Database Connection Pooling: Why Your App Slows Down Under Load
Most apps work fine with a handful of users. Under load, they slow to a crawl because of connection pool misconfiguration.
What Is a Connection Pool?
Opening a DB connection is expensive: TCP handshake, auth, session setup — 20-100ms per connection. A pool maintains open connections that are reused across requests.
The Classic Mistake
// Bad: new connection per request = 50ms overhead every time
app.get('/users', async (req, res) => {
const client = new pg.Client(connectionString);
await client.connect();
const result = await client.query('SELECT * FROM users');
await client.end();
res.json(result.rows);
});
// Good: pool created once, reused for every request
const pool = new pg.Pool({ connectionString, max: 20 });
app.get('/users', async (req, res) => {
const result = await pool.query('SELECT * FROM users');
res.json(result.rows);
});
Pool Sizing
const pool = new pg.Pool({
max: 20, // Maximum connections
min: 2, // Keep 2 warm always
idleTimeoutMillis: 30000, // Remove idle after 30s
connectionTimeoutMillis: 5000, // Fail fast if exhausted
});
Sweet spot: 10-25 connections per app instance. Too small = queuing. Too large = DB overwhelmed.
Serverless: Use a Pooler
Serverless functions can't use traditional pools — hundreds of cold starts can exhaust DB connections. Use PgBouncer, Supabase pooler, or Prisma Accelerate as a proxy.
Monitor Pool Health
setInterval(() => {
metrics.gauge('db.pool.total', pool.totalCount);
metrics.gauge('db.pool.idle', pool.idleCount);
metrics.gauge('db.pool.waiting', pool.waitingCount);
}, 5000);
Connection pooling, Prisma setup, and DB observability come pre-configured in the AI SaaS Starter Kit.
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