feat(networking): add checkpoint 12 reverse proxy and traffic routing lab

Author: wendev27

checkpoints/checkpoint-12-reverse-proxy-and-traffic-routing.md

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+# Checkpoint 12 — Reverse Proxy & Traffic Routing with Nginx
+
+## Objective
+
+Understand how reverse proxies work in modern infrastructure by using Nginx to route incoming requests to multiple Docker containers.
+
+This checkpoint demonstrates how a single public endpoint can forward requests to the correct internal service.
+
+---
+
+# What We Built
+
+We created a small multi-container application composed of:
+
+- Frontend container (Nginx static site)
+- Backend container (Node.js API)
+- Nginx reverse proxy container
+- Docker Compose network
+
+Users interact with only one public port:
+
+http://localhost:8080
+
+Nginx receives the request and decides where to send it.
+
+---
+
+# Architecture
+
+```text
+Browser
+    │
+    │
+localhost:8080
+    │
+    ▼
+┌─────────────────────┐
+│       NGINX         │
+│   Reverse Proxy     │
+└─────────┬───────────┘
+          │
+    ┌─────┴─────┐
+    │           │
+    ▼           ▼
+
+Frontend      Backend
+(Container)   (Container)
+
+"/"           "/api"
+
+Port 80       Port 4000
+```
+
+---
+
+# Request Flow
+
+### Frontend Request
+
+```text
+GET /
+```
+
+Flow:
+
+```text
+Browser
+    ↓
+localhost:8080
+    ↓
+Nginx
+    ↓
+frontend container
+```
+
+---
+
+### Backend Request
+
+```text
+GET /api
+```
+
+Flow:
+
+```text
+Browser
+    ↓
+localhost:8080/api
+    ↓
+Nginx
+    ↓
+backend container
+```
+
+---
+
+# Key Concepts Learned
+
+## Reverse Proxy
+
+A reverse proxy sits between users and backend services.
+
+Responsibilities:
+
+- Receives requests
+- Routes traffic
+- Hides internal services
+- Centralizes access
+- Improves scalability
+
+---
+
+## Docker Networking
+
+Docker Compose automatically created an isolated network for all containers.
+
+Containers communicate using service names:
+
+```nginx
+proxy_pass http://backend:4000;
+```
+
+instead of:
+
+```text
+localhost:4000
+```
+
+---
+
+## Why Reverse Proxies Matter
+
+Modern applications rarely expose every service directly.
+
+Instead:
+
+```text
+Internet
+    ↓
+Load Balancer
+    ↓
+Nginx
+    ↓
+Containers
+```
+
+This architecture is used by:
+
+- Netflix
+- YouTube
+- Spotify
+- Amazon
+- Vercel
+- AWS
+- Kubernetes clusters
+
+---
+
+# Commands Used
+
+Start containers:
+
+```bash
+docker compose up --build
+```
+
+Stop containers:
+
+```bash
+docker compose down
+```
+
+View running containers:
+
+```bash
+docker ps
+```
+
+View logs:
+
+```bash
+docker logs reverse-proxy-lab-nginx-1
+
+docker logs reverse-proxy-lab-backend-1
+
+docker logs reverse-proxy-lab-frontend-1
+```
+
+---
+
+# Nginx Routing Rules
+
+```nginx
+location / {
+    proxy_pass http://frontend;
+}
+
+location /api {
+    proxy_pass http://backend:4000;
+}
+```
+
+---
+
+# Verification
+
+The system successfully:
+
+✅ Built all Docker images
+
+✅ Created the Docker network
+
+✅ Started frontend, backend, and Nginx containers
+
+✅ Routed requests to `/`
+
+✅ Routed requests to `/api`
+
+✅ Verified communication between containers
+
+---
+
+# Important Discoveries
+
+- Users do not talk directly to containers.
+- Nginx decides where traffic goes.
+- Docker provides networking between containers.
+- URL paths determine routing behavior.
+- Reverse proxies are foundational to cloud systems.
+- Kubernetes Ingress works using similar concepts.
+
+---
+
+# Mental Models
+
+| Component        | Mental Model       |
+| ---------------- | ------------------ |
+| Docker Container | House              |
+| Docker Network   | Neighborhood       |
+| Nginx            | Receptionist       |
+| Reverse Proxy    | Traffic Controller |
+| URL Path         | Destination        |
+| Browser          | Visitor            |
+
+---
+
+# Reflection
+
+Before this checkpoint, containers felt isolated.
+
+After this checkpoint, I understood that:
+
+- Containers communicate through Docker networks.
+- Nginx routes requests using rules.
+- One public endpoint can manage multiple services.
+- Reverse proxies are essential in production systems.
+
+This checkpoint bridges Docker fundamentals and future topics such as:
+
+- Load balancing
+- Kubernetes Services
+- Kubernetes Ingress
+- Cloud infrastructure
+- Scalable systems

labs/networking/reverse-proxy-lab/nginx.conf

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+server {
+    listen 80;
+
+    location / {
+        proxy_pass http://frontend:80;
+    }
+
+    location /api {
+        proxy_pass http://backend:4000;
+    }
+}