redesign

Preventive: modification to detect and correct latent faults in the software before they become effective faults Adding additional internal checks

Multiple-choice A credit card company upgrades its system for handling

credit card payments, and this requires a slight change to the type of data that the e-commerce program needs to send to it.

What type of maintenance is required?A. CorrectiveB. AdaptiveC. PerfectiveD. Preventive

Multiple-choice The customer has stated that the e-commerce

program should be able to allow additional credit card companies should the business decide to make them available to shoppers in the future.

However, the maintenance team realizes that the way the system handles credit card payments will not scale up if more credit card options are made available in the future.

What type of maintenance is required?A. CorrectiveB. AdaptiveC. PerfectiveD. Preventive

Multiple-choice An additional product line is added for customers to the

e-commerce site.

What type of maintenance is required?A. CorrectiveB. AdaptiveC. PerfectiveD. Preventive

Multiple-choice When confirmation receipts are emailed to customers

following purchase, if the customer's name exceeds a certain length, then the email does not get sent.

Note: This situation does not occur very frequently (at most, once a week)

What type of maintenance is required?A. CorrectiveB. AdaptiveC. PerfectiveD. Preventive

Types of maintenance

Who Performs the Maintenance?

Separate maintenance team May be more objective May find it easier to distinguish how a system

should work from how it does work

Part of development team Will build the system in a way that makes

maintenance easier May feel over-confident, and ignore the

documentation to help maintenance effort

Maintenance is hard

Interact with developers, customers, and users

Understand the big picture 47% of effort devoted to understanding

the software

Good debugging skills Good testing skills

Maintenance Problems

Staff problems Limited understanding Morale

Technical problems Artifacts and paradigms Testing difficulties

Modeling Maintenance Effort

M = p + Kc-d

M : total maintenance effort p : productive (development) effort C : complexity d : degree of familiarity K : empirical constant

Measuring complexity is difficult

Cyclomatic number (McCabe, 1976) Metric for the structural complexity of

the source code

Cyclomatic Number

Linearly independent path = e - n + 2 e: edges, n : nodes

Software Rejuvenation Tries to increase the overall quality of an

existing system Re-documentation: static analysis adds more

information Restructuring: transform to improve code

structure Reverse engineering: recreate design and

specification information from the code Reengineering: reverse engineer and then

make changes to specification and design to complete the logical model; then generate new system from revised specification and design

Software Rejuvenation

Software rejuvenation is a proactive fault management technique aimed at cleaning up the system internal state to prevent the occurrence of more severe crash failures in the future. Reboot system