node中mysql連接池的connectionLimit指什么，它和mysql的最小連接數(shù)和最大連接數(shù)的關(guān)系是什么？

問題1：node中mysql連接池的connectionLimit指什么，它和mysql的最小連接數(shù)和最大連接數(shù)的關(guān)系是什么
問題2：mysql max_connections是什么，max_used_connections是什么，兩者的關(guān)系？為啥max_connecitons可以小于max_used_connections？
問題3：mysql pool.on('connection',callback)在什么情況下觸發(fā)？例如我的這段代碼，什么時候觸發(fā)connection事件？

var mysql = require('mysql');
var http = require('http');
var pool  = mysql.createPool({
    host     : 'localhost',
    user     : 'guest',
    password : '****',
    database : 'test',
    connectionLimit: 2
});
pool.on('acquire', function(connection){
    console.log('connection %d accuired', connection.threadId);
});
pool.on('connection', function (connection) {
    connection.query('SET SESSION auto_increment_increment=1')
});
pool.on('enqueue', function () {
    console.log('Waiting for available connection slot');
});
// pool.end(function (err) {
//     console.log('end!')
// });
http.createServer(function(req,res){
    if(req.url==='/url1') {
        pool.getConnection(function(err, connection) {
            connection.query('select * from teacher', function(error,results,fields) {
                if (error) throw error;
                res.end(JSON.stringify(results));
                // connection.release();
            });
        })
    }
    if(req.url==='/url2') {
        pool.getConnection(function(err, connection) {
            connection.query('select * from student', function(error,results,fields) {
                if (error) throw error;
                res.end(JSON.stringify(results));
                connection.release();
            });
        })
    }
}).listen(8001);