Randomly raising Internal Server Error 500. How to debug? - ajax

Question

Recently I moved my zend app from simple hosting to azure, configured to process requests via nginx->haproxy->apache chain. nginx, haproxy and apache are docker containers. While loading the application makes 20-30 AJAX requests to different controllers. Here's the common init method I use in controllers:
public function init()
{
$this->sa = System_Auth::getInstance();
$this->data = $this->getRequest()->getParams();
$this->current = $this->sa->getCurrent();
$this->data['customer_id'] = $this->current->customer ? $this->current->customer->id : $this->current->id;
$this->_helper->contextSwitch()
->addActionContext('test', 'json')
->setAutoJsonSerialization(true)
->initContext();
$this->_helper->layout()->disableLayout();
$this->_helper->viewRenderer->setNoRender(true);
}
So there is nothing interesting in it. But every time I load application about 3 or 4 AJAX requests return 500 error. Each time different requests failed with 500 error. Apache and nginx logs are empty. And I can't dump data passed to controller because failed controllers aren't even called.
Anyone has idea?

Answer 1

OK quys, finally it was fixed.
Where happened:
HaProxy and round-robined mariaDB Cluster Master-Master-Master
What happened:
Some queries randomly failed each time
Why:
sql_mode=only_full_group_by was found on one of three mysql servers
Earlier I cleared sql_mode variable on all servers, but it seems later it was restored when one of servers was restarted. And variables aren't replicable, you should manually change them on each server*
*It may be possible but I don't know right now