Senior Capstone — Independent System Design

Senior Capstone

This is the final project. No architecture is provided. No database schema. No API design. No implementation guidance.

The Challenge

Build a globally scalable marketplace supporting:

• Millions of users
• Thousands of sellers
• Search and recommendations
• Real-time inventory
• Orders and payments
• Notifications
• Analytics
• Administrative tooling

Your Deliverables

1. Requirements document
2. Architecture design
3. Capacity estimation
4. Database design
5. API design
6. Frontend architecture
7. Backend architecture
8. Authentication and authorization
9. Caching strategy
10. Messaging and background processing
11. Search implementation
12. Security plan
13. Testing strategy
14. Deployment plan
15. Monitoring and alerting
16. Disaster recovery plan
17. Scaling strategy
18. Cost considerations
19. Architecture tradeoffs document
20. Working implementation of core features

What This Tests

• Independent engineering judgment
• System design ability
• Tradeoff analysis
• Production thinking
• Communication skills
• Technical depth and breadth

Project Architecture

Before writing any code, let's understand what files we need and where they go. Think of this like a blueprint for a house — you don't start building without a plan.

The File Tree

Every project needs a folder structure. Here is ours:

├── server.js
├── database.js
├── src/App.tsx
├── docker-compose.yml
├── .github/workflows/
├── monitoring/

What Each File Does

Let me explain each file like you're 5 years old:

server.js — Production API server

Complete backend with auth, API routes, error handling, and monitoring.

database.js — Database layer

Production database with pooling, migrations, and health checks.

src/App.tsx — Frontend application

Complete React/Astro frontend with all pages and components.

docker-compose.yml — Container orchestration

Runs all services: app, database, cache, reverse proxy.

.github/workflows/ — CI/CD pipeline

Automated testing, building, and deployment.

monitoring/ — Observability

Prometheus metrics, Grafana dashboards, alerting rules.

Why This Structure?

You might wonder: why use folders at all? Why not put everything in one place?

Because as projects grow, you might have:

• Multiple CSS files
• Many JavaScript files
• Images, fonts, and other assets

If they're all in one folder, it's chaos. Folders keep things organized.

// BAD — everything in one folder
index.html
styles.css
main.css
dark.css
app.js
utils.js
helper.js

// GOOD — organized in folders
index.html
css/
  styles.css
  main.css
js/
  app.js
  utils.js

The Golden Rule

When you start a project, ALWAYS:

1. Create the folder structure FIRST
2. Then fill in the files one by one
3. Never write everything in one giant file

This is how professional developers work. Always.

Setting Up the Project

We are going to create an empty project from scratch. This is the exact process professional developers use every single day.

Open Your Terminal

Your terminal (also called "command line" or "console") is where you type commands to talk to your computer.

• Mac: Open "Terminal" app
• Windows: Open "PowerShell" or "Command Prompt"
• VS Code: Press Ctrl+` (backtick) to open the built-in terminal

Step-by-Step Commands

Type each of these commands, one at a time, pressing Enter after each:

mkdir capstone-project && cd capstone-project && git init && npm init -y

What Just Happened?

Let me explain each command like you're 5:

mkdir [folder] — "Make a new box." This creates a folder on your computer.

cd [folder] — "Go inside the box." Now when you type commands, they happen inside this folder.

git init — "Start keeping a diary." Git will now track every change you make. If you break something, you can go back in time.

npm init -y — "Create an ID card for this project." It creates package.json with default settings. The -y means "yes to all questions."

touch [file] — "Create an empty file." Think of it as a blank piece of paper.

Verify It Worked

Type this command to see your files:

ls -la

The .gitignore File

Open .gitignore and add this:

node_modules/
.DS_Store
*.log
.env

Why? Because node_modules can be HUGE — thousands of files. We don't want to save all of them in Git. We can always recreate them with npm install.

Your First Git Commit

Now save everything to Git:

git add .
git commit -m "Initial project setup"

Congratulations! You just set up a project like a real developer.

Building the Application

Now let's build the application step by step. Follow each step in order.

Build Steps

1. Choose your capstone project from the roadmap
2. Design the architecture (draw it on paper first)
3. Set up the project structure and tooling
4. Build the backend API with all features
5. Build the frontend with all pages
6. Add authentication and authorization
7. Write comprehensive tests (unit, integration, E2E)
8. Set up CI/CD pipeline
9. Add Docker containerization
10. Set up monitoring and logging
11. Write comprehensive documentation
12. Deploy to production
13. Prepare presentation and demo
14. Commit and deploy

How Each Step Works

Step 1 — Create the entry point

Every application starts with an entry file. This is the file that runs first when someone opens your app.

Think of it like the front door of a house. Everything starts when you walk through it.

Step 2 — Set up the structure

Before writing features, we set up the basic layout. This is like putting up the walls before painting them.

Step 3 — Add the main functionality

Now we add what makes the app actually DO something. This is the "brain" of the application.

Step 4 — Connect everything

We wire the pieces together. The HTML connects to CSS. The JavaScript connects to HTML. Everything talks to each other.

Step 5 — Test and fix

We try everything, find bugs, and fix them. Real developers spend 30% of their time testing.

Save and Test

After each step, save your files and refresh the browser. If something breaks, check the browser console (F12 → Console tab).

Commit after each major step:

git add .
git commit -m "Describe what you just built"

Pro Tip

Build in small steps. Don't write 100 lines and then test. Write 10 lines, test, fix, then write 10 more. This catches errors early when they're easy to fix.

Gathering Requirements

Functional Requirements

Before writing any code, answer:

1. Who are the users? (customers, sellers, admins)
2. What can they do? (browse, buy, sell, manage)
3. What are the core flows? (search → add to cart → checkout → delivery)
4. What are the edge cases? (out of stock, payment failure, returns)
5. What are the integration points? (payment providers, shipping, email)

Non-Functional Requirements

Requirement	Target
Availability	99.9%
Latency	< 200ms for reads
Throughput	10k RPS
Data consistency	Eventual consistency OK
Durability	No data loss
Security	PCI compliance for payments

Capacity Estimation

Users: 10M registered, 1M daily active
Products: 5M listings
Orders: 100K per day
Search queries: 500K per day
Storage: ~10TB (images, videos)
Bandwidth: ~100GB per day

Architecture Decision Records

# ADR: Database Choice

## Context
We need to store product catalog, user data, and orders.

## Decision
Use PostgreSQL for transactional data (orders, users).
Use MongoDB for product catalog (flexible schema).
Use Elasticsearch for search.

## Alternatives
- All PostgreSQL: simpler but less flexible for catalog
- All MongoDB: less suitable for transactional integrity

## Consequences
- Two databases to maintain
- Need data synchronization between systems
- Better suited to each use case

Evaluating Your Design

Architecture Scorecard

Dimension	Score / 10	Notes
Requirements coverage
Scalability
Reliability
Security
Maintainability
Cost efficiency
Complexity management
Tradeoff documentation
Code quality
Testing

Questions to Justify

1. Why did you choose this database?
2. Why not microservices for this component?
3. How does your system handle failure?
4. What is the biggest bottleneck and how do you address it?
5. What would you do differently with 10x the traffic?
6. What would you cut if you had half the time?
7. How do you ensure data consistency across services?
8. What monitoring would you add in production?
9. How do you handle deployments without downtime?
10. What is your disaster recovery strategy?

The Goal

The goal is not to build a perfect system. The goal is to demonstrate independent engineering judgment — the ability to make reasonable decisions, justify them, and acknowledge tradeoffs.

Mistakes Everyone Makes

Every developer makes these mistakes. Knowing them ahead of time saves hours of debugging.

Mistake 1: Forgetting to Connect Files

<!-- WRONG — CSS is not linked, page looks ugly -->
<link rel="stylesheet" href="style.css">

<!-- RIGHT — file path matches actual location -->
<link rel="stylesheet" href="css/styles.css">

How to check: Right-click the page → Inspect → Network tab → look for red (failed) resources.

Mistake 2: JavaScript Before HTML

<!-- WRONG — JavaScript runs before HTML exists -->
<script src="app.js"></script>
<body>...</body>

<!-- RIGHT — JavaScript runs after HTML loads -->
<body>...</body>
<script src="app.js"></script>

Mistake 3: Not Using preventDefault()

// WRONG — page reloads when you submit
form.addEventListener('submit', () => {
  // This code runs, but then the page reloads!
});

// RIGHT — preventDefault stops the reload
form.addEventListener('submit', (e) => {
  e.preventDefault(); // Now the page doesn't reload
});

Mistake 4: Modifying State Without Re-rendering

// WRONG — changes data but screen stays the same
todos.push(newTodo);

// RIGHT — always update the screen after changing data
todos.push(newTodo);
render(); // Now the screen shows the new todo

Mistake 5: Not Handling Empty Input

// WRONG — adds empty tasks
addTodo(""); // Adds a blank todo

// RIGHT — check for empty input
if (text.trim()) {
  addTodo(text);
}

Mistake 6: Ignoring Errors

How to check for errors:

1. Press F12 in your browser
2. Click "Console" tab
3. Look for red text
4. Click on it to see what went wrong

If you see red text, DON'T IGNORE IT. That's the computer telling you something is broken.

Debugging Checklist

When something doesn't work:

1. Check the console — F12 → Console (red text = error)
2. Check the Network — F12 → Network (red lines = failed requests)
3. Check file paths — Are CSS/JS files in the right folder?
4. Check spelling — Typos in IDs, class names, function names
5. Check the basics — Is the server running? Is the URL correct?

The 5-Minute Rule

If you've been stuck for 5 minutes:

1. Stop
2. Re-read the error message
3. Search the error on Google/Stack Overflow
4. If still stuck, ask for help

Don't waste hours on something that might be a simple typo.

Testing Your App

A real developer NEVER assumes code works. They TEST it.

Why Test?

• Users will find every bug you missed
• Bugs found later cost 10x more to fix
• Testing gives you confidence to make changes

Manual Testing Checklist

Go through this checklist for EVERY feature:

□ Page loads without errors (check browser console — F12)
□ All buttons work when clicked
□ All forms submit correctly
□ Input validation works (empty, special characters)
□ Data saves and loads correctly
□ Responsive on mobile (resize browser)
□ No console errors
□ All links work
□ Loading states appear during API calls
□ Error messages show when things fail

How to Check for Errors

Press F12 in your browser. Click the "Console" tab.

• Empty = Good! No errors.
• Red text = Bad. Click it to see what went wrong.

Edge Cases to Test

Real users do weird things. Test these:

1. Empty input — What happens if you submit nothing?
2. Very long input — Paste a 1000-character string
3. Special characters — Type < > & " ' / 4. Rapid clicking — Click a button 10 times fast
4. No internet — Turn off WiFi and try
5. Refresh mid-action — Reload while something is happening

Browser Testing Tools

Tool	How to Open	What It Shows
Console	F12 → Console	JavaScript errors and logs
Network	F12 → Network	API requests and responses
Elements	F12 → Elements	Live HTML and CSS
Device Mode	Ctrl+Shift+M	Mobile screen simulation

Git Commit

After testing, commit your work:

git add .
git commit -m "Complete [project name] with testing"

Summary — Skills You Now Have

Technical Skills

1. Project Structure — You know how to organize files in folders
2. Git — You can initialize, add, commit, and push
3. Version Control — You can track changes and go back in time
4. HTML/CSS/JavaScript — You can build interactive web pages
5. DOM Manipulation — You can change what's on screen with code
6. Event Handling — You can respond to clicks, inputs, and other actions
7. State Management — You can track and update application data
8. Debugging — You can find and fix errors using browser tools
9. Testing — You can verify your app works correctly

Thinking Skills

1. Architecture — You planned the project BEFORE writing code
2. Problem Solving — You broke a big problem into small steps
3. Debugging — You systematically found and fixed errors
4. Documentation — You wrote notes for future you (and other developers)

The Pattern

Every frontend application follows this pattern:

State (data) → Render (draw) → Event (respond) → Update State → Render again

React, Vue, Angular, Svelte — they ALL do this. You just learned the fundamental pattern that powers the entire web.

What's Next?

Look at the next project in the roadmap. Each new project builds on what you learned here. The skills compound — every project makes you a better developer.

Remember: every expert was once a beginner who didn't give up.

Quick Reference

File Overview

• server.js: Production API server
• database.js: Database layer
• src/App.tsx: Frontend application
• docker-compose.yml: Container orchestration
• .github/workflows/: CI/CD pipeline
• monitoring/: Observability

Setup Commands

mkdir capstone-project && cd capstone-project && git init && npm init -y

Common Patterns

// Select an element
const el = document.getElementById('myId');

// Add event listener
el.addEventListener('click', (e) => {
  e.preventDefault();
});

// Update the DOM
el.innerHTML = '<p>New content</p>';
el.textContent = 'Text only';

Debugging

F12 → Console     → Check for errors
F12 → Network     → Check API requests
F12 → Elements    → Inspect HTML/CSS
Ctrl+Shift+M      → Mobile view

Git Commands

git init                  # Start a repo
git add .                 # Stage all changes
git commit -m "message"   # Save changes
git push                  # Upload to GitHub