Connection Pool

A connection pool is a managed set of pre-established connections (to a database, cache, or external service) reused across requests instead of created and destroyed each time. Pooling slashes latency and resource consumption in high-throughput applications.

Why Connection Pools Matter

Establishing a new database connection is expensive — TCP handshake, Transport Layer Security (TLS) negotiation, and authentication can take 50-200ms. Without a pool:

• Every request pays the connection setup cost
• Under load, you can exhaust the database’s maximum connections
• Connection storms during traffic spikes can crash your database

With a pool, connections are created once and reused thousands of times over.

How a Connection Pool Works

1. The pool creates a set of connections at startup (or lazily on first use)
2. When a request needs a connection, it borrows one from the pool
3. When the request is done, the connection is returned to the pool (not closed)
4. If all connections are in use, new requests wait in a queue

// Without pool: new connection per request (~100ms overhead)
app.get('/api/data', async (req, res) => {
  const client = new Client(DATABASE_URL);
  await client.connect();
  const result = await client.query('SELECT ...');
  await client.end();
  res.json(result.rows);
});

// With pool: reuses connections (~0ms overhead)
const pool = new Pool({ connectionString: DATABASE_URL, max: 20 });

app.get('/api/data', async (req, res) => {
  const result = await pool.query('SELECT ...');
  res.json(result.rows);
});

Pool Configuration

Related Terms

• Timeout
• Worker
• Background Job