Software development life cycle -- waterfall model

The classic waterfall model is basic Software development lifecycle Model . It's very simple , But idealism . This model used to be very popular , But now it is not used . But it's very important , Because all other software development lifecycle models are based on the classic waterfall model .
The classic waterfall model divides the life cycle into a set of stages . This model believes that a phase can be started after the last phase . in other words , The output of one stage will be the input of the next stage . therefore , You can think of the development process as a sequential flow in a waterfall . ad locum , These stages do not overlap . The different sequential stages of the classic waterfall model are shown in the figure below ：

Stage Introduction

Now? , Let's take a brief look at each stage ：

1. feasibility study ： The main objective of this stage is to determine whether the development software is financially and technically feasible .
   The feasibility study involves understanding problems , Then determine various possible strategies to solve the problem . Analyze the advantages and disadvantages of these different solutions , Choose the best solution , And perform all other phases according to this solution strategy .

2. Requirements analysis and specification ： The purpose of the requirements analysis and specification stage is to understand the exact requirements of customers and record them correctly . This phase consists of two different activities .

   • Requirements collection and Analysis ： First , Collect all requirements about software from customers , Then analyze the collected requirements . The goal of the analysis section is to eliminate imperfections （ Incomplete requirements omit some parts of the actual requirements ） And inconsistent （ Inconsistent requirements are requirements that contradict some parts of the requirements with some other parts ）.
   • Requirement specification ： The requirements of these analyses are documented in the software requirements specification （SRS） In the document .SRS The document acts as a contract between the development team and the customer . Any future disputes between customers and developers can be checked SRS Document to solve .

3. Design ： The goal of this stage is to SRS The requirements obtained in are converted into a format that can be encoded in a programming language . It includes high-level and detailed design and overall software architecture . Software design documents are used to record all these works （SDD）

4. Coding and unit testing ： In the coding phase , Use any suitable programming language to convert the software design into source code . therefore , Every designed module is coded . The purpose of the unit test phase is to check whether each module works properly .

5. Integration and system testing ： The integration of different modules takes place shortly after coding and unit testing . The integration of various modules is carried out step by step . In each integration step , Add previously planned modules to the partially integrated system , And test the generated system . Last , After all modules have been successfully integrated and tested , Get a complete working system and test it .

   System testing consists of three different testing activities , As follows ：

   • Alpha test ： Alpha Tests are system tests performed by the development team .
   • Beta test ： Beta The test is a system test performed by a group of friendly customers .
   • The acceptance test ： After software delivery , The customer performs acceptance tests , To determine whether to accept or reject the delivered software .

6. maintain ： Maintenance is the most important stage in the software life cycle . The energy used for maintenance accounts for% of the total workload spent on developing complete software 60%. There are basically three types of maintenance ：

   • Corrective maintenance ： This type of maintenance is performed to correct errors not found during product development .
   • Integrity maintenance ： This type of maintenance is carried out according to the requirements of customers to enhance the function of the system .
   • Adaptive maintenance ： It usually requires adaptive maintenance to migrate software to a new environment , For example, working on a new computer platform or a new operating system .

Advantages of classic waterfall model

The classic waterfall model is an idealistic model of software development . It's very simple , Therefore, it can be regarded as the basis of other software development life cycle models . Here is this SDLC Some of the main advantages of the model ：

• This model is very simple , Easy to understand .
• The phases in this model process one phase at a time .
• Each stage in the model is clearly defined .
• The model has very clear and easy to understand milestones .
• The process , Actions and results are well documented .
• Strengthen good habits ： Define first, design first ,
  Post design code .
• This model is suitable for small projects and requirements
  Understand the project .

Disadvantages of classical waterfall model

The classical waterfall model has various shortcomings , Basically , We can't use it in real projects , But we use other software development lifecycle models based on the classic waterfall model . Here are some of the main drawbacks of this model ：

• No feedback path ： In the classic waterfall model , The evolution of software from one stage to another is like a waterfall . It assumes that developers will not make any mistakes at any stage . therefore , It does not contain any error correction mechanism .

• Difficult to adapt to change requests ： This model assumes that all customer requirements can be defined exactly at the beginning of the project , But in fact, customers' needs will change over time . After the requirements specification stage , It's hard to adapt to any change request .

• There is no overlap of stages ： This model suggests that the new phase can only be started after the completion of the previous phase . But in the real world , This is unsustainable . In order to improve efficiency and reduce costs , The stages may overlap .