Process_Models

1 ) WATERFALL MODEL :

  The model involves a linear sequence of phases. The output of one phase is the input to the next. An erroneous output in one phase results in a a faulty output for the subsequent phases. 

  It has phases like : Communication , Planning , Modeling , Construction , Deployment .

  Communication phase : It involves project initiation, requirements gathering. 
  		Project initiation involves tasks like identification of business needs , objectives , Defining major features and functions.
  		Requirements Gathering consists of identifying content requirement and functional requirement.
 
 Planning : It has to do with estimating, scheduling and tracking. 
 	  Estimation determines how much money,time, effort and resources will be reuired to build a product. Feasibility study enables us to do such estimation. Aspects such as Technical Feasibility , Economical and Operational feasibility are used to evaluate the cost and benefits of the requested system.
 	  Scheduling : A product delivery schedule along with intermediate stage completion schedules is decided. 
 	  Tracking : Tracking is done to establish a relation between work completed according to the established schedule and make necessary arrangements.

 Modelling : Modeling stage involves establishment of data and program structure. The goal is to translate the design into operational software. 
 	   System Analysis : It involves the way things are to be done i.e  definition of the system. Dividing it into smaller and managable parts. Understanding the function and interrelationship of various subsystems.
	   System Design : It defines the way how a system will meet the requirements which are identified during analysis.
 Construction : In this phase actual development of software by coding and testing is carried out. 
 Deployment : It involves delivery, support and feedback. 

2) Rapid Application Development Model : The model gives a quick approach for software development. This model emphasizes a short development cycle . It is a high speed adaptation of the waterfall model. It achieves rapid application development by component based construction.   It puts importance to planning but more on development.  In contrast to the waterfall model, which emphasizes rigorous specification and planning, RAD approaches emphasize the necessity of adjusting requirements in reaction to knowledge gained as the project progresses.This causes RAD to use prototypes in addition to or even sometimes in place of design specifications.

Advantages : As users interact with evolving prototypes	, the functionality achieved can be higher than Waterfall model.
  Early identification of key risk factors and adjusting to them in the early part of the process.Projects get completed in time and within budget.

Disadvantages : The risk factor increases as there may be a need to consider a new approach. The rapid application of the product may sometimes result in a poor design. 

3) Incremental Model : It is used to produce software in increments. There are a series of releases called increments which add more functionality to the software. It adds new features in the course of time . Each new update produces a new release of deliverable software product. With each increment provided to the user, it adds to the evaluation by the user and thereby the refinements in the software. The incremental model applies the waterfall model incrementally.The incremental philosophy is also used in the agile process model.

Advantages :
    After each iteration, regression testing should be conducted. During this testing, faulty elements of the software can be quickly identified because few changes are made within any single iteration.
    It is generally easier to test and debug than other methods of software development because relatively smaller changes are made during each iteration. This allows for more targeted and rigorous testing of each element within the overall product.
    Customer can respond to features and review the product for any needful changes.
    Initial product delivery is faster and costs lower.

Disadvantages :

    Resulting cost may exceed the cost of the organization.
    As additional functionality is added to the product, problems may arise related to system architecture which were not evident in earlier prototypes.

4) Evolutionary Process models : A software can be viewed as evolving with time. They are built in a manner that enables software engineers to develop increasingly more complex versions of the software. A software engineer needs a process model that has been explicitly designed to accomodate a product that evolves over time. Evolutionary models are iterative. This provides for production of an increasingly more complete version of software.

Advantages : It provides flexibility in modifying the requirements.
	   The prototypes are very visual , hence there are no ambiguities.
Disadvantages : It is hard to trace the process due to the ad-hoc nature .
	      Systems are often poorly structured.
	      Not cost effective to produce documents.
5) Spiral model : It combines the iterative nature of the Prototyping model with the controlled and systematic aspects of the Waterfall model.
   The model represented by spiral contains four stages and each stage is represented by one quadrant of the Cartesian diagram . Each loop in the spiral represents a phase of the software. Each loop is split into four sectors : Object setting (set specific object for that phase), Risk assesment and reduction, Development and validation(select a development model based on the risk levels ), Planning (decide if a next loop is required).

  Advantages : Explicit consideration of risks. More detailed processes for each development phase.
  Disadvantages : Cost is high. Sometime difficult to implement or too time consuming.

6)Component based development model :
  Commercial software components are used to build a software. It gives a targetted functionality with well defined interfaces.
 It has many characteristics of spiral model. Regardless of the technology used , it must research and evaluate available component based products for current application.
Components are integrated into the architecture. 
Advantages : This model leads to software reuse.
Disadvantages : Component integration issues are to be dealt. The software has to be designed to accomodate components.
Comprehensive testing is to be done to ensure proper functionality.

7) Formal methods model : 
Comprises set of activities that leads to formal mathematical specification of computer software.
Enables a software engineer to specify, develop and verify a computer based system by applying
mathematical notation.
Advantages : Problems of ambiguity, incompleteness and inconsistency can be managed by this method.
Provides a base for verification therefore enable a software engineer to discover and correct undetected errors also.
Disadvantages : It is difficult to use the models as a communication mechanism for technically unsophisticated customers.
Extensive training is required for applying this method as few developers have the necessary background to work with this method

8) Unified Process Model : 
Comprises best features and characteristics of conventional software process models.
Emphasize importance of customer communication and streamlined methods for describing the customers view of
system.
Phases of Unified Process
Inception : Involves customer communication and planning activities.
Elaboration : Encompasses the customer communication and modeling activities of the generic process model. Architectural representation using, 
Use Case Model, Analysis Model, Design Model, Implementation Model and Deployment Model.
Construction : Develops or acquires the software components that will make each use case operational for
end users.
Transition : Software is given to end user for beta testing and user feedback reports both defects and necessary
changes.
Production : Coincides with the deployment activity of process. The on going use of software is monitored, support for the operating environment is provided, and
defect reports and requests for changes are submitted and evaluated.

9) Aspect oriented software development model : 
Complex software are being implemented as set of localized features, functions and information content referred as components.
But it becomes crosscutting concerns when it flows across multiple systems.
Aspectual requirements define these crosscutting concerns that have impact across the software architecture.
The model provides a process and methodological approach for defining, specifying, designing and constructing aspects.

10) Agile process model :
Agile model believes that every project needs to be handled differently and the existing methods need to be tailored to best suit the project requirements.
In agile the tasks are divided to time boxes (small time frames) to deliver specific features for a release.
Iterative approach is taken and working software build is delivered after each iteration. 
Each build is incremental in terms of features .The final build holds all the features required by the customer.
Advantages : 
Is a very realistic approach to software development.
Promotes teamwork and cross training.
Disadvantages : 
Not suitable for handling complex dependencies.
More risk of sustainability, maintainability and extensibility.