workshop/aula-15/demo-app/app.js

'use strict';

const express = require('express');
const { Pool } = require('pg');

const app = express();
const PORT = process.env.PORT || 3000;

const pool = new Pool({
  host: process.env.PG_HOST || 'localhost',
  port: parseInt(process.env.PG_PORT || '5432', 10),
  user: process.env.PG_USER || 'demo',
  password: process.env.PG_PASSWORD || 'demo',
  database: process.env.PG_DATABASE || 'demo',
});

// --- Database seeding ---

async function seedDatabase() {
  const client = await pool.connect();
  try {
    const tableCheck = await client.query(`
      SELECT EXISTS (
        SELECT FROM information_schema.tables WHERE table_name = 'users'
      )
    `);

    if (tableCheck.rows[0].exists) {
      const countResult = await client.query('SELECT COUNT(*) FROM users');
      if (parseInt(countResult.rows[0].count, 10) > 0) {
        console.log('Database already seeded, skipping.');
        return;
      }
    }

    console.log('Seeding database...');

    await client.query(`
      CREATE TABLE IF NOT EXISTS users (
        id SERIAL PRIMARY KEY,
        name VARCHAR(100) NOT NULL,
        email VARCHAR(150) NOT NULL,
        created_at TIMESTAMP DEFAULT NOW()
      )
    `);

    await client.query(`
      CREATE TABLE IF NOT EXISTS posts (
        id SERIAL PRIMARY KEY,
        user_id INTEGER REFERENCES users(id),
        title VARCHAR(200) NOT NULL,
        body TEXT NOT NULL,
        created_at TIMESTAMP DEFAULT NOW()
      )
    `);

    // Insert 50 users
    for (let i = 1; i <= 50; i++) {
      await client.query(
        'INSERT INTO users (name, email) VALUES ($1, $2)',
        [`User ${i}`, `user${i}@example.com`]
      );
    }

    // Insert 10 posts per user (500 posts total)
    for (let userId = 1; userId <= 50; userId++) {
      for (let p = 1; p <= 10; p++) {
        await client.query(
          'INSERT INTO posts (user_id, title, body) VALUES ($1, $2, $3)',
          [userId, `Post ${p} by User ${userId}`, `Content of post ${p} by user ${userId}. Lorem ipsum dolor sit amet.`]
        );
      }
    }

    console.log('Database seeded: 50 users, 500 posts.');
  } finally {
    client.release();
  }
}

// --- Routes ---

app.get('/health', (_req, res) => {
  res.json({ status: 'ok' });
});

// Fast route: single query, returns 10 users
app.get('/fast', async (_req, res) => {
  const start = Date.now();
  try {
    const result = await pool.query('SELECT * FROM users LIMIT 10');
    const duration = Date.now() - start;
    res.json({
      route: '/fast',
      description: 'Single query - SELECT users LIMIT 10',
      query_count: 1,
      duration_ms: duration,
      data: result.rows,
    });
  } catch (err) {
    res.status(500).json({ error: err.message });
  }
});

// Slow route: N+1 query pattern
app.get('/slow', async (_req, res) => {
  const start = Date.now();
  try {
    const usersResult = await pool.query('SELECT * FROM users');
    const users = usersResult.rows;
    let queryCount = 1;

    const usersWithPosts = [];
    for (const user of users) {
      const postsResult = await pool.query(
        'SELECT * FROM posts WHERE user_id = $1',
        [user.id]
      );
      queryCount++;
      usersWithPosts.push({
        ...user,
        posts: postsResult.rows,
      });
    }

    const duration = Date.now() - start;
    res.json({
      route: '/slow',
      description: 'N+1 pattern - 1 query for users + 1 query per user for posts',
      query_count: queryCount,
      user_count: users.length,
      total_posts: usersWithPosts.reduce((sum, u) => sum + u.posts.length, 0),
      duration_ms: duration,
      data: usersWithPosts,
    });
  } catch (err) {
    res.status(500).json({ error: err.message });
  }
});

// Fixed route: single JOIN query
app.get('/fixed', async (_req, res) => {
  const start = Date.now();
  try {
    const result = await pool.query(`
      SELECT
        u.id AS user_id,
        u.name,
        u.email,
        u.created_at AS user_created_at,
        p.id AS post_id,
        p.title,
        p.body,
        p.created_at AS post_created_at
      FROM users u
      LEFT JOIN posts p ON p.user_id = u.id
      ORDER BY u.id, p.id
    `);

    // Group results by user
    const usersMap = new Map();
    for (const row of result.rows) {
      if (!usersMap.has(row.user_id)) {
        usersMap.set(row.user_id, {
          id: row.user_id,
          name: row.name,
          email: row.email,
          created_at: row.user_created_at,
          posts: [],
        });
      }
      if (row.post_id) {
        usersMap.get(row.user_id).posts.push({
          id: row.post_id,
          title: row.title,
          body: row.body,
          created_at: row.post_created_at,
        });
      }
    }

    const usersWithPosts = Array.from(usersMap.values());
    const duration = Date.now() - start;

    res.json({
      route: '/fixed',
      description: 'Single JOIN query - the correct way',
      query_count: 1,
      user_count: usersWithPosts.length,
      total_posts: usersWithPosts.reduce((sum, u) => sum + u.posts.length, 0),
      duration_ms: duration,
      data: usersWithPosts,
    });
  } catch (err) {
    res.status(500).json({ error: err.message });
  }
});

// --- Startup ---

async function main() {
  // Wait for PostgreSQL to be ready (with retries)
  let retries = 10;
  while (retries > 0) {
    try {
      await pool.query('SELECT 1');
      console.log('Connected to PostgreSQL.');
      break;
    } catch (err) {
      retries--;
      if (retries === 0) {
        console.error('Failed to connect to PostgreSQL after retries:', err.message);
        process.exit(1);
      }
      console.log(`Waiting for PostgreSQL... (${retries} retries left)`);
      await new Promise((resolve) => setTimeout(resolve, 3000));
    }
  }

  await seedDatabase();

  app.listen(PORT, () => {
    console.log(`Demo app listening on port ${PORT}`);
  });
}

main().catch((err) => {
  console.error('Fatal error:', err);
  process.exit(1);
});