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Feature Flags
Simon Willison, @simonw
!
PyCon 2014
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“Release early, release often”
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No content
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https://www.flickr.com/photos/dottiemae/5236273049/
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A way of exposing features to a
sub-set of your userbase
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Decouple deployment from release
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(+ risk-free instant rollback)
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FEATURE_FLAGS = {!
'mapbox': 'Uses MapBox maps rather than Google Maps',!
}!
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class FeatureFlag(Model):!
slug = SlugField(unique = True)!
description = CharField(max_length = 140)!
enabled_for_all = BooleanField(default = False)
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class FeatureFlag(Model):!
slug = SlugField(unique = True)!
description = CharField(max_length = 140)!
enabled_for_all = BooleanField(default = False)!
users = ManyToManyField(User)!
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class FeatureFlag(Model):!
slug = SlugField(unique = True)!
description = CharField(max_length = 140)!
enabled_for_all = BooleanField(default = False)!
users = ManyToManyField(User)!
groups = ManyToManyField(Group)
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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()!
)!
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@flag_required('topic_tracking')!
def topic_track(request, slug):!
# ... view code ... !
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if user.has_flag('topic_tracking'):!
# ...!
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{% flag new_eventbrite %}!
Lanyrd is an 
company
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{% flagactive login_facebook %}(alpha-only){% endflagactive %}
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No content
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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
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MySQL
on EC2
PostgreSQL
on SoftLayer
with no downtime
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Feature flags at
Eventbrite
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Gargoyle
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• User / Staff
• Event
• Organizer
• API key
• Country
• IP address (or range)
• eventbrite.com v.s. .de v.s. .fr
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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())
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Gotchas
• Unexpected flag combinations
• Old feature flag code
• Performance!
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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
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Meanwhile, at Facebook…
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Meanwhile, at Facebook…
500M+ gatekeeper checks/second - back in 2012!
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https://www.flickr.com/photos/capcase/4970062156/
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• Use feature flags, not long-lived branches!
• The ROI on feature flags is ridiculous
• Try Gargoyle (or Gutter), or roll your own