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Software Maintenance

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A consequence of the widespread utilisation of computer based technology over

the past few decades has been the emergence of vast, highly complex computer

systems whose content and structure are increasingly resistant to modification

and change. However fallible such legacy systems remain, many are

"mission-critical" whereby their failure may lead to the collapse of the

business or industry in which they serve. In such cases, it is ultimately not

possible to decommission the system in question. The present report

investigates the nature of such systems and examines why legacy systems cause

problems to Software Maintenance Managers? This report also provides a brief

overview as to how such problems can be minimised and controlled.

Keywords: Legacy systems, legacy system migration, mission critical systems,

re-engineering, software wrapping, software evolution.

1. Introduction

The literature describes legacy systems in terms of being an existing

software application that is predominately within the maintenance phase of its

lifecycle. Such systems are typically old and heavily modified from their

original designs by years of maintenance, usually by many different people

[Moor00]. Although legacy systems are technically obsolete, having been written

in assembly or early third generation languages such as COBAL Fortran and Coral,

they generally represent considerable investment, and maintain significant value

to their users [Benn95] [Brod95].

Legacy systems typically form the backbone of information flow within an

organisation, and as such, are essential for the function of its business.

Failure in these systems is likely to have serious consequences hence why legacy

software is often considered of a "mission critical nature" [Benn95] [Bisb99].

As can be expected, systems of this nature pose a number of problems to the

users, and to the Software Maintenance Manager responsible for the upkeep of the

system. Such problems range from the cost of maintenance to the utilisation of

obsolete skills and technologies. However, several solutions have been proposed

and documented in the literature in response to, and to minimise, these

problems. Generally, they are classified under four categories: maintenance,

re-development, wrapping and migration [Bisb99] [Lee97].

Therefore, the remainder of this report is structured as follows. The next

section addresses the problems posed by legacy systems. Section 3 describes,

with reference to the above 4 categories, methods and techniques used to

minimise these problems. The concluding section presents a summary of findings

and briefly discusses possible future research directions.

2. Legacy Problems

Obsolete or not, a recent study in the usage of legacy systems, estimated

that more than 100 billion lines of working legacy code exit within the

framework of modern business and industry [Coyl00]. With much of this code

found within systems of a "mission critical" nature, the very existence of such

code perpetrates considerable problems to the person responsible for system


2.1 Hardware Issues

These systems are likely to run on hardware that has now been superseded.

Not only are they likely to be large and slow to operate, they are liable to be

expensive to maintain. Such costs mainly stem from the obsolete nature of the

system. With a high demand and relative low supply of necessary parts and

qualified personnel, the actual cost of maintaining the hardware driving the

legacy system is certainly a problem to which the person in charge of

maintenance should be aware [Bisb99] [Somm01]. For example, various studies

have shown such maintenance consumes between 50 and 70 percent of a budget for a

typical organisation utilising legacy systems [Lien80] [Nose90].

2.2 Software Issues

The actual maintenance of the software is liable to be time consuming and

expensive. Again, there is an issue of skill shortages resulting in similar

problems as discussed in relation to hardware issues above. People tend not to

learn languages that are used in legacy systems because they are essentially

obsolete, therefore placing high demands on those individuals who are skilled in

legacy languages.

Another maintenance issue concerns the often poor state of the software

documentation which gives rise to a lack of understanding as to the internal



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