What is SDLC and why is it needed? What are the main stages SDLC includes?

#What SDLC models exist and how do they differ?

---

SDLC is the overall cycle, and SDLC models are different ways of organizing it. They define in what order and how to move between stages. The choice of model depends on the project, requirements, and risks.

---

#1. Waterfall

This is a linear model where each stage strictly follows the previous one. First requirements, then design, then code, then tests, and only then deployment. Changing something "after the fact" is hard - you have to go back up the "waterfall". Used where requirements are stable (for example, banking systems, government contracts). The main drawback is that flexibility is almost impossible if the client changes their mind.

---

#2. V-Model

Similar to Waterfall, but with an emphasis on testing. Each development stage (analysis, design, coding) is immediately linked to a verification stage (at the bottom of the letter "V"). For example, requirements have acceptance tests, design has integration tests. This improves quality but reduces flexibility. Used where full test coverage matters - medicine, aviation, defense.

---

#3. Spiral

The process runs in iterations - like the turns of a spiral. On each turn: risk analysis → design → development → testing → evaluation → next turn. It allows requirements to be refined gradually and risks to be reduced at early stages. Suitable for large and expensive systems where mistakes are very costly.

---

#4. Agile

Agile is not a single model but an entire philosophy, where development runs in short cycles - iterations (sprints). Each sprint lasts 1-4 weeks and ends with a ready, working piece of the product. The team gets feedback, adjusts priorities, and moves on. Agile gives maximum flexibility but requires high discipline and constant communication. In practice, Scrum or Kanban is most often used.

---

#5. Iterative / Incremental

An intermediate option between waterfall and agile. The project is split into parts (iterations), each of which goes through all the SDLC stages - from analysis to testing. Each iteration adds new features, improving the product step by step. This reduces risks and allows a working product to be obtained already at early stages.

---

#Conclusion

• Waterfall - strict and predictable, but inflexible.
• V-Model - focus on testing and quality control.
• Spiral - iterations plus risk analysis.
• Agile - speed, flexibility, and constant adaptation.
• Iterative - the product develops step by step, accumulating features.

Each model solves one task: making the development process manageable, but in different ways.