accelerate your rails site with automatic generation-based action caching

Accelerate Your Rails Site with Automatic Generation-Based Action CachingRod Cope, CTO and FounderOpenLogic, Inc.

Wednesday, July 29, 2009

OpenLogic Company Confidential

Goal

Built-in caching is hard:

Learn how to automate it tomake the pain go away

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Agenda

IntroductionBuilt-in cachingAutomatic generation-based cachingBonus materialConclusion & recommendations

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Introduction

Rod CopeCTO & Founder of OpenLogic25 years of software development experienceIBM Global Services, Anthem, Ericsson, many more

OpenLogic, Inc.Governance, SLA support, security updates, and

indemnification for over 500 Open Source packagesDozens of Fortune 500 customersOSS Census (osscensus.org)

Global, community effort to catalog use of open source

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OpenLogic Company Confidential 5

No Spoon Feeding



Don’t Panic

scotduke.com

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Reasons for Caching

Lots of hits, speed is kingGoogle

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Reasons for Caching

Lots of hits, speed is kingGoogle

Complex hits, richness is kingOpenLogic’s OLEX

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Rails Caching Background

Page cachingUberfast, little controlHighly dynamic site? Move on.

Action cachingFast, lots of controlExpiration hell

Fragment cachingExpiration hell

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Problems with Rails Caching

Caching is easy to implement“Site’s borked again - turn off caching.”Hard to clear (the right) caches when you’re mixing and matching different types, styles, developers, plugins, etc.

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Automatic Generation-Based CachingGeneration-based

Partition the cache so that every state change increments the “generation”

AutomaticAny action that could possibly change state bumps the countLet memcached overwrite old generations

Very conservativeNo domain knowledgeDon’t cache errors, AJAX, redirects, flash notices, etc.

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Overview

Yourcode

Database memcachedRender View

Cache helper

Rails plumbing

Rails plumbing

Non-cached Cached

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Key Value

/packages/1 <html><body>Rails</body></html>

/packages/2 <html><body>Ruby</body></html>

Cache = Hash



Key Value

/gen/1/packages/1 <html>Rails</html>

/gen/1/packages/2 <html>Ruby</html>

Generational Cache



Key Value

/gen/1/packages/1 <html>Rails</html>

/gen/1/packages/2 <html>Ruby</html>

/gen/2/packages/1 <html>Ruby on Rails</html>

Generational Cache



memcached is your friend

Very fastDon’t worry about removing old entriesmemcached will automatically drop the oldest keys when it runs low on memoryRun it on your web servers if you have the RAM

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Cache Helper

No cache-related code sprinkled throughout every model, view, and/or controllerUse a global “around” filterAutomatically increment (scoped) generation count upon POST, PUT, or DELETEHandle event recording and playback (optional)

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Cache Helper Code (in “around” filter)key = make_auto_cache_key(request.request_uri)output = Rails.cache.read(key)

if output render :text => output return

else yield unless response.redirected_to || flash[:notice]

Rails.cache.write(key, response.body) endend

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Cache Key

def make_auto_cache_key(key) ol_gen = Rails.cache.fetch(OL_GEN) { "1" } "olex/#{ol_gen}"end

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“Clear” the Cache

POST, PUT, or DELETE “clears” the cache

def maybe_clear_auto_cache return if request.get? gen = Rails.cache.fetch(OL_GEN) { "1" } Rails.cache.write(OL_GEN, (gen.to_i + 1).to_s)end

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Controller Customization

Some controllers may need special cache control (e.g., user-specific cache, disable cache for certain methods)

olex_auto_cache_is_specific_to_user :only => :show

Note: the cache may still need to be cleared when skipping cache usage!

skip_filter :olex_auto_cache after_filter :maybe_clear_olex_auto_cache

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Cache Partitioning

Make it impossible for users of different corporations and permission levels to see each other’s stuff, access the same cache, clear somebody else’s cache, etc.Make sure any “global” changes clear all cachesUse a cache hierarchy

/olex/<gen #>/corp/<corp #>/<corp gen #>/roles/<role #s>/URLMD5 the key (memcached has a 250 char limit)

Extra credit: Use the key as an etag

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/olex/13/corp/72/2/roles/2,7,19/packages/2<html>

Rails (unsupported)</html>

Partitioned Cache

Global generation





Partitioned Cache

Corporate account ID





Partitioned CacheCorporation #72’s generation





Partitioned Cache

Current user’s role IDs





Partitioned Cache

URL





/olex/13/corp/72/2/roles/2,7,19,22/packages/2<html>

Rails (manager secret!)</html>


Rails (supported)</html>

/olex/14/guest/packages/2 <html>Rails</html>

Partitioned Cache



Cache Key

def make_auto_cache_key(key, user = nil) ol_gen = Rails.cache.fetch(OL_GEN) { "1" } ol_prefix = "olex/#{ol_gen}" if user.nil? && self.respond_to?("current_user") user = current_user end ca = get_corporate_account(user) ...

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Cache Key

...if user.is_corp_user? corp_gen = Rails.cache.fetch(corp_gen_key(ca)){"1"} final_key = make_corp_key(user, ca, key, corp_gen)else final_key = "guest#{key}"end

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Corporate Cache Key

def make_corp_key(user, ca, key, corp_gen) roles = role_string(user) "/corp/#{ca.id}/#{corp_gen}/roles/#{roles}#{key}"end

def role_string(user) Rails.cache.fetch("roles/#{user.id}") do user.role_string endend

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Benefits

Never have to explicitly expire anythingCan’t get an expired page

Generation numbers also stored in memcachedCache hierarchy is like a directory structure

Easy to grokCan’t run out of “disk space”!

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Does it really work?

In production for over a year - a few minor issuesSee Gotchas

Huge performance improvement for us, almost no developer pain

e.g., needed cookie-based “Welcome Joe!”Great for “passive” caching

Write caching code once, enjoy it foreverLots of room for more aggressive caching

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Gotchas

Easy to forget to make AJAX calls use RESTmethod:'get','post', 'put', or 'delete'

Test with and without caching, and test the caching itself!Caching doesn’t help the first hit

Mitigate by pre-caching when feasible

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Recommendations

The “safety first” caching implementation - use itIt’s still action caching, so don’t expect page caching performanceMeasure the result - not all pages will benefit and there is some overhead

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Bonus

Asynchronous pre-caching

Event recording and playback

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Async pre-caching pages at logon

Use Workling/Starling for asyncWhen user logs on, make async callAsync call runs through list of URL’s and hits them on behalf of the currently logged on userNeed secret back-door logon in application.rb

Use big scary session key in environment.rb

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Event Recording

Q: What if you want to record the fact that something happened even though the page was automatically cached and served?A: Watch it while being cached, record what happens in memcached, play it back later when serving cached version

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Summary

Rails built-in caching takesa lot of on-going work

Easy to screw it up

Automatic caching can be writtenonce and enjoyed forever

Much harder to screw up

Still possible to use more aggressive techniques selectively

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Any questions for Rod?

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Resources

memcached: http://www.danga.com/memcached/Workling: http://github.com/purzelrakete/workling/tree/masterStarling: http://rubyforge.org/projects/starling/OLEX: http://olex.openlogic.com

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Credits

Unless otherwise indicated, photos were licensed from BigStockPhoto.com

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OpenLogic is hiring!

Rails guru?Live near Denver, Colorado?Love Open Source?

Come see me!

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accelerate your rails site with automatic generation-based action caching

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