Inside GitHub with Chris Wanstrath

Hello.Hi everyone.

My name is Chris Wanstrath. I go by @defunkt online.

inside githubAnd today I’m going to talk about GitHub.

inside githubThat’s me.

GitHub is what we like to call “social coding.”

You can see what your friends are doing from your dashboard or news feed

Everyone has a proﬁle showing off their code and activity

And you can do things like leave comments on commits.

But it wasn’t always like this.

Originally we just wanted to make a git hosting site.In fact, that was the ﬁrst tagline.

git repository hostinggit repository hosting.That’s what we wanted to do: give us and our friends a place to share git repositories.

a brief historylet’s start with a brief history

It’s not easy to setup a git repository. It never was.But back in 2007 I really wanted to.

I had seen Torvalds’ talk on YouTube about git.But it wasn’t really about git - it was more about distributed version control.It answered many of my questions and clariﬁed DVCS ideas.I still wasn’t sold on the whole idea, and I had no idea what it was good for.

CVS is stupidBut when Torvalds says “CVS is stupid”

and so are you“and so are you,” the natural reaction for me is...

To start learning git.

At the time the biggest and best free hosting site was repo.or.cz.

Right after I had seen the Torvalds video, the god project was posted up on repo.or.czI was interested in the project so I ﬁnally got a chance to try it out with some other people.

Namely this guy, Tom Preston-Werner.Seen here in his famous “I put ketchup on my ketchup” shirt.

I managed to make a few contributions to god before realizing that repo.or.cz was not different.git was not different.Just more of the same - centralized, inﬂexible code hosting.

This is what I always imagined.No rules. Project belongs to you, not the site. Share, fork, change - do what you want.Give people tools and get out of their way. Less ceremony.

So, we set off to create our own site.A git hub - learning, code hosting, etc.

We started with the code browsing and commit viewing...

But once we added the current version of the dashboard, we knew this was different.

And eventually “git repository hosting” gave way to “social coding”

Unleash Your Code Join 500,000 coders with over 1,500,000 repositoriesWhat’s special about GitHub is that people use the site in spite of git.Many git haters use the site because of what it is - more than a place to hostgit repositories, but a place to share code with others.

2007 octoberThe ﬁrst commit was on a Friday night in October, around 10pm.

2008 januaryWe launched the beta in January at Steff’s on 2nd street in San Francisco’s SOMA district.The ﬁrst non-github user was wycats, and the ﬁrst non-github project was merb-core.They wanted to use the site for their refactoring and 0.9 branch.

2008 aprilA few short months after that we launched to the public.

Along the way we managed to pick up Scott Chacon, our VP of R&D

Tekkub, our level 80 support druid

Melissa Severini, who keeps us all in check

Kyle Neath, who makes the site pretty

Ryan Tomayko, who helps keep the site running smoothly.

Zach Holman, head of enterprise

Rick Olson, Rails extraordinaire

Eston Bond, Design Generalissimo

Corey Donohoe, Director of Shipology

And Brian Lopez, our bleeding edge cowboy

Oh yeah, and the other founders: PJ and Tom.

github.comThat’s where we’re at today.So let’s talk about the technical details of the website: github.com

.com as opposed to ﬁ, which I’m not going to get into today.You’ll have to invite PJ out if you want to hear about that.

We also have a store

A job board

And do git training

the web siteAs everyone knows, a web “site” is really a bunch of different components.Some of them generate and deliver HTML to you, but most of them don’t.Our site consists of four major code “frameworks” or “apps”

rails #GitHub.com, Gist, etc 1

resque #Background processing, 50ish different job types currently 2

smoke #All git calls happen over the wire 3

utils #Exception logging, stats, helper apps, etc 4

railsWe use Ruby on Rails 2.2.2 as our web framework.It’s kept up to date with all the security patches and includes custom patches we’ve addedourselves, as well as patches we’ve cherry-picked from more recent versions of Rails.

railsGitHub is about 20,000 lines of Rails code, not counting Rails itself, plugins, or gems.

We found out Rails was moving to GitHub in March 2008, after we had reached out tothem and they had turned us down.So it was a bit of a surprise.

rails pluginsWe currently have 27 Rails plugins installed, and that number is always changing.

shopify / active_merchant

lgn21st / s3_swf_upload

technoweenie /serialized_attributes

query_trace

query_analyzer

rubygemsGitHub depends on about 50 RubyGems

albino

ar-extensions

aws-s3

faraday

github-markup

rdiscount

jekyll

gollum

redis-rb

rackOne of the big features in Rails 2.3 is Rack support.

We badly wanted this, but didn’t want to invest the time upgrading.So using a few open source libraries we’ve wrapped our Rails 2.2.2 instance in Rack.

Now we can use awesome Rack middleware like Rack::Bug in GitHub

Coders created and submitted dozens of Rack middleware for the Coderack competition last year.I was a judge so I got the see the submissions already. Some of my favoritewere

nerdEd / rack-validate

webficient / rack-tidy

talison / rack-mobile-detectsets the X_MOBILE_DEVICE header to the mobile device, ifrecognized

unicornWe use unicorn as our application server- master / worker- 16 workers- preforking

unicorn- instant restart after kill- hard 30s request timeouts- control ram growth

unicorn- 0 downtime deploys- protects against bad rails startup- migrations handled old fashioned way

nginxFor serving static content and slow clients, we use nginxnginx is pretty much the greatest http server everit’s simple, fast, and has a great module system

nginxLimit ZoneLimit simultaneous connections from a client

nginxLimit RequestsLimit frequency of connections from a clientAnti-DDOS

nginxI see many people using Rack to do what the Limit modules do.Don’t.

nginxmemcachedmemcached supportcan serve directly from memcached

nginxPush Modulecomet!

gitThe next major part of GitHub is git

gritWe wrote an open source library called Gritwhich lets us use git from Ruby

mojombo / grityou can get it hereit originally shelled out to git and just parsed the responses.which worked well for a long time.

gritFile.read()Eventually we realized, however, that File.read() can be 100 times faster

gritsystem()Than shelling out

One of the ﬁrst things Scott worked on was rewriting the core parts of Gritto be pure RubyBasically a Ruby implementation of Git

mojombo / gritAnd that’s what we run now

smokeKinda.Eventually we needed to move of our git repositories off of our web serversToday our HTTP servers are distinct from our git servers. The two communicate using smoke

smoke“Grit in the cloud”Instead of reading and writing from the disk, Grit makes Smoke callsThe reading and writing then happens on our ﬁle servers

bert-rpcRather than use Protocol Buffers or Thrift or JSON-RPC, Smoke uses BERT-RPC

bert-rpcbert : erlang ::json : javascriptBERT is an erlang-based protocolBERT-RPC is really great at dealing with large binariesWhich is a lot of what we do

bert-rpcwe have four ﬁle servers, each running bert-rpc serversour front ends and job queue make RPC calls to the backend servers

mojombo / bertrpcYou can grab bert-rpc on GitHub

mojombo / bertOr if you just want to play with BERT

chimneyWe have a proprietary library called chimneyIt routes the smoke. I know, don’t blame me.

chimneyAll user routes are kept in RedisChimney is how our BERT-RPC clients know which server to hitIt falls back to a local cache and auto-detection if Redis is down

chimneyIt can also be told a backend is down.Optimized for connection refused but in reality that wasn’t the real problem - timeouts were

proxymachineAll anonymous git clones hit the front end machinesthe git-daemon connects to proxymachine, which uses chimney to proxy yourconnection between the front end machine and the back end machine (which holdsthe actual git repository)very fast, transparent to you

mojombo / proxymachineproxymachine can be used to proxy any kind of tcp connectionopen source

sshSometimes you need to access a repository over sshIn those instances, you ssh to an fe and we tunnel your connection tothe appropriate backendTo ﬁgure that out we use chimney

node.js

node.jsdownloads

node.jsdownloadshttp => https <img>

node.jsdownloadshttp => https <img>event streams

jobsWe do a lot of work in the background at GitHub

resqueCurrently we use a system called Resque.

defunkt / resqueYou can grab it on GitHub

resque- dealing with pushes- web hooks- creating events in the database- generating GitHub Pages- clearing & warmingcaches- search indexing

queuesIn Resque, a queue is used as both a priority and a localization techniqueBy localization I mean, “where your workers live”

queuescritical,high,lowthese three run on our front end serversResque processes them in this order

queuespageGitHub Pages are generated on their own machine using the `page` queue

queuesarchiveAnd tarball and zip downloads are created on the ﬂy using the `archive` queueon our archiving machines

searchOn GitHub, you can search code, repositories, and people

solrSolr is basically an HTTP interface on top of Lucene. This makes it pretty simpleto use in your code.We use solr because of its ability to incrementally add documents toan index.

Here I am searching for my name in source code

solrWe’ve had some problems making it stable but luckily the guys at Pivotalhave given us some tipsLike bumping the Java heap size.Whatever that means

databaseOur database story is pretty uninteresting

mysqlWe use mysql 5

master / slaveAll reads and writes go to the masterWe use the slave for backups and failover

cachingOn the site we do a ton of cachingusing memcached

fragmentsWe cache chunks of HTML all overUsually they are invalidated by some action

fragmentsFormerly we invalidated most of our fragments using a generation scheme,where you put a number into a bunch of related keys and increment itwhen you want all those caches to be missed (thus creating new cacheentries with fresh data)

fragmentsBut we had high cache eviction due to low ram and hardware constraints, and foundthat scheme did more harm than good.We also noticed some cached data we wanted to remain forever was being evicted dueto the slabs with generational keys ﬁlling up fast

pageWe cache entire pages using nginx’s memcached moduleLots of HTML, but also other data which gets hit a lot and changes rarely:

page- network graph json- participation graph dataAlways looking to stick more into page caches

objectWe do basic object caching of ActiveRecord objects such asrepositories and users all over the placeCaches are invalidated whenever the objects are saved

associationsWe also cache associations as arrays of IDsGrab the array, then do a get_multi on its contents to get a list of objectsThat way we don’t have to worry about caching stale objects

walkerWe also have a proprietary caching library called Walker

walkerIt originally walked trees and cached them when someone pushedBut now it caches everything related to git:

walker- commits- diffs- commit listing- branches- tags- everything

Every git-related page load hits Walker a lot

walkerFor most big apps, you need to write a caching layerthat knows your business domainGeneric, catch-all caching libraries probably won’t do

eventsAn example of this is our events system

This is one fragment

Each of these is a fragment

They’re also cached as objects

As well as a list of ids

And that’s just for the dashboard...

optimizationsSo what other optimizations have we done

asset serversWell we do the common trick of serving assets from multiple subdomains

asset serversassets0.github.comassets1.github.comand so forth

sha asset idInstead of using timestamps for asset ids, which may end up hitting the diskmultiple times on each request, we set the asset id to be the sha of the last commitwhich modiﬁed a javascript or css ﬁle

sha asset id/css/bundle.css?197d742e9fdec3f7/js/bundle.js?197d742e9fdec3f7Now simple code changes won’t force everyone to re-download the css or js bundles

bundlingFor bundling itself, we use

bundlingyui’s compressor for css and

bundlinggoogle’s closure compiler for javascriptwe don’t use the most aggressive setting because it means changingyour javascript to appease the compression gods,which we haven’t committed to yet

scripty 301Again, for most of these tricks you need to really payattention to your app.One example is scriptaculous’ wiki

scripty 301When we changed our wiki URL structure, we setup dynamic 301 redirectsfor the old urls.Scriptaculous’ old wiki was getting hit so much we put the redirect into nginx itself -this took strain off our web app and made the redirects happen almost instantly

ajax loadingWe also load data in via ajax in many places.Sometimes a piece of information will just take too long to retrieveIn those instances, we usually load it in with ajax

If Walker sees that it doesn’t have all the information it needs, it kicks off a jobto stick that information in memcached.

We then periodically hit a URL which checks if the information is in memcached or not.If it is, we get it and rewrite the page with the new information.

We use this same trick on the Network Graph

Fork Queue

ajax loadingand anywhere else it makes sense.

comet loadingvery soon this will all be comet, though

monitoringwhat do we use for monitoring?

nagiosOur support team monitors the health of our machines and coreservices using nagios.I don’t really touch the thing.

Here’s a screenshot from my IE browser, complete with the ICQ plugin

resque webWe monitor our queue using Resque’s included Sinatra app

haystackWe use an in-house app called Haystack to monitor arbitrary information,tracked as JSON.

Here’s an example of Haystack’s “exceptions” view

collectdWe also use collectd to monitor load, RAM usage, CPU usage, and otherapp-related metrics

pingdompingdom sends us SMSes when the site is downit’s nice

tendertender is what we use for customer support

it works incredibly well, and they’re constantly improving it

testingOur testing setup is pretty standard

test unitWe mostly use Ruby’s test/unit.We’ve experimented with other libraries including test/spec, shoulda, and RSpec, but in the endwe keep coming back to test/unit

git fixturesAs many of our fixtures are git repositories, we specify in the test what shawe expect to be the HEAD of that fixture.This means we can completely delete a git repository in one test, then have it back inpristine state in another. We plan to move all our fixtures to a similar git-system in the future.

machinistWe use machinist for our ﬁxtures

notahat / machinist

running_manGives us setup_onceUse it to cache machinist ﬁxtures on a per-test-class basis

technoweenie / running_man

ci joeWe use ci joe, a continuous integration server, to run on tests after each push.He then notiﬁes us if the tests fail.

defunkt / cijoeYou can grab him at github

stagingWe also always deploy the current branch to stagingThis means you can be working on your branch, someone else can be working on theirs,and you don’t need to worry about reconciling the two to test out a featureOne of the best parts of Git

security

github.com/securityhaving a security page really helps

security@github.comwe get weekly emails to our security email (that people ﬁnd on the security page)and people are always grateful when we can reassure them or a answer their question

regular auditsif you can, ﬁnd a security consultant to poke your site for XSS vulnerabilitieshaving your target audience be developers helps, too

24/7 monitoring24/7 monitoring is cool too

backupsbackups are incredibly importantdon’t just make backups: ensure you can restore them, as well

sqlwe keep nightly, off-site backups of our sql databases

gitand the same for all our git repositories

the future

pull requests

organizations

...and more

Questions?thanks for coming

Thanks.thanks for coming

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Inside GitHub with Chris Wanstrath

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Inside GitHub with Chris Wanstrath Presentation Transcript