What is connection pooling and how does it work in Node.js?

Question

Accepted Answer

**Connection pooling** — reuses database connections instead of creating new ones for each request.

**Why it matters:**
- Creating connections is expensive.
- Limits total connections to DB.
- Improves performance under load.

**PostgreSQL with pg:**
```javascript
const { Pool } = require("pg");

const pool = new Pool({
    host: "localhost",
    port: 5432,
    database: "mydb",
    user: "admin",
    password: "secret",
    max: 20,            // max connections
    idleTimeoutMillis: 30000,
    connectionTimeoutMillis: 2000,
});

// Use pool
const result = await pool.query("SELECT * FROM users");

// Get a client for transactions
const client = await pool.connect();
try {
    await client.query("BEGIN");
    await client.query("INSERT INTO users VALUES ($1)", [name]);
    await client.query("COMMIT");
} catch (err) {
    await client.query("ROLLBACK");
    throw err;
} finally {
    client.release();
}
```

**Key points:**
- Set max connections based on DB limits.
- Always release clients back to pool.
- Monitor pool stats with pool.totalCount, pool.idleCount.

Node.js: What is connection pooling and how does it work in Node.js?