Next.js Prisma Singleton Pattern - Fix Hot Reload Connection Leaks

Problem

In Next.js dev mode, every file save triggers a hot reload that creates a new PrismaClient() instance. Database connections pile up until you hit a Too many connections error.

Solution

// lib/prisma.ts
import { PrismaClient } from '@prisma/client'

const globalForPrisma = globalThis as unknown as {
  prisma: PrismaClient | undefined
}

const prisma = globalForPrisma.prisma ?? new PrismaClient()

if (process.env.NODE_ENV !== 'production') globalForPrisma.prisma = prisma

export default prisma

Key Points

• globalThis survives hot reloads. Storing the Prisma instance here ensures the existing connection is reused when modules are re-evaluated.
• In production, modules load only once, so storing on globalThis is unnecessary. The NODE_ENV !== 'production' guard handles this.
• Five lines of code prevent dozens of zombie database connections during development.