many WP installations on the same server

By default it should. But that requires that you use unique table prefixes (They all cannot be wp_).

To use it on many sites where the table prefixes are the same, you can add a constant to wp-config.php called WP_CACHE_KEY_SALT, such as:

define('WP_CACHE_KEY_SALT', md5(DB_NAME . __FILE__));

md5(DB_NAME . __FILE__) can be replaced by whatever you want, just as long as it is unique per site.

See: https://plugins.trac.www.ads-software.com/browser/memcached/tags/2.0.2/object-cache.php#L13

Does this pertain to a WordPress Multi-Site instance or is this about multiple different instances of WordPress? Right now, I have a multi-site instance with a hard coded WP_CACHE_KEY_SALT in the wp-config. It seems to work fine.

One thing that I have noticed is that when I have different WP multi-site instances, I cannot point them to the same ElastiCache cluster even if the WP_CACHE_KEY_SALT is different in each of their wp-config.php files.

The caches seem to get combined. What should I do?

Thanks,

Seth