How do I stop Mongoose creating a new connection on every request in Next.js?

Module reloads in dev and separate serverless instances each re-run your connect code. Cache the connection promise on globalThis so it survives hot reload.

// lib/mongoose.ts
import mongoose from "mongoose";

const uri = process.env.MONGODB_URI;
if (!uri) throw new Error("MONGODB_URI is not set");

type Cache = { conn: typeof mongoose | null; promise: Promise<typeof mongoose> | null };
const g = globalThis as unknown as { _mongoose?: Cache };
const cache: Cache = (g._mongoose ??= { conn: null, promise: null });

export async function connectDB() {
  if (cache.conn) return cache.conn;
  cache.promise ??= mongoose.connect(uri!, {
    bufferCommands: false,
    maxPoolSize: 10,
  });
  try {
    cache.conn = await cache.promise;
  } catch (e) {
    cache.promise = null; // allow retry
    throw e;
  }
  return cache.conn;
}

Use it:

import { connectDB } from "@/lib/mongoose";
import { User } from "@/models/User";

export async function GET() {
  await connectDB();
  return Response.json(await User.find().lean());
}

Model re-registration

Hot reload also causes OverwriteModelError. Guard it:

export const User = mongoose.models.User || mongoose.model("User", schema);

Serverless (Vercel) tips

• Keep maxPoolSize small (5 to 10) because every concurrent function instance opens its own pool.
• On Atlas, allow the needed IPs, or use a connection pooling-friendly plan.
• With the native mongodb driver, the same idea applies: cache the MongoClient promise on globalThis in development.

Pitfall: calling mongoose.connect at module top level without caching still works in production but causes the duplicate connections in dev.