While the xf_css_cache table is subject to 'thundering herd' issues; it still provides a benefit during cache rebuilds and the frequent invalidation is a XF issue with it's usage
Implement short (~5 minute) caching of individual css templates & parse_less_color/parse_less_func functions (parse_less_func is from Advanced BbCodes Pack). Minor, but measurable performance improvement
When caching is enabled, this hits redis instead of the xf_css_cache table.
New option to default disable XenForo's individual templating css caching.
XenForo2 provides per-template css caching which is vulnerable to "thundering herd" cache slamming by introducing unexpected write load which is vulnerable to MySQL locking via insert ... on duplicate key update.
This is only an issue for sites which experience large numbers of concurrent users
The full CSS output is still cached by whatever caching layer is defined, which saves vastly more time than individual template caching.
New retry_reads_on_master option, when a read to the slave-redis instance fails (ie due to the redis instance still loading data), reads will fail-back to the master instance.
Confirm php 7.3 compatibility for pure-php redis connector
Compat bugfix for phpredis-v3 for exists() function. phpredis-v3 returns a bool, credis/phpredis-v4 return an int, this ensures all backend redis connectors return the same type for the exists function.