SDLC Models

Software development models or methodologies

1)       Waterfall model

2)       V model

3)       Incremental model

4)       RAD model

5)       Agile model

6)       Iterative model

7)       Spiral model

The Waterfall Model was first Process Model to be introduced. It is 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 fully 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. In waterfall model phases do not overlap.

Advantages of waterfall model:

·         Simple and easy to understand and 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 of waterfall model:

·         Once an application is in the testing stage, it is very difficult to go back and change something that was not well-thought out in the concept stage.

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

·         High amounts of risk and uncertainty.

·         Not a good model for complex and object-oriented projects.

·         Poor model for long and ongoing projects.

·         Not suitable for the projects where requirements are at a moderate to high risk of changing.

When to use the waterfall model:

·         Requirements are very well known, clear and fixed.

·         Product definition is stable.

·         Technology is understood.

·         There are no ambiguous requirements

·         Ample resources with required expertise are available freely

·         The project is short.

V-model means Verification and Validation 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 of the product is planned in parallel with a corresponding phase of development.

The various phases of the V-model are as follows:

Requirements like BRS and SRS begin the life cycle model just like the waterfall model. But, in this 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 (HLD) phase focuses on system architecture and design. It provide overview of solution, platform, system, product and service/process. 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 (LLD) phase is where the actual software components are designed. It defines the actual logic for each and every component of the system. Class diagram with all the methods and relation between classes comes under LLD. Component 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.

Coding: This is at the bottom of the V-Shape model. Module design is converted into code by developers.

Advantages of V-model:

·         Simple and easy to use.

·         Testing activities like planning, test designing happens well before coding. This saves a lot of time. Hence higher chance of success over the waterfall model.

·         Proactive defect tracking – that is defects are found at early stage.

·         Avoids the downward flow of the defects.

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

Disadvantages of V-model:

·         Very rigid and least flexible.

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

·         If any changes happen in midway, then the test documents along with requirement documents has to be updated.

When to use the V-model:

·         The V-shaped model should be used for small to medium sized projects where requirements are clearly defined and fixed.

·         The V-Shaped model should be chosen when ample technical resources are available with needed technical expertise.

High confidence of customer is required for choosing the V-Shaped model approach. Since, no prototypes are produced, there is a very high risk involved in meeting customer expectations.

In incremental model the whole requirement is divided into various builds. Multiple development cycles take place here, making the life cycle a “multi-waterfall” cycle.  Cycles are divided up into smaller, more easily managed modules.  Each module passes through the requirements, design, implementation and testing phases. A working version of software is produced during the first module, so you have working software early on during the software life cycle. Each subsequent release of the module adds function to the previous release. The process continues till the complete system is achieved.

Advantages of Incremental model:

·         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.

·         Customer can respond to each built.

·         Lowers initial delivery cost.

·         Easier to manage risk because risky pieces are identified and handled during it’d iteration.

Disadvantages of Incremental model:

·         Needs good planning and design.

·         Needs a clear and complete definition of the whole system before it can be broken down and built incrementally.

·         Total cost is higher than waterfall.

When to use the Incremental model:

·         Requirements of the complete system are clearly defined and understood.

·         Major requirements must be defined; however, some details can evolve with time.

·         There is a need to get a product to the market early.

·         A new technology is being used

·         Resources with needed skill set are not available

·         There are some high risk features and goals.

RAD model is Rapid Application Development model. It is a type of incremental model. In RAD model the components or functions are developed in parallel as if they were mini projects. The developments are time boxed, delivered and then assembled into a working prototype.  This can quickly give the customer something to see and use and to provide feedback regarding the delivery and their requirements.

The phases in the rapid application development (RAD) model are:

Business modeling: The information flow is identified between various business functions.

Data modeling: Information gathered from business modeling is used to define data objects that are needed for the business.

Process modeling: Data objects defined in data modeling are converted to achieve the business information flow to achieve some specific business objective. Description are identified and created for CRUD of data objects.

Application generation: Automated tools are used to convert process models into code and the actual system.

Testing and turnover: Test new components and all the interfaces.

Advantages of the RAD model:

·         Reduced development time.

·         Increases reusability of components

·         Quick initial reviews occur

·         Encourages customer feedback

·         Integration from very beginning solves a lot of integration issues.

Disadvantages of RAD model:

·         Depends on strong team and individual performances for identifying business requirements.

·         Only system that can be modularized can be built using RAD

·         Requires highly skilled developers/designers.

·         High dependency on modeling skills

·         Inapplicable to cheaper projects as cost of modeling and automated code generation is very high.

When to use RAD model:

·         RAD should be used when there is a need to create a system that can be modularized in 2-3 months of time.

·         It should be used if there’s high availability of designers for modeling and the budget is high enough to afford their cost along with the cost of automated code generating tools.

·         RAD SDLC model should be chosen only if resources with high business knowledge are available and there is a need to produce the system in a short span of time (2-3 months).

Agile development model is also a type of Incremental model. Software is developed in incremental, rapid cycles. This results in small incremental releases with each release building on previous functionality. Each release is thoroughly tested to ensure software quality is maintained. It is used for time critical applications.  Extreme Programming (XP) is currently one of the most well known agile development life cycle model.

Advantages of Agile model:

·         Customer satisfaction by rapid, continuous delivery of useful software.

·         People and interactions are emphasized rather than process and tools. Customers, developers and testers constantly interact with each other.

·         Working software is delivered frequently (weeks rather than months).

·         Face-to-face conversation is the best form of communication.

·         Close, daily cooperation between business people and developers.

·         Continuous attention to technical excellence and good design.

·         Regular adaptation to changing circumstances.

·         Even late changes in requirements are welcomed

Disadvantages of Agile model:

·         In case of some software deliverables, especially the large ones, it is difficult to assess the effort required at the beginning of the software development life cycle.

·         There is lack of emphasis on necessary designing and documentation.

·         The project can easily get taken off track if the customer representative is not clear what final outcome that they want.

·         Only senior programmers are capable of taking the kind of decisions required during the development process. Hence it has no place for newbie programmers, unless combined with experienced resources.

When to use Agile model:

·         When new changes are needed to be implemented. The freedom agile gives to change is very important. New changes can be implemented at very little cost because of the frequency of new increments that are produced.

·         To implement a new feature the developers need to lose only the work of a few days, or even only hours, to roll back and implement it.

·         Unlike the waterfall model in agile model very limited planning is required to get started with the project. Agile assumes that the end users’ needs are ever changing in a dynamic business and IT world. Changes can be discussed and features can be newly effected or removed based on feedback. This effectively gives the customer the finished system they want or need.

·         Both system developers and stakeholders alike, find they also get more freedom of time and options than if the software was developed in a more rigid sequential way. Having options gives them the ability to leave important decisions until more or better data or even entire hosting programs are available; meaning the project can continue to move forward without fear of reaching a sudden standstill.

An Iterative life cycle model does not attempt to start with a full specification of requirements. Instead, development begins by specifying and implementing just part of the software, which can then be reviewed in order to identify further requirements. This process is then repeated, producing a new version of the software for each cycle of the model.

Advantages of Iterative model:

·         In iterative model we can only create a high-level design of the application before we actually begin to build the product and define the design solution for the entire product. Later on we can design and built a skeleton version of that, and then evolved the design based on what had been built.

·         In iterative model we are building and improving the product step by step. Hence we can track the defects at early stages. This avoids the downward flow of the defects.

·         In iterative model we can get the reliable user feedback. When presenting sketches and blueprints of the product to users for their feedback, we are effectively asking them to imagine how the product will work.

·         In iterative model less time is spent on documenting and more time is given for designing.

Disadvantages of Iterative model:

·         Each phase of an iteration is rigid with no overlaps

·         Costly system architecture or design issues may arise because not all requirements are gathered up front for the entire lifecycle

When to use iterative model:

·         Requirements of the complete system are clearly defined and understood.

·         When the project is big.

·         Major requirements must be defined; however, some details can evolve with time.

The spiral model is similar to the incremental model, with more emphasis 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 spirals, 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.

Advantages of Spiral model:

·         High amount of risk analysis hence, avoidance of Risk is enhanced.

·         Good for large and mission-critical projects.

·         Strong approval and documentation control.

·         Additional Functionality can be added at a later date.

·         Software is produced early in the software life cycle.

Disadvantages of Spiral model:

·         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.

When to use Spiral model:

·         When costs and risk evaluation is important

·         For medium to high-risk projects

·         Long-term project commitment unwise because of potential changes to economic priorities

·         Users are unsure of their needs

·         Requirements are complex

·         New product line

·         Significant changes are expected (research and exploration)