System Design - lesson 4 | HackIndia Skills

System Design - lesson 4 | HackIndia Skills | HackIndia

Example

Express cache example

This simple in-memory cache is fine for learning and a demo. It will reset when the server restarts, and each server instance has its own cache.

For a real multi-server app, use a shared cache like Redis, then mention Redis in your design. If you ask AI to generate this code, check TTL, user data leakage, and error handling.

// server.js
import express from "express";

const app = express();
const cache = new Map();
const TTL_MS = 60 * 1000;

async function getEventsFromDb() {
  return [
    { id: 1, name: "HackIndia campus demo", city: "Bengaluru" },
    { id: 2, name: "AI build night", city: "Pune" }
  ];
}

app.get("/api/events", async (req, res) => {
  const key = "events:list";
  const cached = cache.get(key);

  if (cached && Date.now() < cached.expiresAt) {
    return res.json({ source: "cache", data: cached.data });
  }

  const data = await getEventsFromDb();
  cache.set(key, { data, expiresAt: Date.now() + TTL_MS });
  res.json({ source: "db", data });
});

app.listen(3000, () => console.log("API on port 3000"));

Add cache and load balancing

Remember this

Where cache fits

Pick cache data

Common cache mistake

Where load balancing fits

Express cache example

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