Give your pony wings

Benjamin Wohlwend @piquadrat_ch slides - https://speakerdeck.com/piquadrat/make-your-pony-fly

• I like my code to be lazy
• Making your code not do too much work makes your code faster
• make a stackoverflow clone - for pony owners
• use Django - models for Question, Answer, Tag, User ...
• views for QuestionList, QuestionView
• templates ...
• go live - happy until ...
• someone linked from ponynews and
• we have a performance problem (or maybe several problems)
• so how do you find the issues
• django-debug-toolbar, django-dev-server, ...
• django-debug-toolbar is great - lots of features - SQL queries shown - hundreds of queries for one page
• django-devserver - shows number of sql queries, number of duplicates, time for request

The big three

• reduce database queries
• do less work
• if all else fails - cache

Less SQL queries

• select_related()
  • use to preload objects referenced by ForeignKey
  • eg looking up user in template
• prefetch_related()
  • similar for reverse ForeignKey and ManyToMany
  • so get all tags for all questions in one query

Overall, go from 121 queries to 3 queries!

Do less work

• move work out of request/response cycle (eg celery)
• only fetch data that we need - QuerySet.defer() can skip unneeded large columns (or only())
• let others do the work (eg do calculations in the database with annotate/aggregate)

Caching

• doing properly gives you great scaling
• but cache invalidation is really hard
• can also make code hard to test and understand
• so recommend only use it when necessary
• cache_page decorator - put it on view - timeout
• template caching with {% cache %}
• ORM caching with jonny-cache or django-cache-machine
• caching http proxy like Varnish

Much more in book - High Performance Django