server.js

require('dotenv').config();
const express = require('express');
const { Pool } = require('pg');
const cors = require('cors');
const rateLimit = require("express-rate-limit");
const redisClient = require('./redisClient');
const RedisStore = require('rate-limit-redis').default;

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

// PostgreSQL pool
const pool = new Pool({
  user: process.env.DB_USER,
  host: process.env.DB_HOST,
  database: process.env.DB_NAME,
  password: process.env.DB_PASSWORD,
  port: process.env.DB_PORT
});

// Rate limiter for /news with redis
// This ensures rate limits work across multiple servers to scale later by Distributed Rate Limiting for Distributed Cache Pattern
const newsLimiter = rateLimit({
  store: new RedisStore({
    sendCommand: (...args) => redisClient.sendCommand(args),
  }),
  windowMs: 1 * 1000, //1 seconds
  max: 10, //10 requests per window
  standardHeaders: true,
  legacyHeaders: false,
  handler: (req, res) => {
    res.status(429).json({ success: false, message: "Too many requests. Wait 30s." });
  }
});

// Middleware
app.use(express.json());
app.use(cors());
app.use(express.static('public'));

// Apply to /news route
app.use("/news", newsLimiter);

// /news endpoint with filtering and pagination
app.get('/news', async (req, res) => {
  try {
    const { country = '', category = '', page = 1, pageSize = 20 } = req.query;

    const pageNum = parseInt(page, 10) || 1;
    const pageSizeNum = parseInt(pageSize, 10) || 20;

    const cacheKey = `news${country}:${category}:${pageNum}:${pageSizeNum}`;

    // Redis Check
    const cachedData = await redisClient.get(cacheKey);

    if(cachedData){
      console.log(`Cache Hit: ${cacheKey}`);
      return res.json(JSON.parse(cachedData));
    }
    console.log(`Cache Miss: ${cacheKey}`);

    const offset = (pageNum - 1) * pageSizeNum;

    // Build dynamic filters
    const filters = [];
    const values = [];

    if (country) {
      values.push(country);
      filters.push(`source_country = $${values.length}`);
    }
    if (category) {
      values.push(category);
      filters.push(`news_type = $${values.length}`);
    }

    let query = 'SELECT * FROM news WHERE 1=1';
    if (filters.length > 0) {
      query += ' AND ' + filters.join(' AND ');
    }

    // Append ORDER BY and hardcode LIMIT/OFFSET for performance
    query += ` ORDER BY published_at DESC LIMIT ${pageSizeNum} OFFSET ${offset}`;

    const result = await pool.query(query, values);

    // Count total rows for pagination (same filter logic)
    let countQuery = 'SELECT COUNT(*) FROM news WHERE 1=1';
    if (filters.length > 0) {
      countQuery += ' AND ' + filters.join(' AND ');
    }

    const countResult = await pool.query(countQuery, values);
    const totalCount = parseInt(countResult.rows[0].count, 10);
    const totalPages = Math.ceil(totalCount / pageSizeNum);

    const response = {
      success: true,
      page: pageNum,
      pageSize: pageSizeNum,
      totalCount,
      totalPages,
      data: result.rows
    };

    // cache the response page
    await redisClient.set(cacheKey,JSON.stringify(response), {EX: 3600});

    res.json(response);
  } catch (err) {
    console.error(err);
    res.status(500).json({ success: false, error: err.message });
  }
});

// Start server
app.listen(PORT, () => {
  console.log(`Server running on http://localhost:${PORT}`);
});