Object-oriented Software in Ada 95

2 Software design
This chapter looks at software production in the large. In particular it looks at problems that occur in the
development of large and not so large software systems. The notation used by UML (Unified Modelling
Language) is introduced as a mechanism for documenting and describing a solution to a problem that is to
be implemented on a computer system.
2.1 The software crisis
In the early days of computing, it was the hardware that was very expensive. The programs that ran on these
computers were by today’s standards incredibly small. In those distant times computers only had a very limited
amount of storage; both random access memory and disk storage.
Then it all changed. Advances in technology enabled computers to be built cheaper, with a far greater capacity
than previous machines. Software developers thought, “Great! We can build bigger and more comprehensive
programs”. Software projects were started with an increase in scope and great optimism.
Soon, with projects running over budget and not meeting their client’s expectations, the truth dawned: large
scale software construction is difficult. The early techniques that had been used in small scale software
construction did not scale up successfully for large scale software production.
This can be likened to using a bicycle to travel a short dis tance. Whilst this is adequate for the purpose, the use
of a bicycle is inappropriate if a long distance has to be travelled in a short space of time. You cannot just peddle
faster and faster.
2.2 A problem, the model and the solution
In implementing any solution to a problem, we must first understand the problem that is to be solved. Then, when
we understand the problem fully, a solution can be formulated.
There are many different ways of achieving an understanding of a problem and its solution. Usually, this
involves modelling the problem and its solution using either a standard notation or a notation invented by the
programmer. The advantage of using a standard notation is that other people may inspect and modify the
description of the problem and its proposed solution. For example, in building a house, an architect will draw up a
plan of the various components that are to be built. The client can view the plans and give their approval or
qualified approval subject to minor modifications. The builders can then use the plan when they erect the house.
Architect’s plan (model)
Finished house
Writing a computer program involves the same overall process. First, we need to understand the task that the
computer program will perform. Then we need to implement a solution using the model that we have created.
An easy pitfall at this point is to believe that the model used for the solution of a small problem can be scaled
up to solve a large problem. For example, to cross a small stream we can put a log over the stream or if athletic we
can even jump over the stream. This approach to crossing a stream however, will not scale up to crossing a large
river. Likewise to build a 100-storey tower block, an architect would not simply take the plans for a 2-storey
house and instruct the builders to build some extra floors.
© M A Smith - May not be reproduced without permission