I'm facing a really weird behaviour while testing persistent connections from php to mysql. I have a small script that looks like this:
<?php
$db = new mysqli('p:db-host','user','pass','schema');
$res = $db->query('select * from users limit 1');
print_r($res->fetch_assoc());
My setup is :
- OS: CentOS 7.3
- PHP/7.1.18
- php-fpm
- nginx/1.10.2
- MySQL-5.6.30
I tried to do some requests with ab:
$ ab -c 100 -n 500 http://mysite/my_test_script.php
PHP-FPM was configured to have 150 workers ready, and i saw what i was expecting, 150 established connections to mysql, which stayed open after the ab finished. I launched ab once again, and the behaviour was still the same, 150 connections, no new connections where opened. All fine. Then i created a script which did the the same exact requests, same IP, same HTTP headers, but used curl to make the request, and BOOM i had 300 connections on mysql instead of 150. I launched the script again, i got still 300 connections. Subsequent runs of the same script didn't increase the number of connections. Did anyone ever faced anything like this? Does anyone know what could make php open more connections than needed? Am I missing something obvious?
If it's not clear what i'm asking, please comment below and i will try to better my explain problem.
P.S. I tried this with PDO too, same behaviour.
EDIT: My tests where not accurate
After further testing i noticed that my first tests where not accurate. I was in a multi-tenant environment and different connections ( different schema ) where initialized when i launched ab. In my case the php documentation was a bit missleading, it says:
PHP checks if there's already an identical persistent connection (that remained open from earlier) - and if it exists, it uses it. If it does not exist, it creates the link. An 'identical' connection is a connection that was opened to the same host, with the same username and the same password (where applicable).
http://php.net/manual/en/features.persistent-connections.php
Maybe its i obvious to everyone, I don't know, it was not for me. Passing the 4th parameter to mysqli made php consider connections not identical. Once i changed my code to something like this:
<?php
$db = new mysqli('p:db-host','user','pass');
$db->select_db('schema');
$res = $db->query('select * from users limit 1');
print_r($res->fetch_assoc());
The application started to behave as i expected, one connection per worker.
