Add problem 4

src/problem4/README.md

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+# Problem 4
+
+A small platform has been deployed using Docker Compose.
+Users are reporting that the API is unreliable and sometimes inaccessible.
+
+Your task is to investigate and stabilize the system.
+
+## Setup
+
+Requirements:
+
+* Docker
+* Docker Compose
+
+Run:
+
+docker compose up --build
+
+The application will be available at:
+[http://localhost:8080](http://localhost:8080)
+
+## Your Tasks
+
+1. Identify the issues affecting the system
+2. Fix them
+3. Explain the root causes
+
+## Submission
+
+Submit a short report including:
+
+* What problems you found
+* How you diagnosed them
+* The fixes you applied
+* What monitoring/alerts you would add
+* How you would prevent this in production

src/problem4/api/Dockerfile

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+FROM node:20-alpine
+
+WORKDIR /app
+COPY package.json .
+RUN npm install
+
+COPY src ./src
+
+CMD ["node", "src/index.js"]

src/problem4/api/package.json

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+{
+  "name": "api",
+  "version": "1.0.0",
+  "dependencies": {
+    "express": "^4.18.2",
+    "pg": "^8.11.3",
+    "ioredis": "^5.3.2"
+  }
+}

src/problem4/api/src/index.js

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+const express = require("express");
+const { Pool } = require("pg");
+const Redis = require("ioredis");
+
+const app = express();
+
+const pool = new Pool({
+  host: process.env.DB_HOST,
+  user: "postgres",
+  password: "postgres",
+  database: "postgres",
+  port: 5432,
+});
+
+const redis = new Redis({ host: process.env.REDIS_HOST, port: 6379 });
+
+app.get("/api/users", async (req, res) => {
+  try {
+    const db = await pool.connect();
+    const result = await db.query("SELECT NOW()");
+    db.release();
+
+    await redis.set("last_call", Date.now());
+    res.json({ ok: true, time: result.rows[0] });
+  } catch (err) {
+    console.error(err);
+    res.status(500).json({ ok: false, error: err.message });
+  }
+});
+
+app.get("/status", (req, res) => {
+  res.json({ status: "ok" });
+});
+
+app.listen(3000, () => console.log("API running on 3000"));

src/problem4/docker-compose.yml

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+version: "3.9"
+
+services:
+  nginx:
+    image: nginx:1.25
+    ports:
+      - "8080:80"
+    volumes:
+      - ./nginx/conf.d:/etc/nginx/conf.d
+    depends_on:
+      - api
+
+  api:
+    build: ./api
+    environment:
+      - DB_HOST=postgres
+      - REDIS_HOST=redis
+    depends_on:
+      - postgres
+      - redis
+
+  postgres:
+    image: postgres:15
+    environment:
+      POSTGRES_PASSWORD: postgres
+
+  redis:
+    image: redis:7

src/problem4/nginx/conf.d/default.conf

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+server {
+    listen 80;
+
+    location = / {
+        return 200 "Welcome to the platform\n";
+    }
+
+    location /api/ {
+        proxy_pass http://api:3001;
+    }
+
+}

src/problem4/postgres/init.sql

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+ALTER SYSTEM SET max_connections = 20;