Department : MCA
Semester : IV
Subject : Principles of Software Engineering
Paper : 21
Faculty : Avinash Kumar
Syllabus covered in this blog:
Waterfall Model
Linear/Sequential Model (Waterfall Model)
Classical waterfall
model is the basic software development life cycle model. It is very simple but idealistic.
Earlier this model was very popular but nowadays it is not used. But it is very
important because all the other software development life cycle models are
based on the classical waterfall model.
Classical waterfall
model divides the life cycle into a set of phases. This model considers that
one phase can be started after completion of the previous phase. That is the
output of one phase will be the input to the next phase. Thus the development
process can be considered as a sequential flow in the waterfall. Here the
phases do not overlap with each other.
- Feasibility Study:
The main goal of this
phase is to determine whether it would be financially and technically feasible
to develop the software. The feasibility study involves understanding the
problem and then determines the various possible strategies to solve the problem.
These different identified solutions are analyzed based on their benefits and
drawbacks, the best solution is chosen and all the other phases are carried out
as per this solution strategy.
- Requirements analysis and specification:
The aim of the
requirement analysis and specification phase is to understand the exact
requirements of the customer and document them properly. This phase consists of
two different activities.
- Requirement gathering and analysis:
- Requirement specification:
These analyzed
requirements are documented in a software requirement specification (SRS)
document. SRS document serves as a contract between development team and
customers. Any future dispute between the customers and the developers can be
settled by examining the SRS document.
- Design:
The aim of the design phase is to transform the requirements
specified in the SRS document into a structure that is suitable for
implementation in some programming language.
- Coding and Unit testing:
In coding phase software design is
translated into source code using any suitable programming language. Thus each
designed module is coded. The aim of the unit testing phase is to check whether
each module is working properly or not.
- Integration and System testing:
Integration of different modules are
undertaken soon after they have been coded and unit tested. Integration of
various modules is carried out incrementally over a number of steps. During
each integration step, previously planned modules are added to the partially
integrated system and the resultant system is tested. Finally, after all the
modules have been successfully integrated and tested, the full working system
is obtained and system testing is carried out on this. System testing consists
of three different kinds of testing activities:
o Alpha testing:
Alpha testing is the
system testing performed by the development team.
o Beta testing:
Beta testing is the
system testing performed by a friendly set of customers
o Acceptance testing:
After the software has
been delivered, the customer performed the acceptance testing to determine
whether to accept the delivered software or to reject it.
- Maintenance:
Maintenance is the most important phase of a software life
cycle. The effort spent on maintenance is the 60% of the total effort spent to
develop a full software. There are basically three types of maintenance:
o Corrective Maintenance:
o
This
type of maintenance is carried out to correct errors that were not discovered
during the product development phase.
o Perfective Maintenance:
o This type of maintenance is carried
out to enhance the functionalities of the system based on the customer’s
request.
o Adaptive Maintenance:
Adaptive
maintenance is usually required for porting the software to work in a new
environment such as work on a new computer platform or with a new operating
system.
Advantages
of Classical Waterfall Model
Classical
waterfall model is an idealistic model for software development. It is very
simple, so it can be considered as the basis for other software development
life cycle models. Below are some of the major advantages of this SDLC model:
- This model is very simple and is easy to understand.
- Phases in this model are processed one at a time.
- Each stage in the model is clearly defined.
- This model has very clear and well understood milestones.
- Process, actions and results are very well documented.
- Reinforces good habits like define-before- design, design-before-code.
- This model works well for smaller projects and projects where requirements are well understood.
Drawbacks of Classical Waterfall Model
Classical
waterfall model suffers from various shortcomings, basically we can’t use it in
real projects, but we use other software development lifecycle models which are
based on the classical waterfall model. Below are some major drawbacks of this
model:
No
feedback path:
In
classical waterfall model evolution of a software from one phase to another
phase is like a waterfall. It assumes that no error is ever committed by
developers during any phases. Therefore, it does not incorporate any mechanism
for error correction.
Difficult
to accommodate change requests:
This
model assumes that all the customer requirements can be completely and
correctly defined at the beginning of the project, but actually customers’
requirements keep on changing with time. It is difficult to accommodate any
change requests after the requirements specification phase is complete.
No
overlapping of phases:
This
model recommends that new phase can start only after the completion of the
previous phase. But in real projects, this can’t be maintained. To increase the
efficiency and reduce the cost, phases may overlap.
Iterative Waterfall Model
In
a practical software development project, the classical
waterfall model is hard to use. So, Iterative waterfall model
can be thought of as incorporating the necessary changes to the classical
waterfall model to make it usable in practical software development projects.
It
is almost same as the classical waterfall model except some changes are made to
increase the efficiency of the software development. The iterative waterfall
model provides feedback paths from every phase to its preceding phases, which
is the main difference from the classical waterfall model.
When
errors are detected at some later phase, these feedback paths allow correcting
errors committed by programmers during some phase. The feedback paths allow the
phase to be reworked in which errors are committed and these changes are
reflected in the later phases.
There
is no feedback path to the stage – feasibility study, because once a project
has been taken, does not give up the project easily. It is good to detect
errors in the same phase in which they are committed. It reduces the effort and
time required to correct the errors.
Phase Containment of Errors:
The principle of detecting errors as
close to their points of commitment
as possible is known as Phase containment of errors.
Advantages of Iterative Waterfall
Model
Feedback Path:
In the classical waterfall model,
there are no feedback paths, so there is no mechanism for error correction. But
in iterative waterfall model feedback path from one phase to its preceding
phase allows correcting the errors that are committed and these changes are
reflected in the later phases.
Simple:
Iterative waterfall model is very
simple to understand and use. That’s why it is one of the most widely used
software development models.
Drawbacks of Iterative Waterfall
Model
Difficult to incorporate change
requests:
The major drawback of the iterative
waterfall model is that all the requirements must be clearly stated before
starting of the development phase. Customer may change requirements after some
time but the iterative waterfall model does not leave any scope to incorporate
change requests that are made after development phase starts.
Incremental delivery not supported:
In the iterative waterfall model,
the full software is completely developed and tested before delivery to the
customer. There is no scope for any intermediate delivery. So, customers have
to wait long for getting the software.
Overlapping of phases not supported:
Iterative waterfall model assumes
that one phase can start after completion of the previous phase, But in real
projects, phases may overlap to reduce the effort and time needed to complete
the project.
Risk handling not supported:
Projects may suffer from various
types of risks. But, Iterative waterfall model has no mechanism for risk
handling.
Limited customer interactions:
Customer interaction occurs at the
start of the project at the time of requirement gathering and at project
completion at the time of software delivery. These fewer interactions with the
customers may lead to many problems as the finally developed software may
differ from the customers’ actual requirements.
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