Rails performance: Controlled expiration on Fragment Caching

Rails performance: Controlled expiration on Fragment CachingCopyright Dimelo SA www.dimelo.com

Rails performance: Controlled expiration on Fragment Caching Renaud MORVANCopyright Dimelo SA www.dimelo.com

Rails performance: Controlled expiration on Fragment CachingCopyright Dimelo SA www.dimelo.com

Given a slow partial on a websiteCopyright Dimelo SA www.dimelo.com

Load is skyrocketing when traffic increaseCopyright Dimelo SA www.dimelo.com

A known solution: fragment caching Easy to implement: <% cache("mycachekey") do %> All the topics in the system: <%= render :partial => "topic", :collection => Topic.find(:all) %> <% end %> But you have to sweep it !!!Copyright Dimelo SA www.dimelo.com

Sweeping is hard « There are only two hard things in Computer Science: cache invalidation and naming things. »Copyright Dimelo SA www.dimelo.com

Sweeping is hard • Memcache is the prefered way to do fragment caching • Memcache implement key expirations <% cache("mycachekey", :expires_in => 3.minutes) do %> <% end %> • Great solution for medium term, no sweeper to manageCopyright Dimelo SA www.dimelo.com

But ... here is the load proﬁle when traffic keep on increasingCopyright Dimelo SA www.dimelo.com

Fragment cache is no silver bullet • It synchronizes cached part accross process • The slower the calculation the worse the load • Multiple process try to recalculate key def cache key, options = {}, &block unless Rails.cache.read key, options yield.tap do |result| Rails.cache.write key, result, options end end endCopyright Dimelo SA www.dimelo.com

Solution: backgrounded sweeping • hard • messy • buggy • cron/observer debugging is painful • code is split between multiple ﬁlesCopyright Dimelo SA www.dimelo.com

My solution: Atomic fragment caching • Don’t use memcached built-in expiration • Use software expiration • When expiration time is reached, increase expiration and trigger cache recalculation • Short expired period => few concurrent recalculationCopyright Dimelo SA www.dimelo.com

Rails performance: Controlled expiration on Fragment Caching

by Dimelo R&D Team

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