Software Development Process Models
INTRODUCTION
There is various software development approaches defined and designed which are used/employed during development process of software, these approaches are alsoreferred as "Software Development Process Models".
Each process model follows a particular life cycle in order to ensure success in process of software development.
A software development process, also known as a software development life cycle (SDLC), is a structure imposed on the development of a software product. Similar terms include software life cycle and software process. It is often considered a subset of systems development life cycle. There are several models for such processes, each describing approaches to a variety of tasks or activities that take place during the process. Some people consider a lifecycle model a more general term and a software development process a more specific term. For example, there are many specific software development processes that 'fit' the spiral lifecycle model. ISO 12207 is an ISO standard for software lifecycle processes. It aims to be the standard that defines all the tasks required for developing and maintaining software.
Software development models
Several models exist to streamline the development process. Each one has its pros and cons, and it's up to the development team to adopt the most appropriate one for the project. Sometimes a combination of the models may be more suitable.
TYPES OF SDLC MODELS
 Waterfall model
 V Model
 Spiral model
 Evolutionary Development model
 Model driven development
 User experience
 Top-down and bottom-up design
 Chaos model
 Evolutionary prototyping
 Prototyping
 ICONIX Process (UML-based object modeling with use cases)
 Unified Process
 V-model
 Extreme Programming
 Software Development Rhythms
 Specification and Description Language
 Incremental funding methodology
 Verification and Validation (software)
 Service-Oriented Modeling Framework
WATERFALL MODEL
Waterfall approach was first Process Model to be introduced and followed widely in Software Engineering to ensure success of the project. In "The Waterfall" approach,the whole process of software development is divided into separate process phases.
The phases in Waterfall model are: Requirement Specifications phase, Software Design, Implementation and Testing & Maintenance. All these phases are cascaded to each other so that second phase is started as and when defined set of goals are achieved for first phase and it is signed off, so the name "Waterfall Model". All the methods and processes undertaken in Waterfall Model are more visible.
History of the Waterfall Model
In 1970 Royce proposed what is now popularly referred to as the waterfall model as an initial concept, a model which he argued was flawed (Royce 1970). His paper then explored how the initial model could be developed into an iterative model, with feedback from each phase influencing previous phases, similar to many methods used widely and highly regarded by many today.
Despite Royce's intentions for the waterfall model to be modified into an iterative model, use of the "waterfall model" as a purely sequential process is still popular, and, for some, the phrase "waterfall model" has since come to refer to any approach to software creation which is seen as inflexible and non-iterative.
The stages of "The Waterfall Model" are:
Requirement Analysis & Definition:
All possible requirements of the system to be developed are captured in this phase. Requirements are set of functionalities and constraints that the end-user (who will be using the system) expects from the system. The requirements are gathered from the end-user by consultation, these requirements are analyzed for their validity and the possibility of incorporating the requirements in the system to be development is also studied. Finally, a Requirement Specification document is created which serves the purpose of guideline for the next phase of the model.
System & Software Design:
Before a starting for actual coding, it is highly important to understand what we are going to create and what it should look like? The requirement specifications from first phase are studied in this phase and system design is prepared. System Design helps in specifying hardware and system requirements and also helps in defining overall system architecture. The system design specifications serve as input for the next phase of the model.
Implementation & Unit Testing:
On receiving system design documents, the works divided in modules/units and actual coding is started. The system is first developed in small programs called units, which are integrated in the next phase. Each unit is developed and tested for its functionality; this is referred to as Unit Testing. Unit testing mainly verifies if the modules/units meet their specifications.
Integration & System Testing:
As specified above, the system is first divided inunits which are developed and tested for their functionalities. These units are integrated into a complete system during Integration phase and tested to check if all modules/units coordinate between each other and the system as a whole behaves as per the specifications. After successfully testing the software, it is delivered to the customer.
Operations & Maintenance:
This phase of "The Waterfall Model" is virtually never ending phase (Very long). Generally, problems with the system developed (which are not found during the development life cycle) come up after its practical use starts, so the issues related to the system are solved after deployment of the system. Not all the problems come in picture directly but they arise time to time and needs to be solved; hence this process is referred as Maintenance.