To ensure high availability on the zabbix database, I am using keepalived to move a virtual IP from one mysql-server to another, in case of failure or maintenance. Obviously the zabbix_server detects that it's database is no longer responding and so it should just reconnect. I have found out that this does not always work, and I discovered it because suddenly I was not getting any alerts from Zabbix. When I simply restart the zabbix-server, I get all alerts, at once, which were supposed to have been sent out earlier.