Software development lifecycle - Process Models

Process Models
There are several methodologies or models that can be used to guide the software development lifecycle. Some of these include:

1. Water fall model
2. V-Model
3. Spiral Model
4. Incremental model

1. Water fall model
This is the best-known and oldest process, where developers (roughly) follow these steps in order. They state requirements, analyze them, design a solution approach, architect a software framework for that solution, develop code, test (perhaps unit tests then system tests), deploy, and maintain. After each step is finished, the process proceeds to the next step, just as builders don't revise the foundation of a house after the framing has been erected. The process has no provision for correcting errors in early steps (for example, in the requirements), so the entire (expensive) engineering process may be executed to the end, resulting in unusable or unneeded software features, just as a house built on an incorrect foundation might be uninhabitable after it is handed over to the customer. The original description of the methodology did include iteration, but that part of the process is usually overlooked.
The weakness of the Waterfall Model is at hand:
It is very important to gather all possible requirements during the first phase of requirements collection and analysis. If not all requirements are obtained at once the subsequent phases will suffer from it. Reality is cruel. Usually only a part of the requirements is known at the beginning and a good deal will be gathered during the complete development time.
Iterations are only meant to happen within the same phase or at best from the start of the subsequent phase back to the previous phase. If the process is kept according to the school book this tends to shift the solution of problems into later phases which eventually results in a bad system design. Instead of solving the root causes the tendency is to patch problems with inadequate measures.
There may be a very big "Maintenance" phase at the end. The process only allows for a single run through the waterfall. Eventually this could be only a first sample phase which means that the further development is squeezed into the last never ending maintenance phase and virtually run without a proper process
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.

2. V-model
The V-model is the improved version of water-fall model. A further development of the waterfall model issued into the so called "V-Model". If you look at it closely the individual steps of the process are almost the same as in the waterfall model. Therefore I will not describe the individual steps again, because the description of the waterfall steps may substitute this. However, there is on big difference. Instead of going down the waterfall in a linear way the process steps are bent upwards at the coding phase, to form the typical V shape. The reason for this is that for each of the design phases it was found that there is a counterpart in the testing phases which correlate to each other.


The time in which the V-model evolved was also the time in which software testing techniques were defined and various kinds of testing were clearly separated from each other. This new emphasis on software testing (of course along with improvements and new techniques in requirements engineering and design) led to the evolution of the waterfall model into the V-model. The tests are derived directly from their design or requirements counterparts. This made it possible to verify each of the design steps individually due to this correlation.
Another idea evolved which was the traceability down the left side of the V. This means that the requirements have to be traced into the design of the system, thus verifying that they are implemented completely and correctly. Another feature can be observed when you compare the waterfall model to the V-model. The "Operation & Maintenance" phase was replaced in later versions of the V-model with the validation of requirements. This means that not only the correct implementation of requirements has to be checked but also if the requirements are correct. In case there is the need of an update of the requirements and subsequently the design and coding, etc. there are two options. Either this has to be treated like in the waterfall model in a never ending maintenance phase, or in going over to another V-cycle. The earlier versions of V-models used the first option. For later versions a series of subsequent V-cycles was defined.

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

3. The Spiral Model

The process begins at the center position. From there it moves clockwise in traversals. Each traversal of the spiral usually results in a deliverable. It is not clearly defined what this deliverable is. This changes from traversal to traversal. For example, the first traversals may result in a requirement specification. The second will result in a prototype, and the next one will result in another prototype or sample of a product, until the last traversal leads to a product which is suitable to be sold. Consequently the related activities and their documentation will also mature towards the outer traversals. E.g. a formal design and testing session would be placed into the last traversal.

These regions are:
a. The planning task - to define resources, responsibilities, milestones and schedules.
b. The goal determination task - to define the requirements and constraints for the product and define possible alternatives.
c. The risk analysis task - to assess both technical and management risks.
d. The engineering task - to design and implement one or more prototypes or samples of the application
The most outstanding distinction between the spiral model and other software models is the explicit risk evaluation task. Although risk management is part of the other processes as well, it does not have an own representation in the process model. For other models the risk assessment is a sub-task e.g. of the overall planning and management. Further there are no fixed phases for requirements specification, design or testing in the spiral model. Prototyping may be used to find and define requirements. This may then be followed by "normal" phases as they can be found in other process models to handle design and testing.

The advantages of the spiral model are that it reflects the development approach in many industries much better than the other process models do. It uses a stepwise approach which e.g. goes hand in hand with the habit of maintaining a number of hardware sample phases in cases where the product to be produced is not only software for a given environment, but also contains the development of hardware. This way the developers and the customer can understand and react much better to risks in the evolutionary process. By having an iterative process which reduces formalisms and omit-table activities in the earlier phases the use of resources is optimized. Further, any risks should be detected much earlier than in other process models and measures can be taken to handle them.
The disadvantages of the spiral model are that the risk assessment is rigidly anchored in the process. First of all it demands risk-assessment expertise to perform this task and secondly in some cases the risk assessment may not be necessary in this detail. For completely new products the risk assessment makes sense. But I dare to say that the risks for programming yet another book keeping package are well known and do not need a big assessment phase. Also if you think of the multitude of carry over projects in many industries i.e. applying an already developed product to the needs of a new customer by small changes, the risks are not a subject generating big headaches. Generally speaking the spiral model is not much esteemed and not much used; although it has many advantaged and could have even more if the risk assessment phases would be tailored down to the necessary amount.

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.

4. 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.

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