Kickin' Ass with Cache-Fu (with notes)

CHRIS WANSTRATH ERR FREE [ http://errfree.com ] today i want to talk about...

Video games! I’m a long time gamer Or, speciﬁcally....

E3. E3 = Electronics Entertainment Expo - Biggest annual video game conference up through 2006 - Every May - 60,000 - 80,000 industry people - Publishers spend millions - Staples Center in LA - Blogs, magazines, crappy cable TV shows, websites - The two biggest gaming websites are...

IGN, owned by News Corp, who also owns MySpace Super good at SEO and breaking news The other one is...

GameSpot, owned by CNET GS gets 1.5m uniques a day, over 15m pageviews Here’s the obligatory alexa graph...

The spikes are E3 Twitter’s catching up! Okay, so I used to work for GameSpot....

funnier if you can see how long my hair is now. and, how not nerdy i am.

I was a PHP developer for GameSpot - Hubs - Tagging system - User videos - User proﬁles Last year I convinced my boss to send me to E3...

I got sent down because of Rails, but that’s another story - Booth between Nintendo and Sony. - Microsoft was in another hall. - Unveiling of Wii & PS3 Our booth had a studio attached to it ($400k on new eq) and a bunch of computers inside it...

- I sat at one of these computers - The editors would run around writing stories and interviewing people and playing games - GS had exclusive right to internet streaming of sony and nintendo conferences - Nintendo.com and Sony.com were pointing at Gamespot for press conference streaming - Also streamed Microsoft (they streamed on Xbox Live too) - Live blogging (twittering) from the press conferences for kids at school - Imagine million of teenage boys constantly hitting refresh...

- We used Netscalers -- switch, ﬁrewall, and “accelerator” - Normally our Netscalers would gzip outgoing requests - Had to turn o gzip compression because Netscalers’ CPUs were running so hot - Couldn’t gzip and serve requests fast enough - 3000 req/sec - ~70 app servers - ~15 database servers - apache2, php4 w/APC - when the smoke cleared...

50,000,000 pages no downtime pretty cool, but how?

memcached. thanks, in a large part, to memcached this is what my talk is really about...

Memcaching Rails CHRIS WANSTRATH ERR FREE [ http://errfree.com ] memcaching rails i will talk about: - what memcached is - when and where to use it - tricks, code - libraries, tools, and hopefully answer questions please ask questions whenever i promise i cant answer them all. also: i don’t want you guys to think that i think i’m an expert i’m more of a foot soldier, or a ninja i’m oficially renaming this session...

Memcaching Rails CHRIS WANSTRATH ERR FREE [ http://errfree.com ]

chris wanstrath railsconf 2007 kickin’ ass with cache-fu kickin ass with cache-fu what is memcached...

{} conceptually, it’s like a hash...

class Memcache < Hash undef :each, :keys end a hash that... you can’t enumerate over you can’t ﬁnd all the keys for you can still GET, SET, and DELETE keys...

class Memcache < DRbHash undef :each, :keys end distributed by distributed, we mean you start a daemon on each app server keys are stored on dierent servers transparently, quickly let’s look at a daemon...

$ memcached -vv <3 server listening <7 new client connection <7 get app-test:Story:1 >7 END <7 set app-test:Story:2 0 >7 STORED <7 delete app-test:Story:1 >7 DELETED here is some sample debugging output single memcache daemon, getting, setting, deleting truncated but you get the idea

$ memcached -vv <3 server listening <7 new client connection <7 get app-test:Story:1 >7 END <7 set app-test:Story:2 0 >7 STORED <7 delete app-test:Story:1 >7 DELETED developed by...

livejournal alleviate database stress they were growing too fast, couldnt scale their databases you can distribute reads but everyone needs to write, which can block hdd too slow -- avoid sql queries / disk access by caching in RAM cache anything: generated images, intense number crunching, html, whatever fast, C, non-blocking IO, O(1) lookups scales -- drop in a new daemon and youre good to go it’s also used by...

these guys chowhound serves about .5m pages a day in, of course, rails...

you’ve got a rails site should you use memcached?

YAGNI ya ain’t gonna need it none of the big guys built memcache into their infrastructure (just ask twitter) build it in later focus on your app ﬁrst in small apps it is slower than sql hardware is the real special sauce memcached wont help if you cant keep up with the IO requests you probably don’t need memcache...

UYRDNI er-den-ee...

UYRDNI (unless you really do need it) this would be: - millions of hits - millions of rows - millions of both if you’re getting these, you can use memcached to help with the heavy lifting...

before we get ahead of ourselves, we need to look at the basic pattern...

class Presentation < ActiveRecord::Base def self.get_cache(id) if data = @cache.get(id) data else data = find(id) @cache.set(id, data) data end end end pretend @cache is our memcache object try to ﬁnd an id in memcache if it’s nil, we ﬁnd it in the database we set it to the cache we return second time we call this method, it returns the cached data a simpler way to write this...

class Presentation < ActiveRecord::Base def self.get_cache(id) @cache.get(id) || @cache.set(id, find(id)) end end almost could rewrite it like this but #get can return false so, with this basic pattern, we can cache...

Fragments Actions Sessions & Objects fragments - tag cloud - user info / hcard actions - site index - seldom-changing content - front door - everything but the layout sessions - if you’re not using the cookie store on edge objects - user object - article object - avoid sql what tools can you use...

memcache-client by eric hodel it’s the good ruby memcache api bare metal - conﬁgure servers - instantiate memcache object - get basic daemon stats - set / get / delete - uses marshal to store data - speeds up object creation on cache hit because unmarshal faster than #new cake to install...

memcache-client $ gem install memcache-client gem install of course, you need memcached running...

topfunky memcached real easy way to get it installed let’s get it ready for rails...

CachedModel - from eric hodel - used for activerecord object caching - overwrites ﬁnd() - caches single objects - no complex queries - clears cache on update - simple and clean

Fragment Cache Store one on rubyforge patches rails to work with memcache-client lets you set a time based expiry on fragments useful because all your mongrels share the same cache no caching on disk

Session Store do this by hand environment.rb after you setup memcache-client...

you can put all these pieces together memcache-client cached model etc

fragment store session store or...

you can use a rails plugin which handles almost everything rails related

cache_fu cache fooooo

cache_fu ( acts_as_cached 2.0 ) acts as cached 2 rails plugin used in production on chowhound and chow other sites too out of the box, this plugin can handle caching...

Fragments Actions Sessions & Objects the things we talked about earlier - can automatically setup memcache sessions - can make all fragment caching use memcache - can do the same for action caching - can give any ruby object (activerecord) get_cache set_cache and expire_cache i’m not going to go in depth into the basics if you want that information...

acts_as_cached i have another (outdated) pdf on my blog with some basic api info on it also a google group you can join so ﬁrst, sessions and fragments...

config/memcached.yml defaults: ttl: 1800 namespace: railsconf sessions: false fragments: false servers: localhost:11211 here’s a snippet of the yaml conﬁg ﬁle that cache_fu uses each environment has its own section, like database.yml, which inherits from this default ttl is the default time to live, or expiry namespace is the namespace all the keys live under -- lets you have dierent apps sharing the same servers so to turn on memcache as our fragment and session store...

config/memcached.yml defaults: ttl: 1800 namespace: railsconf sessions: true fragments: true servers: localhost:11211 bam more realistically you’d have something like...

config/memcached.yml production: benchmarking: false sessions: true fragments: true servers: - 192.185.254.121:11211 - 192.185.254.138:11211 - 192.185.254.160:11211 might want to stick with something simple in dev mode while we’re here, our ﬁrst tip, learned the hard way...

config/memcached.yml production: benchmarking: false sessions: true fragments: true servers: - 192.185.254.121:11211 - 192.185.254.138:11211 - 192.185.254.160:11211 use ip addresses for servers dns requests can make a noticeable dierence on app performance what if your internal dns goes down? takes whole site with you we didnt manage our own dns at cnet it’s happened to me more than i’d care to admit so, that’s fragments and sessions. done what about models...

class Presentation < ActiveRecord::Base acts_as_cached end our class from before it’s acting like caching this adds a bunch of methods, the main two being...

get_cache expire_cache get_cache will by default go to #ﬁnd if it’s a miss it can accept a block which it will use instead on a miss because of this you probably never need set_cache expire_cache issues a DELETE to memcached cachedmodel automatically clears a record’s cache on update or delete with cache_fu, we ask you to do that explicitly but it’s easy...

class Presentation < ActiveRecord::Base acts_as_cached after_save :expire_cache end expire_cache is also an instance method which uses the object’s id now if we save a presentation object, its cache will be cleared for us we did almost all of the caching on chowhound this way strong cache integrity -- rarely stale data why not after_update? you want to clear an object’s cache after create because you may have cached that the object doesnt exist like, caching nil for an id that isnt yet created so we want to do it after save but this brings us to another gotcha...

class Presentation < ActiveRecord::Base def self.get_cache(id) if data = @cache.get(id) data else data = find(id) @cache.set(id, data) data end end end our get_cache method from before what happens if we want to cache nil? how do we express the non-existence of a record? well, cache_fu checks speciﬁcally for nil...

class Presentation < ActiveRecord::Base def self.get_cache(id) if not (data = @cache.get(id)).nil? data else data = find(id) @cache.set(id, data) data end end end so only things that are not nil are returned what good is this? it lets us cache false so when you try to cache nil, cache_fu caches false...

class Presentation < ActiveRecord::Base def self.get_cache(id) if not (data = @cache.get(id)).nil? data else data = find(id) || false @cache.set(id, data) data end end end you dont want to keep running a query on a 4 million row table when you know that the record you’re looking for isnt there it’s really not any dierent than the record being there you want to cache information so you dont need to keep expensively looking it up...

Presentation.get_cache(1337) so this is great for single row ﬁnds but what about that rails idiom we’re all trying to stick to...

skinny models fat controllers we want to write custom ﬁnders instead of cluttering our controller we can implement many of these custom ﬁnders, in our model...

with with_scope in this blog post the example used is find_playing but the inspiration for this post was some chow code which used find_live that me and evan weaver did only show published, public items not deleted or banned items so what if you want to always use this scoped find call when caching? you have two options...

class Presentation < ActiveRecord::Base acts_as_cached :conditions => 'published = 1' end the acts_as_cached call can take any arbitrary parameter will pass it through to the find call it uses so now all our get_cache calls are scoped -- doesnt overwrite the model’s find method goes against the with_scope custom finder idea...

class Presentation < ActiveRecord::Base acts_as_cached :finder => :find_live end and now we’re scoped this isnt a good controller example, though with_scope is not the majority case often you just get crazy ﬁnds in controllers you want to cache...

Topic.find :all, :conditions => [quot;created_at > ?quot;, 1.week.ago], :order => 'post_count desc', :limit => 5 do we want this in our controller? does our caching code go in our controller? doesnt seem very skinny...

seems fatty what we’d typically do here is write a custom ﬁnder pre-rolled ﬁnd method...

class Topic < ActiveRecord::Base def self.weekly_popular(limit = 5) find :all, :conditions => [quot;created_at > ?quot;, 1.week.ago], :order => 'post_count desc', :limit => limit end end write our ﬁnder wraps our custom ﬁnder (you wrote a test, right?) gives us a skinnier controller...

Topic.weekly_popular but it’s still not cached...

DB: 0.00 (0%) on chowhound, this is what we aimed for every second page load should not hit mysql at all big time cache coverage on the backend so we’d cache every custom finder or find call remember that get_cache takes a block we can write a custom cached finder which wraps our finder method...

class Topic < ActiveRecord::Base def self.cached_weekly_popular get_cache(:weekly_popular) do weekly_popular end end end so if it’s cached, we just return it if not, we run the query and cache it...

Topic.cached_weekly_popular still pretty skinny and clean lets us know we’re working with cached data before we go any further, let’s test this...

we dont want to test memcached itself we disable memcached in tests your tests shouldnt depend on external resources to help us out with all this, we’ll use mocha...

ruby mocha mocking and stubbing you’ve no doubt heard of mocha one of the best rubygems it’s in my standard library i’m also going to write my test bdd-style....

bdd test spec with test/spec test/spec is a bdd library which wraps test/unit i like it because it’s clear english similar to rspec cache_fu was written bdd with test/spec...

A Ruby object acting as cached - should be able to retrieve a cached version of itself - should be able to set itself to the cache - should pass its cached self into a block when supplied - should be able to expire its cache - should be able to reset its cache - should be able to tell if it is cached - should be able to set itself to the cache with an arbitrary ttl Finished in 0.028509 seconds. 28 specifications (53 requirements), 0 failures some of the output! so our test...

context quot;Calling #cached_weekly_popularquot; do specify quot;should call #weekly_popular if not cachedquot; do Topic.expects(:fetch_cache).returns(nil) Topic.cached_weekly_popular.should.equal Topic.weekly_popular end specify quot;should return if cachedquot; do Topic.expects(:get_cache).returns(true) Topic.expects(:weekly_popular).never Topic.cached_weekly_popular end end make sure our cached method is calling the method we want make sure we did our caching right fetch_cache is the method we want to force into returning nil set expectations that’s it, custom ﬁnder of course, this pattern is built into cache_fu...

Topic.cached(:weekly_popular) same exact thing used to cache the return value of arbitrary methods key is the method name but, we didnt just waste all that time...

let’s talk about time how long will the weekly_popular method be cached? if it’s more than a day, it could start to become inaccurate and stale we could set a ttl sometimes you need dierent caches on dierent days may want story to appear tomorrow but it’s in the db today how to make sure the cache is cleared when it needs to be? here’s a simple 80 / 20 solution...

def self.cache_key_with_date(id) date = Date.today.to_s.tr(' ', '_') cache_key_without_date(id) + ':' + date end class << self alias_method_chain :cache_key, :date end override the cache_key method add the date at midnight, forces a cache miss insta-publishing works by day, not by time this is class wide if you just want to do it for one key...

class Topic < ActiveRecord::Base def self.date_for_key Date.today.to_s.tr(' ', '_') end def self.cached_weekly_popular key = 'weekly_popular' + date_for_key get_cache(key) { weekly_popular } end end add a method, or something remember: this is a collection, so be careful...

memcached only stores 1 meg of data per key ‘slabs’ if youre caching associations along with objects, slabs can get big fast that said, a 200+ post thread on chowhound only takes up about 300k and those people talk a lot so check and think before caching anything let’s say you have a 200 post thread...

this one has 233 we’ve got all these users. some users appear more than once. we dont want to cache users with forum posts, that can give us stale data would have to clear the cache of every post christine has made whenever she changes her avatar how do we avoid 233+ memcache calls?

Topic.find(1, 2, 3) ﬁnd takes an array, right?

Topic.get_cache(1, 2, 3) so does get_cache this will utilize memcache-client’s get_multi grabs all the keys in parallel cache_fu ﬁlls in and caches the blanks for you when i last looked at the livejournal code, they used get_multi like crazy avoid hitting the cache -- it can add up to be expensive so, in other words...

user_ids = @topic.posts.map(&:user_id).uniq @users = User.get_cache(user_ids) in our controller will give us a hash keyed by the user ids can reference this in our view keep things speedy, minimize memcached calls we only use this in one place on chowhound another thing we can do to keep our cache speedy is to use a process cache...

class ApplicationController before_filter :local_cache_for_request end built into cache_fu keeps a local hash of memcache’d objects to speed up subsequent access on a single page view also in cachedmodel for example...

# pulls from memcache @user = User.get_cache(1) # pulls from local cache @user = User.get_cache(1) in a controller cleared out at the start of every new request doesnt carry over responds to expires and sets, so you wont get stale data within a single request sometimes, though, you dont want any caching in a request maybe you think you have stale data and want to see your page straight from the database maybe you want to re-set all keys on a page...

class ApplicationController before_filter :set_cache_override def set_cache_override returning true do ActsAsCached.skip_cache_gets = !!params[:skip_cache] end end end skip_cache_gets tells cache_fu to treat every get as a cache miss everything will be pulled from its source and re-set to the cache call it with...

www.mysite.com/home?skip_cache=1 something like that useful for debugging, in dev mode and on the live site but you need to be careful...

what if you do this on the front door of a big site like gamespot during peak hours? all those expensive queries get re-run but not just for you if a query takes 1 second to run, and it’s not in the cache every request within that 1 second will see the query’s cache as a miss it will be run N times depending on how many requests you get a second that can literally kill a big website...

but forget about the skip_cache what if you expire a tag cloud on a homepage a popular home page, and you’ve got a billion tags the same thing will happen every request will see the tag cloud’s cache as a miss while the original request builds the data this can and has taken down gamespot they have tagging and some crappy programmers but i dont want to name names...

reset_cache so, we’ve got this guy called reset_cache grabs data and sets it to the cache without expiring the key while this is going on, every request gets the old cached data new cached data is set crisis averted you get promoted you quit php and start doing rails another way to do this in cache_fu...

@topic.set_cache instances can set themselves to the cache not that big of a deal be careful what you expire...

class Presentation < ActiveRecord::Base acts_as_cached after_save :reset_cache end can also rock the reset_cache after_save while we’re back in the model you run a migration...

you add a field to your model your cached objects in production wont have this field if you write views or code which access this field, they’ll break the unmarshaled objects have no idea this field exists flush the cache? kind of, but only for that model...

class Presentation < ActiveRecord::Base acts_as_cached :version => 1 end cache_fu supports this version parameter when i set this to 1, all cached presentation objects will be misses fresh caches for all of them if i up it to 2, same thing happens again just a way to keep your keys consistent and unique we always forget to do it -- when your site 500s you’ll know speaking of 500s...

deployment! cache_fu comes with capistrano tasks you can use for deployment also useful is...

monit monit is a great deployment tool for monitoring multiple daemons across multiple servers we’ve found memcached to be pretty damn reliable sometimes we’d see funk after heavy load and we’d just reboot to get things back to normal single nodes rarely out which is good, memcache-client cant recover without re-hashing every key perl api can recover i’ve heard but recently, there’s this new kid on the blog...

libketama from last.fm consistent hashing algorithm pretty cool actually...

1 600 200 400 you assign each server a number let’s say these are my four basically plots the numbers on a circle...

1 600 cache_get :railsconf 200 400 let’s say i want the key ‘railsconf’

1 :railsconf == 100 600 200 400 consistently give the key a number ﬁnd that number’s position in the conceptual circle

1 :railsconf == 200 600 200 400 if that number isnt found, goes to the next highest number which does exist

1 :railsconf == 200 600 200 so we can remove servers and aect only a subset of keys

1 700 :railsconf == 200 600 200 500 300 400 and we can add servers in the same manner i havent played with it yet but want to soon in reality i believe each server gets assigned to more than one point on the circle, to help distribute the cache

l33t h4x0rs • Geoffrey Grosenbach • Rob Sanheim • Ryan King • Lourens Naudé • Michael Moen • Corey Donohoe • PJ Hyett • Eric Hodel thanks majorly to these guys for their work and contributions to the plugin or ruby memcache in general

{} ( thanks. any questions? ) thanks everyone any questions?

http://flickr.com/photos/seibuone/144588686/ http://flickr.com/photos/j00zt1n/255430115/ http://flickr.com/photos/jameswong/145397570/ http://flickr.com/photos/xalpha/58368229/ http://flickr.com/photos/63503896@N00/35723413/ http://flickr.com/photos/mrcrash/145451993/ thanks http://flickr.com/photos/psybernoid/398301743/ http://flickr.com/photos/45royale/422227291/ flickr http://flickr.com/photos/andrson/420810541/ http://flickr.com/photos/joshuaweiland/370931770/ http://flickr.com/photos/zesmerelda/27258314/ http://flickr.com/photos/slice/390271923/ http://flickr.com/photos/cocoen/411960476/ http://flickr.com/photos/pinguino/198885132/ http://flickr.com/photos/davidfmiller/468476118/ http://laughingsquid.com - Scott Beale http://flickr.com/photos/bail56/313536999/ http://flickr.com/photos/65995199@N00/272672183/ FLICKRZ

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Kickin' Ass with Cache-Fu (with notes)

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