Stack Overflow, and its Q&A network Stack Exchange, have been growing exponentially for the last five years. They now encompass ~150 Q&A sites ~9 million users ~13 million questions ~22 million answers In this talk, I will describe: + The physical architecture of Stack Overflow. How many servers are there? What is their purpose and what are their specs? + The logical architecture of the software. How do we scale up? What are the main building blocks of our software? + The tooling system. What supports our extreme optimization philosophy? + The development team. What are our core values? What footprint do we want to leave as developers?

1. Marco Cecconi
@sklivvz
http://sklivvz.com

2. ARCHITECTURE?
WHAT THE HECK IS
THAT?

5. In the past 12 months
• The Stack Exchange network grew from 130 to
150 sites (+15%)
• US site rank for visitors of the whole network
improved from #44 to #19 (-25 ranks)

23. Tight, tight, code cycles

24. BATCAVE
Code-build-test cycle
running on home machine
feature requests

25. BATCAVE DEV.SO
Test on the real servers
git push

26. BATCAVE DEV.SO
META.SE
1-click deploy
Users test on
meta.stackexchange.com
(“baking”)
git push

27. BATCAVE DEV.SO
META.SE
1-click deploy
HALP!
git revert
git push

28. BATCAVE DEV.SO
NETWORK META.SE
1-click deploy
git push
1-click
deploy
It’s live! Tons of users use it…

29. BATCAVE DEV.SO
NETWORK META.SE
1-click deploy
git push
1-click
deploy
…and
provide new
feature requests

30. Move fast and break things*
* Not the home page or question page :-)

31. Move fast and break things*
* Not the home page or question page :-)

33. “Normal” caching

36. Our frienemy:
The Garbage Collector

38. Too many objects instantiated

39. Caching abuse

44. Compliler abuse

47. Framework abuse

51. Complex instance
graph

52. Avoiding common patterns

53. IRepository<Order> orderRepository =
container.Resolve<IRepository<Order>>();
Order order = orderRepository.Get(35);
Dependency injection pattern

55. IRepository<Order> repository =
new ValidatingOrderRepository (
new SecurityRepository<Order> (
new LoggingRepository<Order> (
new CachingRepository<Order> (
new NHibernateRepository<Order> ()
)
)
)
);
Order order = repository.Get(35);

56. A look at our source code

61. Automated TESTERS

64. What we do test:
Open Source and Libraries

70. BOSS MODE CODE

71. BOSS MODE TOOLS

72. • Performance is a feature
• Always. Be. Shipping.
• Use your circumstances.
• Open source your libraries
• 3 obscenely big monitors.
KEY TAKEAWAYS

74. Marco Cecconi
@sklivvz
http://sklivvz.com
perl -wlne'BEGIN{$b=rand$=}$a=qw/To
lzhk_kypam_yr_qryaimtcpdjmu_bmr_amk_enq_ghqhmf
Cnvm/[($_<=>int$b)+1];$a=~y/a-zA-Z/b-zaB-ZA/;print $a'