Software Development Models Explained

A release that follows Waterfall feels predictable until a requirement changes after coding starts. This section documents the Gate-by-Gate Waterfall Checklist as a strict pipeline: Requirements Analysis → Design → Coding → Testing → Deployment → Maintenance, with concrete artifacts you produce at each phase and the points where changes become costly.

Overview

Waterfall is a linear, phase-by-phase development flow where each phase completes before the next begins. Use it when requirements are stable, interfaces are well-defined, and you want a clear audit trail from spec to deployment.

Gate-by-Gate Waterfall Checklist means each phase has a “gate” output (a specific document or build artifact) that must exist before moving forward.

Quick Reference

Parameters

The Waterfall pipeline depends on API-reference quality and developer-guide precision. For an HTTP API contract, document these parameters:

Code Example

Below is a minimal Node.js/Express route that matches a documented Waterfall API contract (including structured errors). In Waterfall, this code is produced only after the API contract and response schemas are approved at Gate 1 and Gate 2.

// app.js
import express from "express";

const app = express();
app.use(express.json());

// Example: POST /v1/courses
// Contract (Gate 1/2):
// - 201 on success with responseSchema { id: string, name: string }
// - 400 on validation with errorFormat { code, message, details }
app.post("/v1/courses", async (req, res) => {
  const { name } = req.body ?? {};

  // Gate 4 testing will cover: missing name, non-string name, and success path.
  if (typeof name !== "string" || name.trim().length < 1) {
    return res.status(400).json({
      code: "VALIDATION_ERROR",
      message: "Field 'name' is required and must be a non-empty string.",
      details: [{ field: "name", issue: "missing_or_empty" }],
    });
  }

  // In Waterfall, persistence/migrations are aligned in Gate 2 design.
  const id = crypto.randomUUID();

  return res.status(201).json({
    id,
    name: name.trim(),
  });
});

export default app;

Response Format

Document the expected JSON response structures as part of the developer guide and API reference.

{
  "success": true,
  "data": {
    "id": "string",
    "name": "string"
  },
  "meta": {
    "requestId": "string"
  }
}

Error response format (400/404/500):

{
  "success": false,
  "error": {
    "code": "string",
    "message": "string",
    "details": [
      {
        "field": "string",
        "issue": "string"
      }
    ]
  },
  "meta": {
    "requestId": "string"
  }
}

Field notes:

• `requestId` enables traceability across logs during Gate 5 deployment.
• `details` is an array to keep validation errors stable across versions.

Notes & Best Practices

• Lock the API contract at Gate 1: endpoints, schemas, and status codes must not change during Gate 3 coding without a documented re-gate (requirements + design).
• Error handling must be schema-driven: keep `code/message/details` stable so Gate 4 tests remain valid....