Connection pooling
A pool keeps a set of open database connections ready to reuse instead of opening a new one per request.
In this page:
Syntax
const pool = mysql.createPool({ host: 'host', user: 'user', database: 'database', connectionLimit: n });
const [rows] = await pool.query('SQL', [values]);Connection pooling
Opening connections is slow, so pools hand out idle ones and take them back when released. You set a maximum size. Requests wait when all connections are busy. Always release connections you check out.
Note:
Size the pool for your database's connection limit.
Example: Connection pooling
class Pool {
constructor(max) { this.max = max; this.inUse = 0; this.waiting = []; }
async acquire() {
if (this.inUse < this.max) { this.inUse++; return; }
await new Promise((r) => this.waiting.push(r));
}
release() {
const next = this.waiting.shift();
if (next) next(); else this.inUse--;
}
}
const pool = new Pool(2);
(async () => {
const job = async (n) => { await pool.acquire(); console.log("job", n, "running, inUse:", pool.inUse); await new Promise((r) => setTimeout(r, 5)); pool.release(); };
await Promise.all([job(1), job(2), job(3)]);
console.log("all done, inUse:", pool.inUse);
})();
// Output:
// job 1 running, inUse: 2
// job 2 running, inUse: 2
// job 3 running, inUse: 2
// all done, inUse: 0⚠️ Run this in your own terminal or Node.js environment.
Related Topics
Common Mistakes
- Not releasing connections
- Making the pool too large
- Creating a new pool per request
Chapter Summary
- Pools reuse connections
- Set a maximum size
- Always release
- One pool per app
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