If you’re using NGinx spawn-cgi or FPM with PHP and calling mysql_pconnect, you are likely going to experience frequent database crashes and “Too many connections” errors.
This took a while to trace, but once you understand the issue, it all makes sense.
mysql_pconnect opens a “persistent” connection to the database. From the documentation: “the connection to the SQL server will not be closed when the execution of the script ends. Instead, the link will remain open for future use (mysql_close() will not close links established by mysql_pconnect()).”
The issue is that FPM keeps a number of php-cgi processes running in the background to process php scripts. These php-cgi processes never die and so MySQL connections keep open forever…
Sooner or later, you are going to run out of MySQL connections (or worse yet - run out of file descriptors) and that’s when all hell breaks loose.
And if that’s not enough, after doing some digging into mysql_pconnect I found a few additional reasons NOT to use mysql_pconnect:
If you use mysql_pconnect on a machine that has a local database and you are connecting to a remote database, PHP will try to use the same mysql connection for both databases.
Temporary tables don’t work with persistent connections (they are only visible to the connection that was used to open the table)
Setting charset variables on a persistent connection, is going to impact all future queries on that connection as well
Calling mysql_pconnect twice (in the same script) with different parameters doesn’t work as expected
PHP 4.1 on Apache running with MySQL persistent connections, is known to memory leak (not flushing properly).
Bottom line, never ever use mysql_pconnect.
Replace all occurrences of mysql_pconnect with mysql_connect in your code and in your php.ini file, prevent persistent connections:
[MySQL] ; Allow or prevent persistent links. mysql.allow_persistent = Off