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The SPAMEX system proposed by SCABB is outlined in the attached
letter. I hope to suggest a suitable software process model for the
development of the SPAMEX system in the following document.
2. The 'Waterfall' Model
The waterfall model consists of several stages of the development
life-cycle, each of which are completed in turn.
The first stage in applying this model to the development of the
SPAMEX system would be to document the system concept and identify the
system requirements. After analysing these requirements, one would
break the system into pieces, for example; TIP user interface,
customer database etc. Each of these components (or subsystems) now
require detailed design before the coding can take place. After each
of the components has been tested and debugged individually, they can
be integrated to form part of the whole SPAMEX system. The system as a
whole can now be tested and deployed although requiring ongoing
The waterfall model was the first of its kind and is still widely
used. It allows documented evidence of progress as each stage must be
approved and 'signed off' before the next stage is undertaken. This
should appeal to SCABB since they have access to these documents and
can track the progress of the development of their software. It would
also benefit the project manager, who would be able to ensure
consistency in the quality of the software and manage accordingly his
investments in time and money.
The model also allows the various stages of the development to be
overlapped in accordance with the wishes of SCABB. This is
particularly useful in this case as the current brief presented by
SCABB is not to the detail required by the developer. Further meetings
between both parties would be essential and ongoing changes in
requirements will be inevitable. However, such iterations are not
possible without significant investments in time and money from both
the developer and SCABB.
As we can see, one of the main characteristics of the waterfall model
is that commitments be made for each stage early on and each one must
be completed and 'signed off' before the next is undertaken. Many
problems may arise from this when applied to the SPAMEX system. For
example, instability and other coding problems may not be discovered
until the testing of the whole system. In such cases re-design may be
required, which is very problematic because from the very beginning,
this model assumes feasibility before implementation.
The waterfall model works well when requirements are stable and well
defined, the present SPAMEX brief is somewhat vague and specific
details may only be attained through extensive client-developer
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requirements; these are essential when applying the waterfall model in
order to avoid the evolution of the subsystems, which may result in an
unstable system development.
The main foreseeable disadvantage to SCABB will be the fact that the
waterfall model is a document-driven model. It is unlikely that
employees at a managerial level at SCABB will be able to understand
the documentation thoroughly; they will not have access to a working
model until the entire system has been coded. If the product doesn't
meet their needs at this late stage, there is little that can be done
without further expenditure.
3. Evolutionary Development
Developing the SPAMEX system in an evolutionary style can take one of
two paths; exploratory development or throw-away prototyping.
Exploratory development is particularly suitable for requirements
which are well defined by the client and well understood by the
developer. This allows the developer to evolve a system from the
initial specification. However, this may not be suitable for the
SPAMEX system since the requirements proposed at present are not
defined well enough for this method to take place.
Throw-away prototyping however is ideal for the development of the
SPAMEX system since it relies on ongoing client-developer interaction
to produce refined software that is developed with a client-focused
orientation. For the development of SCABB, this method allows for the
lack of specific detail in the requirements and uses this brief as a
starting point to develop the system through extensive communication
between the client and the developer.
Evolutionary development takes an outline description such as
described in the initial letter from SCABB. Using this, the developer
can construct a specification, develop the system and validate it
before presenting this initial prototype to SCABB. Following the
feedback given by SCABB, the above stages may be repeated taking into
account the client's new requirements and thus an intermediate
prototype is available to SCABB for trial. These stages may be
repeated any number of times until a final version of the system is
The speed at which such development takes place is reliant upon the
number of prototypes which are created and the speed at which each one
of them is developed. This requires special skills in rapid
prototyping which are expensive and will add to the overall cost of
the system as well as the time which it takes to develop. Furthermore,
if contracts are made to outside developers with skills in rapid
prototyping, their methods may not be compatible with in-house
When applying such a model, there is often a lack of process
visibility since it is hard for either party to foresee the end
result. However, evolutionary development is very useful for the rapid
development of subsystems, for example; the TIP or TAC user interface.
This would allow SCABB to trial the front-end of the system and offer
feedback to the developer without concerning themselves with the inner
workings of the system.
It is true that design issues are cheaper and easier to resolve
through experimentation rather than analysis; this is why evolutionary
development is often favoured as it allows a structured, disciplined
avenue for experimentation.
3. Re-use Orientated Development
Re-use orientated development is based on the systematic re-use of
existing components. Applying this model to the SPAMEX system would
involve integrating existing components into the system rather than
creating them from scratch.
The process starts from the initial requirements specification,
followed by the analysis of each component. In order to accommodate
existing components, changes to the requirements must be made before
the system is designed with re-used components. The SPAMEX system
would now be ready for development, integration and validation.
Using the re-use orientated model has significant advantages, namely
the speed at which the system is delivered. This would benefit SCABB
in particular since the money invested in man-hours would be reduced.
In concern to the developer, the reduction of software developed
in-house would be of primary importance.
In principle, this model should produce more reliable systems since
existing components have undergone extensive testing. However, the use
of existing components means that compromises in the requirements must
be made for the SPAMEX system. This is a major drawback for systems
such as SPAMEX, which ideally require bespoke components based on
business algorithms and specific scenarios. Such precisely tailored
components are not often available when applying re-use orientated
The SPAMEX system will require ongoing evolution due to the lack of
detail in the brief, using re-use orientated development will give the
developer less control over the system's evolution, leaving SCABB with
a product that may not fit their required needs.
4. Hybrid Process Models
Hybrid process models combine elements of the waterfall and
evolutionary models. The models which I have studied are the
incremental model and the spiral model.
4.1. Incremental Development
Applying the incremental model to the development of SPAMEX would
begin by defining the requirements to greater detail than at present
and assigning them to increments. Using these increments, the
developer is now able to design the architecture of the system and
develop, validate and integrate each increment though a series of
iterations until the final system is produced.
This software process model gives better support for process iteration
and allows easier integration of sub-systems. This would greatly
benefit the SPAMEX system as its success depends on the stable
relationship between the TIP, TAC and PAT sub-systems.
Rework in the software construction process is greatly reduced when
using the incremental model due to the identification and resolution
of coding problems early in the development life-cycle. Using the
incremental model means that the project is less likely to fail and
delivery is possible part-by-part. This however, is not necessary for
the development of SPAMEX. A major draw back of the incremental model
is that the increments must be relatively small; this makes it
impractical for larger systems such SPAMEX. Furthermore, when using
the incremental model, mapping requirements to increments is not
always easy, especially when the requirements are not clearly defined,
as in the SPAMEX brief.
4.2. Spiral Development
The spiral model is another hybrid process model that is often
represented visually as a spiral; each loop representing an iteration.
If applied to the SPAMEX system, the following activities would be
repeated until a final system is reached; objective setting, risk
assessment and reduction, development and validation and planning.
The spiral model specifically takes risk into consideration for each
iteration, this is a benefit not only to SPAMEX, but to all software
systems as they all carry a certain amount of risk during development.
The spiral model is reflection of real-world practices rather than a
theoretical model. This however can also be a disadvantage since it is
relatively complex and difficult to follow strictly. It is only really
suitable for large systems, not necessarily SPAMEX since we have
already sub-divided it into TIP, TAC and PAT. An important issue that
developers may be concerned with is the need for expertise in risk
evaluation and reduction which is going to significantly increase the
cost of development.
I have chosen to develop the SPAMEX system using evolutionary
development and throw-away prototyping.
At present, SCABB have proposed only the outline for SPAMEX, to apply
any other process model to a situation such as this would leave the
client with unwanted results. Using throw-away prototyping however
gives SCABB a chance to get a better understanding of the system they
require and allows the developer to produce a much more refined
system. This is done through a series of meetings between client and
developer where SCABB would evaluate a rapidly developed system
prototype and give feedback to the developer. The developer can use
this new information to improve the system and present a more suitable
prototype to SCABB at their next meeting. The result would be a highly
refined system that has been developed with a customer-orientated