Presented by James Meickle, developer evangelist at AppNeta, to the Boston Django Meetup on March 28, 2014.

1. Caching Up and
Down the Stack
1
James Meickle
Developer evangelist, AppNeta
@jmeickle
Django Boston Meetup Group
March 26, 2014

2. 2
4
WHAT IS CACHING?
Uncached
Client
Data
Source

3. 3
4
WHAT IS CACHING?
Uncached Cached
Client
Data
Source
Data
Source
Client
Cache Intermediary

4. 4
4
WHAT IS CACHING?
Uncached Cached
Client
Data
Source
Data
Source
Client
Cache Intermediary
Fast
!
Slow...

5. 5
• Images
• CSS
• JavaScript
• HTML documents
• DNS
4
WHAT GETS CACHED BY
CLIENTS?

6. 6
• HTML documents
• HTML fragments
• Operations on data
• Database queries
• Expensive objects
4
WHAT GETS CACHED BY APPS?

7. 7
• Compiled source
• Packages
• Disk access
• Memory access
• CPU instructions
4
WHAT GETS CACHED BY
SERVERS?

8. 8
• Client-side assets
• Full pages
4
CACHING IN DJANGO:
FRONTEND

9. 9
4
OUR SITES
appneta.com
(static site via Jekyll)

10. 10
4
OUR SITES
internal.tracelytics.com
(dynamic single page Pylons app)

11. 11
4
CLIENT-SIDE ASSETS

12. 12
• Use HTTP caches!
• CDN
• Intermediate proxies
• Browser
• Set policy with cache headers
• Cache-Control
• Expires
4
CLIENT-SIDE ASSETS

13. 13
4
CLIENT-SIDE ASSETS
/tl-layouts_base-compiled-757f5eec3603f60850acfdb86e6701cf104f80ae.css
Request Method: GET
Status Code: 304 Not Modified
Cache-Control: max-age=315360000
Connection: keep-alive
Date: Mon, 18 Feb 2013 22:46:12 GMT
Expires: Thu, 31 Dec 2037 23:55:55 GMT
Last-Modified: Tue, 12 Feb 2013 21:10:20 GMT
Server: nginx/0.8.54

14. 14
4
CLIENT-SIDE ASSETS
appneta.com
internal.tracelytics.com

15. 15
4
CLIENT-SIDE ASSETS (IN
DJANGO)
VS

16. 16
• Full pages
• Partial pages
• Objects
• Queries
4
CACHING IN DJANGO: BACKEND

17. 17
4
WE’RE STILL TALKING ABOUT
PAGES?
Client-side
assets
Pages

18. 18
4
FULL-PAGE HTTP CACHING
Client Varnish
Do HTTP caching, but with your rules.
No internet standards necessary!
Webserver

19. 19
• Why do it server-side?
• Invalidation
• Amount cached
• Changing cache policies
4
FULL-PAGE HTTP CACHING

20. 20
4
FULL-PAGE HTTP CACHING (IN
DJANGO)

21. 21
• Full pages
• Partial pages
• Objects
• Queries
4
CACHING IN DJANGO: BACKEND

22. 22
4
FULL-PAGE HTTP CACHING?
appneta.com

24. 24
4
FRAGMENT CACHING
appneta.com
Ruins
everything

25. 25
4
FRAGMENT CACHING

26. 26
4
FRAGMENT CACHING

27. 27
4
FRAGMENT CACHING

28. 28
4
FRAGMENT CACHING (IN
DJANGO)

29. 29
• Full pages
• Partial pages
• Objects
• Queries
4
CACHING IN DJANGO: BACKEND

30. 30
4
OBJECT CACHING
def get_item_by_id(key):
# Look up the item in our database
return session.query(User)
.filter_by(id=key)
.first()

31. 31
4
OBJECT CACHING
def get_item_by_id(key):
# Check in cache
val = mc.get(key)
# If exists, return it
if val:
return val
# If not, get the val, store it in the cache
val = return session.query(User)
.filter_by(id=key)
.first()
mc.set(key, val)
return val

32. 32
4
OBJECT CACHING
@decorator
def cache(expensive_func, key):
# Check in cache
val = mc.get(key)
# If exists, return it
if val:
return val
# If not, get the val, store it in the cache
val = expensive_func(key)
mc.set(key, val)
return val

33. 33
4
OBJECT CACHING
@cache
def get_item_by_id(key):
# Look up the item in our database
return session.query(User)
.filter_by(id=key)
.first()

34. 34
4
OBJECT CACHING (IN DJANGO)

35. 35
• Full pages
• Partial pages
• Objects
• Queries
4
CACHING IN DJANGO: BACKEND

36. 36
4
QUERY CACHING
Cached
Table
Data
DB client
Query
Cache
SQL server
Retrieve results from memory…
…or from memcached…
…or cache in the DB itself!

37. 37
4
QUERY CACHING
mysql> select SQL_CACHE count(*) from traces;
+----------+
| count(*) |
+----------+
| 3135623 |
+----------+
1 row in set (0.56 sec)
mysql> select SQL_CACHE count(*) from traces;
+----------+
| count(*) |
+----------+
| 3135623 |
+----------+
1 row in set (0.00 sec)

38. 38
4
QUERY CACHING (IN DJANGO)

39. 39
4
QUERY CACHING (IN
TRACEVIEW)
Cassandra in TraceView:
Retrieving data from data warehouse

40. 40
4
QUERY CACHING (IN
TRACEVIEW)
Memcache in TraceView:
97%ile is 50ms!

41. 41
• Invalidation
• Fragmentation
• Stampedes
• Complexity
4
WHAT CAN GO WRONG?

42. 42
4
INVALIDATION
Uncached Cached
Client
Data
Source
Data
Source
Client
Cache Intermediary
Invalidation

43. 43
4
INVALIDATION
Drupal:
Automatically cache everything!

44. 44
4
INVALIDATION
Memcache GET performance

45. 45
4
INVALIDATION
Do you know what you’re storing in
this call?

46. 46
4
INVALIDATION
Memcache SET performance

47. 47
4
FRAGMENTATION
“In the beginning there was NCSA Mosaic, and Mosaic
called itself NCSA_Mosaic/2.0 (Windows 3.1), and
Mosaic displayed pictures along with text, and there was
much rejoicing.”
History of the browser user-agent string

48. 48
• On a cache miss extra work is done
• The result is stored in the cache
• What if multiple simultaneous
misses?
• Every node tries to do the same
work at the same time and your
4
STAMPEDES

49. 49
• What caching scheme?
• How many extra servers?
• What happens if they fail?
• What will you do to debug it?
4
COMPLEXITY

50. 50
• The ‘how’ of caching:
• What are you caching?
• Where are you caching it?
• How bad is a cache miss?
• How and when are you
invalidating?
4
TAKEAWAYS

51. 51
• The ‘why’ of caching:
• Did it actually get faster?
• Is speed worth extra complexity?
• Don’t guess – measure!
• Always use real-world conditions.
4
TAKEAWAYS

52. 52
• ?
4
QUESTIONS

53. 53
• Django documentation on caching:
https://docs.djangoproject.com/en/dev/topics/cache/
• Varnish caching, via Disqus:
http://blog.disqus.com/post/62187806135/scaling-django-to-8-
billion-page-views
• Django cache option comparisons:
http://codysoyland.com/2010/jan/17/evaluating-django-caching-
options/
• More Django-specific tips: http://www.slideshare.net/csky/where-
django-caching-bust-at-the-seams
• Guide to cache-related HTTP headers:
http://www.mobify.com/blog/beginners-guide-to-http-cache-
headers/
4
RESOURCES

54. 54
TraceView
Stop by later tonight to hear more
about AppNeta or TraceView 
(make sure to get a sticker!)
April 1: Come drink on our tab
after the Boston Python Meetup
April 11: PyCon! Swing by our
booth (214) for an awesome shirt
Or, just try us out:
THANK YOU!
http://www.appneta.com/products/traceview/