// Core logic — teaches: REST API, database, redirect, basic auth
// Stack: Express + PostgreSQL + Redis (cache)

// Schema:
// CREATE TABLE links (
//   code    TEXT PRIMARY KEY,
//   url     TEXT NOT NULL,
//   user_id TEXT,
//   hits    INT DEFAULT 0,
//   created_at TIMESTAMPTZ DEFAULT NOW()
// );

// POST /links — create short link
app.post('/links', authenticate, async (req, res) => {
  const { url } = z.object({ url: z.string().url() }).parse(req.body)
  const code = randomBytes(4).toString('base64url')   // e.g. 'aB3xZ9'
  await db.query('INSERT INTO links(code, url, user_id) VALUES($1,$2,$3)', [code, url, req.user.id])
  res.status(201).json({ code, shortUrl: `https://short.ly/${code}` })
})

// GET /:code — redirect
app.get('/:code', async (req, res) => {
  const cached = await redis.get(req.params.code)
  const url = cached ?? (await db.query('SELECT url FROM links WHERE code=$1', [req.params.code])).rows[0]?.url
  if (!url) return res.status(404).json({ error: 'Not found' })
  if (!cached) await redis.setex(req.params.code, 3600, url)
  await db.query('UPDATE links SET hits = hits + 1 WHERE code=$1', [req.params.code])
  res.redirect(301, url)
})

Architecture (teaches: WebSocket, Redis pub/sub, multi-process):

Browser A                 Node Process 1              Redis
   │                           │                        │
   │  ws.send("hello room:1")  │                        │
   └──────────────────────────►│  PUBLISH room:1 hello  │
                                └───────────────────────►│
                                                         │
Browser B                 Node Process 2                 │
   │                           │                         │
   │                           │◄─── SUBSCRIBE room:1 ──┘
   │◄──────────────────────────│  ws.send("hello") to all
                                     connected to Process 2

Key learning: each Node process only knows about its own WebSocket connections.
Redis pub/sub broadcasts to all processes so all users receive messages.