MySQL – How to Find the Last SQL Statement Executed for a Given Connection

max-connectionsmysql-5.6

MySQL running on Windows 7 64 bit.

I'm trying to track down a leak of connections, I'm pretty sure the only ASP.NET app using the server is at fault here.

We received the "Too many connections" error and checking the server we saw many connections with status Sleep.

I know that, fiddling with my.cf, MySQL can be forced to close sleeping connection after a shorter time, but I would rather fix the actual problem instead of the symptoms.

So I was wondering if there's a way to retrieve the SQL statement that was last executed on a given connection.
I think if I could do that on older sleeping connections, I might have an easier time tracking down this leak.

Of course, if anyone can think of an alternative solution for this scenario, any suggestion is very welcome.

Best Answer

You can enable "general query log", which will log each executed query, along with its session/connection ID, so if you grep the connection ID, you will have all the queries on one session.

HOWEVER, 'sleep'ing connections are not the cause of max connection error, but a result of it. They are not harmful to stay there. In fact, they are established connections that can be re-used by future sessions.

What would be more useful in this case is to search the slow query log, and check the innodb status to get information about locks and dead locks that have happened:

SHOW ENGINE INNODB STATUS;

Related Solutions

MySQL – What Are User Connections and When Are They Created and Destroyed

Please notice the makeup of mysql.user:

mysql> show create table mysql.user\G
*************************** 1. row ***************************
       Table: user
Create Table: CREATE TABLE `user` (
  `Host` char(60) COLLATE utf8_bin NOT NULL DEFAULT '',
  `User` char(16) COLLATE utf8_bin NOT NULL DEFAULT '',
  `Password` char(41) CHARACTER SET latin1 COLLATE latin1_bin NOT NULL DEFAULT '',
  `Select_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Insert_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Update_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Delete_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Create_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Drop_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Reload_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Shutdown_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Process_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `File_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Grant_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `References_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Index_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Alter_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Show_db_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Super_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Create_tmp_table_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Lock_tables_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Execute_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Repl_slave_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Repl_client_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Create_view_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Show_view_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Create_routine_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Alter_routine_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Create_user_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Event_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Trigger_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Create_tablespace_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `ssl_type` enum('','ANY','X509','SPECIFIED') CHARACTER SET utf8 NOT NULL DEFAULT '',
  `ssl_cipher` blob NOT NULL,
  `x509_issuer` blob NOT NULL,
  `x509_subject` blob NOT NULL,
  `max_questions` int(11) unsigned NOT NULL DEFAULT '0',
  `max_updates` int(11) unsigned NOT NULL DEFAULT '0',
  `max_connections` int(11) unsigned NOT NULL DEFAULT '0',
  `max_user_connections` int(11) unsigned NOT NULL DEFAULT '0',
  `plugin` char(64) COLLATE utf8_bin DEFAULT '',
  `authentication_string` text COLLATE utf8_bin,
  PRIMARY KEY (`Host`,`User`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_bin COMMENT='Users and global privileges'
1 row in set (0.00 sec)

Notice the column max_user_connections. It puts a moratorium on the number of connections a user may make per hour. Check and make sure the user '557574_prod' isn't connecting to mysql a ton of times within less than an hour.

This is used in conjunction with the option max_user_connections that you can set in /etc/my.cnf

Please change either max_user_connections in /etc/my.cnf or, if the individual user has this column set, that column mysql.user.max_user_connections. If you have no config control over my.cnf and you cannot do SQL against the mysql schema, then you beg and implore your hosting company to raise those limits for you.

I believe you can set the max_user_connections as a session variable, but you may need certain privileges, probably SUPER.

As for establish connections, you need to keep those connections open. Perhaps sending a heartbeat of some kind by doing a SELECT. But guess what ??? The db columns max_questions monitor SELECT queries per hour and max_updates monitors INSERTs, UPDATEs, DELETEs per hour. YOu may need to check these columns in mysql.user. You can quickly check those number with:

SHOW GRANTS;

I am not sure if the the user's per-hour connection limits are per thread or per user. You could answer that quickly by connecting to the DB aggressively using 2 or 3 DB connecitons at the same time and the error message should manifest itself. It is up you how to count the number of connections, perhaps "SHOW STATUS LIKE 'Connections';" It should reach either the max limits of the connection within the one connections or it could be the sum of all connections' connection status value.

Please also check the wait_timeout and interactive_timeout values. The default is 28800 in a standalone server. They may be set already. You cannot set them within a current connection. You must be able to set them as follows: (for new conenctions coming)

SET GLOBAL interactive_timeout = 86400;
SET GLOBAL wait_timeout = 86400;

If you cannot run even these commands, my condolences !!! :(

Mysql – multiple connections on MySQL database with no users on httpd

You will need to look into how your connection pool is configured in your PHP web app. Even a fully functional connection pool will often keep many connections to a database open even when they are not in use. Opening connections to the database can be very expensive for a web app and it makes sense to have some unused ones sitting around.

Most connection pools have settings for things like:

Minimum number of connections to keep open
Maximum number of connections to keep open
Number of connections to open when new connections are needed

There is also the possibility that your web app is not releasing connections back to the connection pool properly. When you are done with your connections, you need to close them to let the pool know that they are available. Not doing so is a a kind of resource leak and you will eventually run out of connections.

Best Answer

Related Solutions

MySQL – What Are User Connections and When Are They Created and Destroyed

Mysql – multiple connections on MySQL database with no users on httpd

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