Challenge 3: Fix a Connection-Per-Request Anti-Pattern — Possible Solution
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WHAT'S WRONG
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Creating a new MongoClient and calling .connect() inside every single
request handler means every incoming request pays the full cost of
establishing a brand-new connection (and spinning up its own internal
connection pool) from scratch, rather than reusing a connection that's
already established. Under real traffic — many requests arriving
close together — this can quickly exhaust the number of connections
MongoDB's server allows at once, causing later requests to fail or
queue up waiting for a connection slot, purely because of how the
application is managing (or failing to manage) its connections, not
because of anything wrong with the actual queries themselves.

THE FIX
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Create the MongoClient ONCE, outside of any individual request
handler — typically when the application starts up — and reuse that
same client instance for every request that comes in afterward:

  // At application startup, NOT inside a route handler:
  const client = new MongoClient("mongodb://localhost:27017");
  await client.connect();
  const db = client.db("shop");

  // Inside each route handler, reuse the already-connected client:
  app.get("/products", async (req, res) => {
    const products = await db.collection("products").find({}).toArray();
    res.json(products);
  });

WHY THIS WORKS AS AN ANSWER
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A MongoClient's internal connection pool is specifically DESIGNED to
be created once and shared across many operations over the
application's entire lifetime — that's the whole point of a
"connection pool" existing at all. Reconnecting per request throws
that design away and pays the full connection-setup cost on every
single request instead of just once at startup, which is exactly the
same underlying mistake node2-5's MySQL connection-pooling material
warned against, just applied to MongoDB's driver instead.