What is database connection pooling and why is it critical for production?

Question

At a scaling startup: "We create a new DB connection per request. Under load we get 'connection refused' errors."

Interview OS Community · Accepted Answer

## Without Pooling
100 requests → 100 connections → 50ms each to establish → DB max 100 → request 101 fails

## With Pool
Startup: create 20 connections. Request: borrow (0ms) → query → return. All 100 requests share 20 connections.

```typescript
// Prisma built-in pooling via URL
DATABASE_URL="postgresql://...?connection_limit=20&pool_timeout=10"
```

## Singleton pattern (prevent multiple pools in dev)
```typescript
const globalForPrisma = globalThis as unknown as { prisma: PrismaClient };
export const prisma = globalForPrisma.prisma || new PrismaClient();
if (process.env.NODE_ENV !== "production") globalForPrisma.prisma = prisma;
```

Pool size guideline: CPU cores × 2 + disk spindles. Typical: 10-30.

Interview OS

What is database connection pooling and why is it critical for production?

Question Details

Suggested Solution

Without Pooling

With Pool

Singleton pattern (prevent multiple pools in dev)

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