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Software Development Life Cycle Models

Software Development Life Cycle Models

 

I was asked to put together this high-level and traditional software life
cycle information as a favor for a friend of a friend, so I thought I might as
well share it with everybody.

The General Model

Software life cycle models describe phases of the software cycle and the
order in which those phases are executed.  There are tons of models, and many
companies adopt their own, but all have very similar patterns.  The general,
basic model is shown below:

General Life Cycle Model

Each phase produces deliverables required by the next phase in the life
cycle.  Requirements are translated into design.  Code is produced during
implementation that is driven by the design.  Testing verifies the deliverable
of the implementation phase against requirements.

Requirements

Business requirements are gathered in this phase.  This phase is the main
focus of the project managers and stake holders.  Meetings with managers,
stake holders and users are held in order to determine the requirements.  Who
is going to use the system?  How will they use the system?  What data should
be input into the system?  What data should be output by the system?  These
are general questions that get answered during a requirements gathering
phase.  This produces a nice big list of functionality that the system should
provide, which describes functions the system should perform, business logic
that processes data, what data is stored and used by the system, and how the
user interface should work.  The overall result is the system as a whole and
how it performs, not how it is actually going to do it.

Design

The software system design is produced from the results of the requirements
phase.  Architects have the ball in their court during this phase and this is
the phase in which their focus lies.  This is where the details on how the
system will work is produced.  Architecture, including hardware and software,
communication, software design (UML is produced here) are all part of the
deliverables of a design phase.

Implementation

Code is produced from the deliverables of the design phase during
implementation, and this is the longest phase of the software development life
cycle.  For a developer, this is the main focus of the life cycle because this
is where the code is produced.  Implementation my overlap with both the design
and testing phases.  Many tools exists (CASE tools) to actually automate the
production of code using information gathered and produced during the design
phase.

Testing

During testing, the implementation is tested against the requirements to
make sure that the product is actually solving the needs addressed and
gathered during the requirements phase.  Unit tests and system/acceptance
tests are done during this phase.  Unit tests act on a specific component of
the system, while system tests act on the system as a whole.

So in a nutshell, that is a very basic overview of the general software
development life cycle model.  Now lets delve into some of the traditional and
widely used variations.

 

Waterfall Model

This is the most common and classic of life cycle models, also referred to
as a linear-sequential life cycle model.  It is very simple to understand and
use.  In a waterfall model, each phase must be completed in its entirety
before the next phase can begin.  At the end of each phase, a review takes
place to determine if the project is on the right path and whether or not to
continue or discard the project.  Unlike what I mentioned in the general
model, phases do not overlap in a waterfall model.

Waterfall Life Cycle Model

Advantages

• Simple and easy to use.


• Easy to manage due to the rigidity of the model – each phase has
  specific deliverables and a review process.


• Phases are processed and completed one at a time.


• Works well for smaller projects where requirements are very well
  understood.

Disadvantages

• Adjusting scope during the life cycle can kill a project


• No working software is produced until late during the life cycle.


• High amounts of risk and uncertainty.


• Poor model for complex and object-oriented projects.


• Poor model for long and ongoing projects.


• Poor model where requirements are at a moderate to high risk of
  changing.

 

V-Shaped Model

Just like the waterfall model, the V-Shaped life cycle is a sequential path
of execution of processes.  Each phase must be completed before the next phase
begins.  Testing is emphasized in this model more so than the waterfall model
though.  The testing procedures are developed early in the life cycle before
any coding is done, during each of the phases preceding implementation.

Requirements begin the life cycle model just like the waterfall model. 
Before development is started, a system test plan is created.  The test plan
focuses on meeting the functionality specified in the requirements gathering.

The high-level design phase focuses on system architecture and design.  An
integration test plan is created in this phase as well in order to test the
pieces of the software systems ability to work together.

The low-level design phase is where the actual software components are
designed, and unit tests are created in this phase as well.

The implementation phase is, again, where all coding takes place.  Once
coding is complete, the path of execution continues up the right side of the V
where the test plans developed earlier are now put to use.

V-Shaped Life Cycle Model

Advantages

• Simple and easy to use.


• Each phase has specific deliverables.


• Higher chance of success over the waterfall model due to the development
  of test plans early on during the life cycle.


• Works well for small projects where requirements are easily understood.

Disadvantages

• Very rigid, like the waterfall model.


• Little flexibility and adjusting scope is difficult and expensive.


• Software is developed during the implementation phase, so no early
  prototypes of the software are produced.


• Model doesn’t provide a clear path for problems found during testing
  phases.

 

Incremental Model

The incremental model is an intuitive approach to the waterfall model. 
Multiple development cycles take place here, making the life cycle a
“multi-waterfall” cycle.  Cycles are divided up into smaller, more easily
managed iterations.  Each iteration passes through the requirements, design,
implementation and testing phases.

A working version of software is produced during the first iteration, so
you have working software early on during the software life cycle.  Subsequent
iterations build on the initial software produced during the first iteration.

Incremental Life Cycle Model

Advantages

• Generates working software quickly and early during the software life
  cycle.


• More flexible – less costly to change scope and requirements.


• Easier to test and debug during a smaller iteration.


• Easier to manage risk because risky pieces are identified and handled
  during its iteration.


• Each iteration is an easily managed milestone.

Disadvantages

• Each phase of an iteration is rigid and do not overlap each other.


• Problems may arise pertaining to system architecture because not all
  requirements are gathered up front for the entire software life cycle.

 

Spiral Model

The spiral model is similar to the incremental model, with more emphases
placed on risk analysis.  The spiral model has four phases: Planning, Risk
Analysis, Engineering and Evaluation.  A software project repeatedly passes
through these phases in iterations (called Spirals in this model).  The
baseline spiral, starting in the planning phase, requirements are gathered and
risk is assessed.  Each subsequent spirals builds on the baseline spiral.

Requirements are gathered during the planning phase.  In the risk analysis
phase, a process is undertaken to identify risk and alternate solutions.  A
prototype is produced at the end of the risk analysis phase.

Software is produced in the engineering phase, along with testing at the
end of the phase.  The evaluation phase allows the customer to evaluate the
output of the project to date before the project continues to the next spiral.

In the spiral model, the angular component represents progress, and the
radius of the spiral represents cost.

 

Spiral Life Cycle Model

Advantages

• High amount of risk analysis


• Good for large and mission-critical projects.


• Software is produced early in the software life cycle.

Disadvantages

• Can be a costly model to use.


• Risk analysis requires highly specific expertise.


• Project’s success is highly dependent on the risk analysis phase.


• Doesn’t work well for smaller projects.

 

And that’s it.  If you have any input, especially your views on advantages
and disadvantages of any particular model, feel free to leave them in the
comments and I can add them to my copy.

Software Development Life Cycle Models

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sections. Certain information included herein may contain statements that are
forward-looking, such as statements relating to plans for future expansion and
other business development activities. Such forward-looking information is
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