How to fix too many in _connectionQueue of Pool? - mysql

Question

My very simple Node.js code doesn't seem like its connection pool work as it's supposed to do. _connectionQueue of Pool object just gets longer and longer infinitely, and app dies. I mean it does make a pool and there are pre-made connections already, but they are not reusable or insert requests are too many and fast? I'm not sure..
I've tried to put some more connectionLimit like following :
let state = { pool: null }
export const connect = () => {
state.pool = mysql.createPool({
connectionLimit: 200,
host: process.env.DATABASE_HOST || 'localhost',
user: process.env.DATABASE_USER || 'root',
password: process.env.DATABASE_PASSWORD || 'password',
database: process.env.DATABASE_NAME || 'database'
})
}
export const get = () => state.pool
Mostly given job of this server is subscription and insertion. It subscribes several MQTT topics and just tries to insert messages into RDB. About 100 messages arrives every second, and that code looks like this.
mqttClient.on('message', function (topic, message) {
if(topic.includes('sensor')){
try {
const data = JSON.parse(message.toString())
if(validate(data.uuid)){
const params = [data.a, data.b, data.c, ...]
sensor.setStatus(params)
}
} catch(err){
console.error(err)
}
}
}
export const setStatus = (params) => {
const SQL = `INSERT INTO ...`
db.get().query(SQL, params, (err, result) => {
if (err) console.error(err)
})
}
Then, I see this through chrome-devtools
Object
pool: Pool
config: PoolConfig {acquireTimeout: 10000, connectionConfig: ConnectionConfig, waitForConnections: true, connectionLimit: 200, queueLimit: 0}
domain: null
_acquiringConnections: []
_allConnections: (200) [PoolConnection, PoolConnection, …]
_closed: false
_connectionQueue: (11561) [ƒ, ƒ, ƒ, ƒ, …]
_events: {}
_eventsCount: 0
_freeConnections: []
_maxListeners: undefined
__proto__: EventEmitter
__proto__: Object
I've put console.log into setStatus like following :
export const setStatus = (params) => {
const SQL = `INSERT INTO ...`
console.log(`allConnections=${db.get()._allConnections.length}, connectionQueue=${db.get()._connectionQueue.length}`)
db.get().query(SQL, params, (err, result) => {
if (err) console.error(err)
})
}
, and got these.
allConnections=200, connectionQueue=29
allConnections=200, connectionQueue=30
allConnections=200, connectionQueue=31
allConnections=200, connectionQueue=32
allConnections=200, connectionQueue=33
allConnections=200, connectionQueue=34
...
It seems like server created a connection pool very well, but not using those connections. Instead, trying to create a new connection more and more all the time and those requests just get stuck in _connectionQueue.

Answer 1

It appears you are creating a new pool every time you'd like to make a query. The common model is to create a pool once when the application starts, then use connections from that pool as needed (one pool, many connections).
Also if you're using a simple DB model you can simplify access to the pool by making it global. Below is an alternate to your code you might try:
app.js
const mysql = require('mysql');
const connection = mysql.createPool({
host: process.env.DB_HOST || '127.0.0.1',
user: process.env.DB_USER || 'local_user',
password: process.env.DB_PASSWORD || 'local_password',
database: process.env.DB_NAME || 'local_database'
});
global.db = connection;
modules.js
export const setStatus = (params) => {
let SQL = `INSERT INTO ...`
db.query(SQL, params, (err, result) => {
if (err) console.error(err)
console.log(result)
})
}
Documentation for further reference :: https://github.com/mysqljs/mysql#pooling-connections
Edit 1 - Log pool events
db.on('acquire', function (connection) {
console.log('Connection %d acquired', connection.threadId);
});
db.on('connection', function (connection) {
console.log('Pool id %d connected', connection.threadId);
});
db.on('enqueue', function () {
console.log('Waiting for available connection slot');
});
db.on('release', function (connection) {
console.log('Connection %d released', connection.threadId);
});