I have a website based on a PHP/Laravel back-end, using MySQL as the database. Recently I have encountered "Too many connections" issues and have been doing some debugging to find out where the bottleneck is.
During my testing, I saw that a certain GET HTTP request was sometimes fired repetitively (dozens of times within a minute), and that this seemed to slow down the global database queries processing, causing connections to remain open for much longer.
I was very surprised, though, to see the timing of responses. Here is my Laravel debugging log output after firing the request 10 times simultaneously from the developer console. The Starting marker is right after entering the controller method, the Returning marker is right before returning the response.
[2019-07-30 09:02:09] dev.DEBUG: Starting query 1
[2019-07-30 09:02:17] dev.DEBUG: Returning from query 1 after 7.82s
[2019-07-30 09:02:18] dev.DEBUG: Starting query 2
[2019-07-30 09:02:26] dev.DEBUG: Returning from query 2 after 7.82s
[2019-07-30 09:02:26] dev.DEBUG: Starting query 3
[2019-07-30 09:02:29] dev.DEBUG: Starting query 4
[2019-07-30 09:02:29] dev.DEBUG: Starting query 5
[2019-07-30 09:02:29] dev.DEBUG: Starting query 6
[2019-07-30 09:02:30] dev.DEBUG: Starting query 7
[2019-07-30 09:02:31] dev.DEBUG: Starting query 8
[2019-07-30 09:02:45] dev.DEBUG: Returning from query 3 after 18.7s
[2019-07-30 09:02:46] dev.DEBUG: Starting query 9
[2019-07-30 09:02:56] dev.DEBUG: Returning from query 4 after 26.4s
[2019-07-30 09:02:56] dev.DEBUG: Returning from query 5 after 26.5s
[2019-07-30 09:02:56] dev.DEBUG: Returning from query 6 after 26.6s
[2019-07-30 09:02:56] dev.DEBUG: Returning from query 7 after 26.1s
[2019-07-30 09:02:57] dev.DEBUG: Returning from query 8 after 26.2s
[2019-07-30 09:02:57] dev.DEBUG: Starting query 10
[2019-07-30 09:03:02] dev.DEBUG: Returning from query 9 after 16.0s
[2019-07-30 09:03:06] dev.DEBUG: Returning from query 10 after 8.7s
Is there any reason why, for 10 simultaneous calls, Laravel would fully process the first one, then the second one, then 3-8 in parallel but with 3 finishing before others, then.. etc?