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Detect "Too many connections" error and show alternate web page

<?php
$link = mysql_connect("localhost", "mysql_user", "mysql_password");
if (mysql_errno() == 1203) {
  // 1203 == ER_TOO_MANY_USER_CONNECTIONS (mysqld_error.h)
  header("Location: http://your.site.com/alternate_page.php");
  exit;
}
?>

<?
ini_set('display_errors', '0');
$link = mysql_connect("localhost", "user", "pass");
if (mysql_errno() == 1040 OR mysql_errno() == 1203) {
    define("DB_HOST", "some_ip:3306");//remote
    define("DB_NAME", "db");  //database_name
    define("DB_USER", "user"); //database user name
    define("DB_PASSWORD","pass");  //database (user) password
}
else
{
    define("DB_HOST", "localhost");
    define("DB_NAME", "db_name2");  //database_name
    define("DB_USER", "user"); //database user name
    define("DB_PASSWORD","pass");  //database (user) password
}
ini_set('display_errors', '1');
//by feha at www.vision.to
?>

You can increase this value in main config file (e.g., /etc/my.cnf) using this syntax:

[mysqld]
set-variable=max_connections=250

A note por PHP developers. You can find this error if your scripts open persistent connections, wich aren't closed even if the script terminates. Use mysql_connect() instead of mysql_pconnect() unless you have a good reason. In particular, check this setting in third-party scripts (such as osCommerce).

Server administrators can disable persistent connections for PHP scripts in php.ini file:

[MySQL]
; Allow or prevent persistent links.
mysql.allow_persistent=Off

Scripts won't fail, they'll just use non-persistent connections silently.

Another symptom for PHP users, the "max_connections" error being returned to the browsers and the "show processlist" filling up with sleeping threads: This can sometimes be alleviated by using the PHP .htaccess option to lower the connect timeout from the default 60 seconds.

php_value mysql.connect_timeout 20

The osCommerce setting mentioned by ?varo is in the catalog/includes/configure.php file:

define('USE_PCONNECT', 'false'); // use persistent connections?

It defaults to true, so mysql_pconnect() is used, and you get the error message "Warning: mysql_pconnect(): Too many connections ..." Change to false for mysql_connect() to be used.

Please note that an instruction of the form
set-variable=max_connections=500
should be placed in the [mysqld] section. Otherwise MySQL will ignore it.

A lot of sites only suffer from this problem when Google or any other search bot is visiting. The best way to resolve this is to add the 'Crawl-delay' parameter in your robots.txt or to set it to a higher number of seconds.

I found that maxing out all available connections is not the only way to have this error. If your disk on your MySQL server is full, you can also get this error. Clearing up disk space will rid of the error.

Full explanation of what I've encountered here: http://www.microshell.com/database/mysql/mysql-too-many-connection-errors/

The following can also be used to change max_connections:

mysql> SET GLOBAL max_connections = 200;

However, this only lasts until the MySQL Server restarts.

On Solaris, you maybe limited to ulimits "open files".

i.e.

do ulimit -a
core file size (blocks, -c) unlimited
data seg size (kbytes, -d) unlimited
file size (blocks, -f) unlimited
open files (-n) 256
pipe size (512 bytes, -p) 10
stack size (kbytes, -s) 8192
cpu time (seconds, -t) unlimited
max user processes (-u) 29995
virtual memory (kbytes, -v) unlimited

Here "open files = 256" which is a very low setting.

At "open files = 1024" I was limited to 214 max connections.

In order the get the 500 + I wanted, I had to set "open files = 4096"

i.e. ulimit -n 4096

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