Waterfall Software Development Life Cycle Model

The simplest software development life cycle model is the waterfall model, which states that the phases are organized in a linear order. A project begins with feasibility analysis. On the successful demonstration of the feasibility analysis, the requirements analysis and project planning begins.
The design starts after the requirements analysis is done. And coding begins after the design is done. Once the programming is completed, the code is integrated and testing is done. On succeeful completion of testing, the system is installed. After this the regular operation and maintenance of the system takes place. The following figure demonstrates the steps involved in waterfall life cycle model.

The Waterfall Software Life Cycle Model
With the waterfall model, the activities performed in a software development project are requirements analysis, project planning, system design, detailed design, coding and unit testing, system integration and testing. Linear ordering of activities has some important consequences. First, to clearly identify the end of a phase and beginning of the others. Some certification mechanism has to be employed at the end of each phase. This is usually done by some verification and validation. Validation means confirming the output of a phase is consistent with its input (which is the output of the previous phase) and that the output of the phase is consistent with overall requirements of the system.
The consequences of the need of certification is that each phase must have some defined output that can be evaluated and certified. Therefore, when the activities of a phase are completed, there should be an output product of that phase and the goal of a phase is to produce this product. The outputs of the earlier phases are often called intermediate products or design document. For the coding phase, the output is the code. From this point of view, the output of a software project is to justify the final program along with the use of documentation with the requirements document, design document, project plan, test plan and test results.
Another implication of the linear ordering of phases is that after each phase is completed and its outputs are certified, these outputs become the inputs to the next phase and should not be changed or modified. However, changing requirements cannot be avoided and must be faced. Since changes performed in the output of one phase affect the later phases, that might have been performed. These changes have to made in a controlled manner after evaluating the effect of each change on the project.This brings us to the need for configuration control or configuration management.
The certified output of a phase that is released for the best phase is called baseline. The configuration management ensures that any changes to a baseline are made after careful review, keeping in mind the interests of all parties that are affected by it. There are two basic assumptions for justifying the linear ordering of phase in the manner proposed by the waterfall model.
For a successful project resulting in a successful product, all phases listed in the waterfall model must be performed anyway.
Any different ordering of the phases will result in a less successful software product.
Project Output in a Waterfall Model
As we have seen, the output of a project employing the waterfall model is not just the final program along with documentation to use it. There are a number of intermediate outputs, which must be produced in order to produce a successful product.
The set of documents that forms the minimum that should be produced in each project are: * Requirement document * Project plan * System design document * Detailed design document * Test plan and test report * Final code * Software manuals (user manual, installation manual etc.) * Review reports

Prototyping Software Software Life Cycle goal of prototyping based development is to counter the first two limitations of the waterfall model discussed earlier. The basic idea here is that instead of freezing the requirements before a design or coding can proceed, a throwaway prototype is built to understand the requirements. This prototype is developed based on the currently known requirements. Development of the prototype obviously undergoes design, coding and testing. But each of these phases is not done very formally or thoroughly. By using this prototype, the client can get an "actual feel" of the system, since the interactions with prototype can enable the client to better understand the requirements of the desired system.
Prototyping is an attractive idea for complicated and large systems for which there is no manual process or existing system to help determining the requirements. In such situations letting the client "plan" with the prototype provides invaluable and intangible inputs which helps in determining the requirements for the system. It is also an effective method to demonstrate the feasibility of a certain approach. This might be needed for novel systems where it is not clear that constraints can be met or that algorithms can be developed to implement the requirements. The process model of the prototyping approach is shown in the figure below.

Prototyping Model
The basic reason for little common use of prototyping is the cost involved in this built-it-twice approach. However, some argue that prototyping need not be very costly and can actually reduce the overall development cost. The prototype are usually not complete systems and many of the details are not built in the prototype. The goal is to provide a system with overall functionality. In addition, the cost of testing and writing detailed documents are reduced. These factors helps to reduce the cost of developing the prototype. On the other hand, the experience of developing the prototype will very useful for developers when developing the final system. This experience helps to reduce the cost of development of the final system and results in a more reliable and better designed system.
Advantages of Prototyping 1. Users are actively involved in the development 2. It provides a better system to users, as users have natural tendency to change their mind in specifying requirements and this method of developing systems supports this user tendency. 3. Since in this methodology a working model of the system is provided, the users get a better understanding of the system being developed. 4. Errors can be detected much earlier as the system is mode side by side. 5. Quicker user feedback is available leading to better solutions.
Disadvantages
1. Leads to implementing and then repairing way of building systems. 2. Practically, this methodology may increase the complexity of the system as scope of the system may expand beyond original plans.

Iterative Enhancement Life Cycle Model
The iterative enhancement life cycle model counters the third limitation of the waterfall model and tries to combine the benefits of both prototyping and the waterfall model. The basic idea is that the software should be developed in increments, where each increment adds some functional capability to the system until the full system is implemented. At each step extensions and design modifications can be made. An advantage of this approach is that it can result in better testing, since testing each increment is likely to be easier than testing entire system like in the waterfall model. Furthermore, as in prototyping, the increments provides feedback to the client which is useful for determining the final requirements of the system.
In the first step of iterative enhancement model, a simple initial implementation is done for a subset of the overall problem. This subset is the one that contains some of the key aspects of the problem which are easy to understand and implement, and which forms a useful and usable system. A project control list is created which contains, in an order, all the tasks that must be performed to obtain the final implementation. This project control list gives an idea of how far the project is at any given step from the final system.
Each step consists of removing the next step from the list. Designing the implementation for the selected task, coding and testing the implementation, and performing an analysis of the partial system obtained after this step and updating the list as a result of the analysis. These three phases are called the design phase, implementation phase and analysis phase. The process is iterated until the project control list is empty, at the time the final implementation of the system will be available. The process involved in iterative enhancement model is shown in the figure below.

The Iterative Enhancement Model
The project control list guides the iteration steps and keeps track of all tasks that must be done. The tasks in the list can be include redesign of defective components found during analysis. Each entry in that list is a task that should be performed in one step of the iterative enhancement process, and should be simple enough to be completely understood. Selecting tasks in this manner will minimize the chances of errors and reduce the redesign work.
The Spiral Life Cycle Model
This is a recent model that has been proposed by Boehm. As the name suggests, the activities in this model can be organized like a spiral. The spiral has many cycles. The radial dimension represents the cumulative cost incurred in accomplishing the steps dome so far and the angular dimension represents the progress made in completing each cycle of the spiral. The structure of the spiral model is shown in the figure given below. Each cycle in the spiral begins with the identification of objectives for that cycle and the different alternatives are possible for achieving the objectives and the imposed constraints.
The next step in the spiral life cycle model is to evaluate these different alternatives based on the objectives and constraints. This will also involve identifying uncertainties and risks involved. The next step is to develop strategies that resolve the uncertainties and risks. This step may involve activities such as benchmarking, simulation and prototyping. Next, the software is developed by keeping in mind the risks. Finally the next stage is planned.

The next step is determined by remaining risks. For example, its performance or user-interface risks are considered more important than the program development risks. The next step may be evolutionary development that involves developing a more detailed prototype for resolving the risks. On the other hand, if the program development risks dominate and previous prototypes have resolved all the user-interface and performance risks; the next step will follow the basic waterfall approach.
The risk driven nature of the spiral model allows it to accommodate any mixture of specification-oriented, prototype-oriented, simulation-oriented or some other approach. An important feature of the model is that each cycle of the spiral is completed by a review, which covers all the products developed during that cycle, including plans for the next cycle. The spiral model works for developed as well as enhancement projects.
Spiral Model Description
The development spiral consists of four quadrants as shown in the figure above
Quadrant 1: Determine objectives, alternatives, and constraints.
Quadrant 2: Evaluate alternatives, identify, resolve risks.
Quadrant 3: Develop, verify, next-level product.
Quadrant 4: Plan next phases.
Although the spiral, as depicted, is oriented toward software development, the concept is equally applicable to systems, hardware, and training, for example. To better understand the scope of each spiral development quadrant, let’s briefly address each one.
Quadrant 1: Determine Objectives, Alternatives, and Constraints
Activities performed in this quadrant include: 1. Establish an understanding of the system or product objectives—namely performance, functionality, and ability to accommodate change. 2. Investigate implementation alternatives—namely design, reuse, procure, and procure/ modify 3. Investigate constraints imposed on the alternatives—namely technology, cost, schedule, support, and risk. Once the system or product’s objectives, alternatives, and constraints are understood, Quadrant 2 (Evaluate alternatives, identify, and resolve risks) is performed.
Quadrant 2: Evaluate Alternatives, Identify, Resolve Risks
Engineering activities performed in this quadrant select an alternative approach that best satisfies technical, technology, cost, schedule, support, and risk constraints. The focus here is on risk mitigation. Each alternative is investigated and prototyped to reduce the risk associated with the development decisions. Boehm describes these activities as follows:
. . . This may involve prototyping, simulation, benchmarking, reference checking, administering user questionnaires, analytic modeling, or combinations of these and other risk resolution techniques.
The outcome of the evaluation determines the next course of action. If critical operational and/or technical issues (COIs/CTIs) such as performance and interoperability (i.e., external and internal) risks remain, more detailed prototyping may need to be added before progressing to the next quadrant. Dr. Boehm notes that if the alternative chosen is “operationally useful and robust enough to serve as a low-risk base for future product evolution, the subsequent risk-driven steps would be the evolving series of evolutionary prototypes going toward the right (hand side of the graphic) . . . the option of writing specifications would be addressed but not exercised.” This brings us to Quadrant 3.
Quadrant 3: Develop, Verify, Next-Level Product
If a determination is made that the previous prototyping efforts have resolved the COIs/CTIs, activities to develop, verify, next-level product are performed. As a result, the basic “waterfall” approach may be employed—meaning concept of operations, design, development, integration, and test of the next system or product iteration. If appropriate, incremental development approaches may also be applicable.
Quadrant 4: Plan Next Phases
The spiral development model has one characteristic that is common to all models—the need for advanced technical planning and multidisciplinary reviews at critical staging or control points. Each cycle of the model culminates with a technical review that assesses the status, progress, maturity, merits, risk, of development efforts to date; resolves critical operational and/or technical issues (COIs/CTIs); and reviews plans and identifies COIs/CTIs to be resolved for the next iteration of the spiral.
Subsequent implementations of the spiral may involve lower level spirals that follow the same quadrant paths and decision considerations.