Feature Flags

A 15 minute presentation given at PyCon 2014 as part of the Advanced Django Patterns workshop http://lanyrd.com/2014/pycon/scxqhp/ describing feature flags, how we rolled our own at Lanyrd and how Eventbrite use Gargoyle.

1. Feature Flags Simon Willison, @simonw ! PyCon 2014
2. “Release early, release often”
4. https://www.flickr.com/photos/dottiemae/5236273049/
5. A way of exposing features to a sub-set of your userbase
6. Decouple deployment from release
7. (+ risk-free instant rollback)
8. FEATURE_FLAGS = {! 'mapbox': 'Uses MapBox maps rather than Google Maps',! }!
9. class FeatureFlag(Model):! slug = SlugField(unique = True)! description = CharField(max_length = 140)! enabled_for_all = BooleanField(default = False)
10. class FeatureFlag(Model):! slug = SlugField(unique = True)! description = CharField(max_length = 140)! enabled_for_all = BooleanField(default = False)! users = ManyToManyField(User)!
11. class FeatureFlag(Model):! slug = SlugField(unique = True)! description = CharField(max_length = 140)! enabled_for_all = BooleanField(default = False)! users = ManyToManyField(User)! groups = ManyToManyField(Group)
12. def is_enabled(self, user):! return (! self.enabled_for_all or! self.users.filter(pk = user.pk).exists() or! self.groups.filter(users__pk = user.pk).exists()! )!
13. @flag_required('topic_tracking')! def topic_track(request, slug):! # ... view code ... !
14. if user.has_flag('topic_tracking'):! # ...!
15. {% flag new_eventbrite %}! <p>Lanyrd is an <img src="/eb.png" alt="Eventbrite"> company</p>! {% endflag %}!
16. {% flagactive login_facebook %}(alpha-only){% endflagactive %}
18. In read_only_mode • Prevent logged-in user actions • Auth middleware ignores user cookie • Mobile app APIs ignore user token • Shows the banner and hide the “sign in” link
19. MySQL on EC2 PostgreSQL on SoftLayer with no downtime
20. Feature flags at Eventbrite
21. Gargoyle
22. • User / Staff • Event • Organizer • API key • Country • IP address (or range) • eventbrite.com v.s. .de v.s. .fr
23. class RequestCountry(ConditionSet):! country = String(label='Country')! def can_execute(self, instance):! return isinstance(instance, (HttpRequest,))! ! def get_field_value(self, request, field_name):! if field_name == 'country':! return getattr(request, 'country')! ! gargoyle.register(RequestCountry())
24. Gotchas • Unexpected flag combinations • Old feature flag code • Performance!
25. Gargoyle performance • Switch = DB row with JSON blob of conditions • Caches switches in local memory AND remote cache • At Eventbrite, we run memcached on every frontend server to reduce network traffic
26. Meanwhile, at Facebook…
27. Meanwhile, at Facebook… 500M+ gatekeeper checks/second - back in 2012!
28. https://www.flickr.com/photos/capcase/4970062156/
29. • Use feature flags, not long-lived branches! • The ROI on feature flags is ridiculous • Try Gargoyle (or Gutter), or roll your own