MySQL Connection Errors

There could be many reasons why a connection to MySQL server can fail, like

  • Networking Problem
  • Server itself could be down
  • Authentication Problems
  • Maximum Connection Errors allowed.

Of all the errors, this thread will discuss Maximum Connection Errors.
This particular parameter max_connect_errors defines the no. of connection errors a particular host can make before it is banned. Yes Banned! This is a feature that MySQL provides to limit erroneous clients.

By default 10 maximum connection errors are allowed per host. You can check using foll. command.

mysql> show variables like 'max_connect_errors';
+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| max_connect_errors | 10    |
+--------------------+-------+
1 row in set (0.02 sec)

You can set the above variable in the same manner as setting max_connections, ie in my.cnf file.

[mysqld]
max_connect_errors=100
...

Once particular host exceeds the no. of errors, a particular command is required to be given to the server to reset the host connect errors and indirectly allow the hosts to connect to the server. This command flush hosts will help you do that.

In our scenario, where there are 300 connections per/second, and each connection-query is taking some processing time before connection is closed, There could be a time where the Server is busy, and starts to queue processes, in such a case the connection would be still held until its query is processed. So when a new connection is required, the server may reject it considering the connections that are currently active. ie. We’d set max_connections to 300, so every new connection request could be denied adding to the connection errors, which could mean the host could be blocked. Considering a Web Server + MySQL Server scenario, the web server’s host could be banned until a flush hosts is fired at the MySQL Server.

So in such a case, it is necessary to increase the no. of simultaneous connections that can be made to the server by changing the max_connections parameter. Still if the load on MySQL server is high, and you are getting Connection errors, its time to load balance the Server.

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