Redis cache store
The Redis cache store is one of the best options to handle session and application cache as it supports: data guarantee, locking, key awareness. (and also can be used for user's sessions caching: Session_handling#Redis_session_driver in the config.php file)
Before Redis is available as a cache store, you will need to install Redis service (daemon) on your Moodle server, locally in case of a single Moodle app node architecture or externally if you are using a cluster of Moodle nodes. only then, you can configure Redis as an application or session level cache store.
If you are configuring a cluster of Moodle servers/nodes, the Redis service (daemon) should be installed on an external server and all Moodle nodes (servers/instances) should point to that external Redis. so all user's data is available when the user is using (connected to) any of the Moodle nodes, in case the user's connection is not persistent (sticky) to a specific node for the entire Moodle session. An external Redis (NoSQL) service can be installed on the main SQL server alongside the MySQL/MariaDB service, just make sure you have enough memory allocated on that server for both services.
A good practice is to give the Redis cache store prefix a proper short name and not leave it empty, as later on, it might conflict with user's session Redis cache you might choose to use, or other Moodle instances that will be installed on the same server. for example "my-school-name_cs_", where you replace "my-school-name" with your short school name. (and if you are also using a user's session store on the config.php file, you might like to prefix it with "my-school-name_us_")
When using a cluster setup with several Moodle instances on each node that belong to different Schools/Institutes/clients, make sure that you use the same Prefix for all Moodle instances that are on different nodes and are of the same School/Institute/Client.
Installing Redis server
- install and configure redis on centos 7
- Session_handling#Redis_session_driver and How to Set Up a Redis Server as a Session Handler for PHP