Software development on a broader level includes all the activities, between the manifestation of the idea for the software to the maintenance of the said software. Sometimes, it may also be referred to the actual task of writing the software code, and also maintaining it over a period of time. Over a period of time, there are different software development models, that have been used for the said purpose. The first of the software model to be used was the waterfall model. The other model, which is commonly used is the spiral model. Both these models are among the most popular models in the field of software development. The older of the two models is the waterfall model. There is often the waterfall model vs spiral model debate, which can be heard in the corridors, when a new software development process is undertaken. Understanding the difference between the two models will make it easier to decide, which is the right model to be used for software development. However, it is equally important to understand each of these models, which will help in better understanding of difference between both of them. The waterfall model is often also referred to as the linear and sequential model, for the flow of activities in this model are rather linear and sequential as the name suggests. In this model, the software development activities move to the next phase only after the activities in the current phase are over. However, like is the case with a waterfall, one cannot return to the previous stage. The phases of this model are:

• Requirement Gathering and Analysis Phase
• Design Phase
• Coding Phase
• System Integration Phase
• Testing and Debugging Phase
• Delivery Phase
• Maintenance Phase

The most important advantage of the waterfall model lies in the fact, that there is minimum planning overhead for the steps that are to follow, since the activities in each of the phase is carried out upfront, it is feasible that one does not have to plan for the entire phase. There is certain amount of discipline that is enforced as one has to only look into one phase of the process at any given point of time. In other models it is often difficult to nail the start and end of the said phase, which is not the case with the waterfall model. The most important of the advantage is that the project does not slip on its schedule. The number of resources working on the project does not keep on increasing with each passing day, as the planning for the same is done at the start of the phase itself. As there are advantages of the waterfall model, there are also some disadvantages of the said model. The first and the foremost disadvantage of this model is the inability of making changes to the system, once the system requirements have been frozen. It is not uncommon to see the requirements changing in the initial phases. In other words, one cannot go back to the previous stage. This is a disadvantage, due to which this model was relegated to the back. The same happens to be the case with the hardware and the software requirement for the software. If the software development process extends for a longer period of time, there are chances that the hardware and the software options chosen for the software may become obsolete at the end of the development process. The spiral model was introduced, due to the shortcomings in the waterfall and prototype models of software engineering. It is a combination of the said two models of software development. From the name of the model, it can be derived that the activities of software development are carried out like a spiral. To explain the model further, the entire software development process is broken down into small projects. The phases of the spiral model are as follows:

• Planning Phase
• Risk Analysis Phase
• Engineering Phase
• Coding and Implementation Phase
• Evaluation Phase

The disadvantage of the waterfall model is the advantage of the spiral model. It is a realistic model, which is often used in the development of large software. There is a systematic approach used in the spiral model, which is integrated into the iterative framework. This helps in ensuring there is no problems in the software. Since changes to the software can be made at any point of time in the software development process. In the spiral model, it is important to have a member in the team, who is an expert at risk assessment. Without correct risk assessment, there are chances that the software can utterly be a failure. The client may have to spend a lot of time with the development team to fix the issues that have cropped up in the software. This may at times become like a overhead for the both the client as well as the software development company. This also leads to the over involvement of the customer in the process of software development, which may cause the customer to ignore certain risks and cause harm to the project. While in the spiral model, the customer is made aware of all the happenings in the software development, in the waterfall model the customer is not involved. This often leads to situations, where the software is not developed according to the needs of the customer. In the spiral model, the customer is involved in the software development process from the word go. This helps in ensuring that the software meets the needs of the customer. In the waterfall model, when the development process shifts to the next stage, there is no going back. This often leads to roadblocks, especially during the coding phase. Many times it is seen that the design of the software looks feasible on paper, however, in the implementation phase it may be difficult to code for the same. However, in the spiral model, since there are different iterations, it is rather easier to change the design and make the software feasible. In the spiral model, one can revisit the different phases of software development, as many times as one wants, during the entire development process. This also helps in back tracking, reversing or revising the process. However, the same is not possible in the waterfall model, which allows no such scope. Often people have the waterfall model or spiral model confusion due to the fact, that the spiral model seems to be a complex model. It can be attributed to the fact that there are many iterations, which go into the model. At the same time, often there is no documentation involved in the spiral model, which makes it difficult to keep a track of the entire process. On the other hand, the waterfall model has sequential progression, along with clear documentation of the entire process. This ensures one has a better hold over the entire process. From the above discussion on spiral model vs waterfall model, it is clear that both the models have their own advantages and shortcomings. While one is stuck between the waterfall model vs spiral model debate, it is best to evaluate the software that is being developed and then decide the right approach. The size of the project and the urgency of the software will have to be taken into consideration for the same. At the same time, the resources available will have an important role to play in the software development process.