Tuesday, July 1, 2008

Software Development Life Cycle (SDLC)

Various Software Life Cycle Models


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. Some of the models as follows.


  • General Model
  • Water fall model/ Linear Sequential/ Classic Life Cycle Model
  • V-Model
  • Rapid Application Development (RAD) model
  • Incremental Model
  • Spiral Model
  • Proto type model
  • Fourth Generation (4GT) Techniques

General Life Cycle 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

Water fall / Linear Sequential /Classic Life Cycle Model


The "waterfall model", documented in 1970 by Royce was the first publicly documented life cycle model. The model was developed to help with the increasing complexity of aerospace products.


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.


The least flexible and most obsolete of the life cycle models. Well suited to projects that has low risk in the areas of user interface and performance requirements, but high risk in budget and schedule predictability and control.

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/stable.

Disadvantages

    • It’s difficult to respond to changing customer requirements.
    • 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-orented projects.
    • Poor model for long run and ongoing projects.

V - 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 we mentioned in the general model, phases do not overlap in a waterfall model.
  • The least flexible and most obsolete of the life cycle models. Well suited to projects that has low risk in the areas of user interface and performance requirements, but high risk in budget and schedule predictability and control.

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/Iterative model


This model does not attempt to start with full specification of requirements. 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.


Key Points

  • Development and delivery is broken down into increments
  • Each increment delivers part of the required functionality
  • Requirements are prioritised and the highest priority requirements are included in early increments
  • Once the development of an increment is started, the requirements are frozen
  • Requirements for later increments can continue to evolve

Advantages

  • System functionality is available earlier and customer does not have to wait as long.
  • Early increments act as a prototype to help elicit requirements for later increments.
  • The highest priority functionalities tend to receive more testing.
  • 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.

Prototype model

In this model, a prototype (an early approximation of a final system or product) is built, tested, and then reworked as necessary until an acceptable prototype is finally achieved from which the complete system or product can now be developed.

Prototype paradigm begins with requirements gathering. Developer and customer meet and define the overall objectives for the software, identify whatever requirements are known, and outline areas where further definition is mandatory.

A quick design occurs which leads to the construction of prototype.

The prototype is evaluated by the customer/user and used to refine the requirements for the software to be developed.

Iteration occurs as the prototype is tuned to satisfy the user requirements, while at the same time enabling developer to better understand what needs to be done.

Spiral - model

  • This model of development combines the features of the prototyping model and the waterfall model. The spiral model is favored for large, expensive, and complicated projects.
  • 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 is gathered and risk is assessed. Each subsequent spiral 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.


Advantages

  • High amount of risk analysis.
  • Risks are explicitly assessed and resolved throughout the process
  • Focus on early error detection and design flaws.
  • 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.

Rapid Application Development (RAD) model

RAD model makes heavy use of reusable software components with an extremely short development cycle.

The RAD is a linear sequential software development process that emphasizes an extremely short development cycle. The RAD software model is a "high speed" adaptation of the linear sequential model in which rapid development is achieved by using a component-based construction approach. Used primarily for information systems applications, the RAD approach encompasses the following phases

  • Business modeling
  • Data modeling
  • Process modeling
  • Application generation
  • Testing

RAD process emphasizes reuse many of the program components have already been tested, which minimizes the testing and development time.


Fourth Generation (4GT) Techniques


Software tool is used to generate the source code automatically for a software system from a high level specification representation.

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