If MySQL is dropping connections in a PHP application, and MySQL connection limit is set above the number of concurrent users in the application, which other factors can contribute to this behavior? Also, analyzing moodle logs (the only app running at these servers), yesterday I had 4 times more activity and it didn’t drop anything, but today there were times where it was frustrating the number of dropped connections.
My main question here is why the database is rejecting connections when it didn’t before with 4 times more activity (and everything it’s the same, I didn’t change anything in between).
Some background: I’ve got 2 servers contracted at my hosting:
- shared server running Debian Linux/PHP 5.3 (FastCGI)
- VPS running Debian Linux/MySQL 5.1.39
On this environment I’m running only moodle 1.9.12 (for the database connection using adoDB and persistent connections), php part on the shared server, database on the VPS. I suspect that, by PHP running on a shared server, other hosting accounts are affecting me (the database rejecting connections I mean, I really don’t care about RAM/CPU).
Reading about the issue I’ve seen in places that persistent connections don’t work well with PHP as CGI/FastCGI and that if both servers are in the local lan it really doesn’t matter using persistent or not persistent connections because of the connection is going to be quick anyway. So now I’m using not persistent connections. I guess that may be part of the problem, but I fail to understand why it worked with more load. Which PHP/Apache settings are involved here?
Since your database and web server are on two different machines, there are two other possible causes: the network in between, and the network layer of the operating system.
If you did not change anything in your configuration and it worked with higher loads before, it is more likely to be an issue with the network connectivity between the two machines. Since the database machine is a VPS you also don’t know how much real network load it is handling. If your ISP has competent support personnel (which unfortunately isn’t always the case) it can’t hurt to ask them if they have an explanation.
The same goes for your “shared” web-server. While it is unlikely, it is not impossible that it is a an issue of too many connections on that machine.
It would also help to know how exactly you are measuring dropped connections. If you are looking at the aborted-connections counter of mySQL, it is not necessarily a measure of an actual problem: http://dev.mysql.com/doc/refman/5.1/en/communication-errors.html. A user aborting a page load already may increase this counter.
If PHP throws an error, because it could not connect to the server or lost connection to the server during a query, then it is an issue.