Caching Up and

Down the Stack

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James MeickleDeveloper evangelist, AppNeta

@jmeickle

Django Boston Meetup GroupMarch 26, 2014

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4WHAT IS CACHING?

Uncached

Client

Data Source

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4WHAT IS CACHING?

Uncached Cached

Client

Data Source

Data Source

Client

Cache Intermediary

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4WHAT IS CACHING?

Uncached Cached

Client

Data Source

Data Source

Client

Cache Intermediary

Fast!

Slow...

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• Images

• CSS

• JavaScript

• HTML documents

• DNS

4WHAT GETS CACHED BY CLIENTS?

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• HTML documents

• HTML fragments

• Operations on data

• Database queries

• Expensive objects

4WHAT GETS CACHED BY APPS?

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• Compiled source

• Packages

• Disk access

• Memory access

• CPU instructions

4WHAT GETS CACHED BY SERVERS?

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• Client-side assets

• Full pages

4CACHING IN DJANGO: FRONTEND

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4OUR SITESappneta.com

(static site via Jekyll)

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4OUR SITESinternal.tracelytics.com

(dynamic single page Pylons app)

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4CLIENT-SIDE ASSETS

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• Use HTTP caches!

• CDN

• Intermediate proxies

• Browser

• Set policy with cache headers

• Cache-Control

• Expires

• ETag

4CLIENT-SIDE ASSETS

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4CLIENT-SIDE ASSETS

/tl-layouts_base-compiled-757f5eec3603f60850acfdb86e6701cf104f80ae.cssRequest Method: GETStatus Code: 304 Not Modified

Cache-Control: max-age=315360000Connection: keep-aliveDate: Mon, 18 Feb 2013 22:46:12 GMTExpires: Thu, 31 Dec 2037 23:55:55 GMTLast-Modified: Tue, 12 Feb 2013 21:10:20 GMTServer: nginx/0.8.54

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4CLIENT-SIDE ASSETSappneta.com

internal.tracelytics.com

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4CLIENT-SIDE ASSETS (IN DJANGO)

VS

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• Full pages

• Partial pages

• Objects

• Queries

4CACHING IN DJANGO: BACKEND

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4WE’RE STILL TALKING ABOUT PAGES?

Client-sideassets

Pages

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4FULL-PAGE HTTP CACHING

Client Varnish

Do HTTP caching, but with your

rules.

No internet standards necessary! Webserver

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• Why do it server-side?

• Invalidation

• Amount cached

• Changing cache policies

4FULL-PAGE HTTP CACHING

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4FULL-PAGE HTTP CACHING (IN DJANGO)

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• Full pages

• Partial pages

• Objects

• Queries

4CACHING IN DJANGO: BACKEND

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4FULL-PAGE HTTP CACHING?

appneta.com

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4FRAGMENT CACHING

appneta.com

Ruins everything

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4FRAGMENT CACHING

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4FRAGMENT CACHING

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4FRAGMENT CACHING

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4FRAGMENT CACHING (IN DJANGO)

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• Full pages

• Partial pages

• Objects

• Queries

4CACHING IN DJANGO: BACKEND

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4OBJECT CACHING

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

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4OBJECT 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

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4OBJECT CACHING

@decoratordef 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

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4OBJECT CACHING

@cachedef get_item_by_id(key): # Look up the item in our database return session.query(User)\ .filter_by(id=key)\ .first()

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4OBJECT CACHING (IN DJANGO)

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• Full pages

• Partial pages

• Objects

• Queries

4CACHING IN DJANGO: BACKEND

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4QUERY CACHING

Cached

TableData

DB client

Query Cache

SQL server

Retrieve results from

memory…

…or from memcached…

…or cache in the DB itself!

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4QUERY 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)

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4QUERY CACHING (IN DJANGO)

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4QUERY CACHING (IN TRACEVIEW)

Cassandra in TraceView:Retrieving data from data warehouse

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4QUERY CACHING (IN TRACEVIEW)

Memcache in TraceView:97%ile is 50ms!

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• Invalidation

• Fragmentation

• Stampedes

• Complexity

4WHAT CAN GO WRONG?

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4INVALIDATION

Uncached Cached

Client

Data Source

Data Source

Client

Cache Intermediary

Invalidation

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4INVALIDATION

Drupal: Automatically cache everything!

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4INVALIDATION

Memcache GET performance

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4INVALIDATION

Do you know what you’re storing in this call?

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4INVALIDATION

Memcache SET performance

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4FRAGMENTATION

“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

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• 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

cache dies

4STAMPEDES

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• What caching scheme?

• How many extra servers?

• What happens if they fail?

• What will you do to debug it?

4COMPLEXITY

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• 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?

4TAKEAWAYS

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• The ‘why’ of caching:

• Did it actually get faster?

• Is speed worth extra

complexity?

• Don’t guess – measure!

• Always use real-world

conditions.

4TAKEAWAYS

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• ?

4QUESTIONS

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• 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/

4RESOURCES

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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/