本文介绍golang 中连接MySQL时,如何设置最大连接数。
文中例子连接MySQL用的package是github.com/go-sql-driver/mysql
.
设置最大连接数的接口是
func (db *DB) SetMaxOpenConns(n int)
设置连接MySQL可以打开的最大连接数。
如果n <= 0,表示打开的连接数没有限制。
默认为0,也就是不限制连接数。
另一个与连接数相关的参数是MaxIdleConns
,表示最大空闲连接数。
如果MaxIdleConns
大于0,且大于MaxOpenConns
,最后MaxIdleConns
将最后等于MaxOpenConns
。
1.测试例子
首先设置最大打开的连接数为1,接着开启20个goroutine,每个goroutine执行sql语句,并打印执行sql使用的连接的connection id。并执行耗时的sql语句占用连接,观察结果。
package main
import (
"database/sql"
"log"
_ "github.com/go-sql-driver/mysql"
)
var DB *sql.DB
var dataBase = "root:Aa123456@tcp(127.0.0.1:3306)/?loc=Local&parseTime=true"
func Init() {
var err error
DB, err = sql.Open("mysql", dataBase)
if err != nil {
log.Fatalln("open db fail:", err)
}
DB.SetMaxOpenConns(1)
err = DB.Ping()
if err != nil {
log.Fatalln("ping db fail:", err)
}
}
func main() {
Init()
for {
for i:=0; i < 20; i++ {
go one_worker(i)
}
select {
}
}
}
func one_worker(i int) {
var connection_id int
err := DB.QueryRow("select CONNECTION_ID()").Scan(&connection_id)
if err != nil {
log.Println("query connection id failed:", err)
return
}
log.Println("worker:", i, ", connection id:", connection_id)
var result int
err = DB.QueryRow("select sleep(10)").Scan(&result)
if err != nil {
log.Println("query sleep connection id faild:", err)
return
}
}
output
2019/10/02 18:14:25 worker: 2 , connection id: 55
2019/10/02 18:14:25 worker: 17 , connection id: 55
2019/10/02 18:14:25 worker: 11 , connection id: 55
2019/10/02 18:14:35 worker: 3 , connection id: 55
2019/10/02 18:14:45 worker: 0 , connection id: 55
2019/10/02 18:14:45 worker: 4 , connection id: 55
2019/10/02 18:14:45 worker: 5 , connection id: 55
2019/10/02 18:15:05 worker: 7 , connection id: 55
2019/10/02 18:15:25 worker: 15 , connection id: 55
2019/10/02 18:15:25 worker: 6 , connection id: 55
2019/10/02 18:15:35 worker: 13 , connection id: 55
2019/10/02 18:15:45 worker: 19 , connection id: 55
2019/10/02 18:15:45 worker: 10 , connection id: 55
2019/10/02 18:15:45 worker: 12 , connection id: 55
2019/10/02 18:15:55 worker: 14 , connection id: 55
2019/10/02 18:16:15 worker: 8 , connection id: 55
2019/10/02 18:16:35 worker: 18 , connection id: 55
2019/10/02 18:16:35 worker: 1 , connection id: 55
2019/10/02 18:17:05 worker: 16 , connection id: 55
2019/10/02 18:17:35 worker: 9 , connection id: 55
从输出结果可以看到,20个goroutine轮流使用一个连接(connection id 为55)执行sql语句。
2.查看连接
使用show processlist查看连接
mysql> show processlist;
+----+------+-----------------+------+---------+------+------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+------+---------+------+------------+------------------+
| 20 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 55 | root | localhost:59518 | NULL | Query | 5 | User sleep | select sleep(10) |
+----+------+-----------------+------+---------+------+------------+------------------+
2 rows in set (0.00 sec)
使用netstat 查看连接
netstat -an | grep 3306
tcp4 0 0 127.0.0.1.3306 127.0.0.1.59518 ESTABLISHED
tcp4 0 0 127.0.0.1.59518 127.0.0.1.3306 ESTABLISHED
tcp46 0 0 *.3306 *.* LISTEN
可以看到,只建立了1个连接。