关于MySQL DNS解析探究之二：unauthenticated user

把这篇没写完的文章写完，2015年的事就不留到2016了

开启DNS解析

mysql> show variables like 'skip_name_resolve';
+-------------------+-------+
| Variable_name     | Value |
+-------------------+-------+
| skip_name_resolve | OFF   |
+-------------------+-------+

经过代码模拟用户查询请求，不断的创建链接执行sql查询，而后经过show processlist命令查看链接状况：

JAVA代码一

/**
 *  MySQL Problem DNS lead to 'unauthenticated  user'
 */
public class MysqlDNSUnauthUser {

    public static final Logger LOG = LoggerFactory
            .getLogger(MysqlDNSUnauthUser.class);
    /**
     * 执行查询操做
     * @return
     */
    public static long query() {

        long start = System.currentTimeMillis();
        Connection conn = null;
        Statement stmt = null;

        String sql = "select 1";
        try {
            // 建立新的链接
            conn = DBUtils.getConnection();
            stmt = conn.createStatement();
            stmt.executeQuery(sql);

        } catch (SQLException e) {
            LOG.error(e.getMessage(), e);
        } finally {
            DBUtils.close(conn, stmt);
        }

        long end = System.currentTimeMillis();
        return end - start;
    }

    public static void main(String[] args) {

        // 描述统计
        DescriptiveStatistics stats = new DescriptiveStatistics();
        int n = 0;
        while(true) {
            n++;
            long time = query();
            stats.addValue(time);
            if(n == 1000) break;
        }

        LOG.info(stats.toString());
    }
}

打印结果以下：

n: 1000
min: 10.0
max: 421.0
mean: 14.202000000000002
std dev: 16.242444797905648
median: 12.0
skewness: 20.270437547223214
kurtosis: 463.94461541900915

发现最大值421ms

mysql> show processlist;
+------+----------------------+-------------------+------+---------+------+-------+------------------+
| Id   | User                 | Host              | db   | Command | Time | State | Info             |
+------+----------------------+-------------------+------+---------+------+-------+------------------+
|    7 | root                 | localhost         | test | Query   |    0 | NULL  | show processlist |
| 2763 | unauthenticated user | 192.168.4.2:54804 | NULL | Connect | NULL | login | NULL             |
+------+----------------------+-------------------+------+---------+------+-------+------------------+

能够发现其中出现unauthenticated user的用户

关闭DNS解析

在/etc/my.cnf中的[mysqld]节点增长skip_name_resolve，重启mysqld服务

mysql> show variables like 'skip_name_resolve';
+-------------------+-------+
| Variable_name     | Value |
+-------------------+-------+
| skip_name_resolve | ON    |
+-------------------+-------+

再次执行JAVA代码一，结果以下：

n: 1000
min: 7.0
max: 413.0
mean: 10.306999999999988
std dev: 16.131117877070835
median: 9.0
skewness: 20.655330340756084
kurtosis: 471.9124123214076

对比上一次开启DNS解析，此次的结果性能有所提升，上一次的均值是14ms，此次为10ms

mysql> show processlist;
+------+----------------------+-------------------+------+---------+------+-------+------------------+
| Id   | User                 | Host              | db   | Command | Time | State | Info             |
+------+----------------------+-------------------+------+---------+------+-------+------------------+
|    2 | root                 | localhost         | NULL | Query   |    0 | NULL  | show processlist |
| 3891 | unauthenticated user | 192.168.4.2:59502 | NULL | Connect | NULL | login | NULL             |
+------+----------------------+-------------------+------+---------+------+-------+------------------+

能够发现仍然出现了unauthenticated user的用户

目前可得出以下结论：

DNS解析确实会影响链接创建的性能，可是DNS解析开启与否，与出现unauthenticated user没有关系

猜测unauthenticated user的出现应该是链接在创建的过程当中出现的临时状态