Memcached for the object cache. Not a second Redis, and not on a public port.
WordPress or a PHP app that already speaks Memcached. Port 11211 stays on localhost. A cache that restarts empty is normal. Orders do not belong only there.
Simple key-value. No persistence, no pub/sub.
Options and transients out of MariaDB. The plugin must match the socket or 127.0.0.1. A miss still builds the page. A down Memcached must not take the site down.
A slab limit, not all of the RAM. PHP-FPM and MariaDB still have to run. Memcached evicts. That is the design, not a fault.
If the app wants Redis, we do not install both to look complete. Two caches for the same keys waste RAM and disagree after a flush.
11211 on the public interface is an open cache
Memcached has no authentication that we rely on. Bind to localhost, firewall the port, and do not put it on a second server unless that link is private. A public Memcached is closed in the same hour.
€55 an hour to install, bind, or stop it filling the RAM
Minimum 1 hour. If the site does not have a Memcached client, we say so and stop. Installing a daemon nothing uses is not a result.
Object cache wanted, and Redis is the wrong tool?
Tell us the app and whether 11211 is already listening. No root password in the first message.