I want to validate a setup with Redis caching in combination with multiple webservers.
The goal is to keep page load times below 15 secs with 600 users running a quiz simultaneously which worked with memcached but we had to switch to Redis.
We have a setup with 4 webservers running Apache and PHP-FPM 7.0 running RHEL6 with a shared document root on NFS. In front of the webservers there is a F5 loadbalancer configured with sticky sessions.
On every webserver there is an independent Redis instance. Due to the fact that we have sticky sessions configured on the loadbalancer the client will have her/his session on the same webserver with access to the Redis cache.
The Moodle docs say this is OK for session caches but is this a valid setup to use as Application cache?
What are the errors we should look for if a shared application cache is not working like it should? Is there a simple way to test it?
We have had it running like this for a while but switched back to session-cache only after some performance (php-fpm timeout errors) issues that could be unrelated. Running a single shared Redis cache is not the question here - I know that should work.
Thanks for any help on this topic.