PostgreSQL Error 53300: Too many connections

Identify: does your log match?

psql: error: connection to server at "10.0.4.12", port 5432 failed: FATAL:  sorry, too many clients already

FATAL:  remaining connection slots are reserved for roles with the SUPERUSER attribute

1. High

   Serverless functions each open a pool

   Every Lambda or Vercel function instance creates its own pool of N connections. 50 concurrent instances × 10 connections exceeds the default max_connections of 100.

2. Medium

   Leaked connections never released

   Code calls pool.connect() and forgets client.release() on an error path. The pool grows until the server is full.

3. Medium

   Dev hot-reload creates new clients

   Next.js or nodemon reloads re-run new PrismaClient() or new Pool() at module scope, leaving old pools connected.

Fix

const client = await pool.connect();
const { rows } = await client.query('SELECT * FROM jobs');
client.release(); // skipped if query throws

const pool = new Pool({ max: 5, idleTimeoutMillis: 10000 });

const client = await pool.connect();
try {
  const { rows } = await client.query('SELECT * FROM jobs');
  return rows;
} finally {
  client.release();
}

Verify: reproduce it on purpose

$ for i in $(seq 1 120); do psql -c "SELECT pg_sleep(30)" >/dev/null & done; wait

Prevent

• Budget connections: instances × pool max + admin headroom < max_connections.
• Set idle_in_transaction_session_timeout to kill stuck sessions.
• Use a single PrismaClient/Pool per process (globalThis in dev).