Executing arbitrary logic when a feature flag status gets retrieved from storage

[nicolasiensen]

I checked Bugsnag's new feature, where we can now include the enabled feature flags to be part of the error reports.

They updated their Ruby SDK to allow developers to include feature flags as part of the error reports like so:

Bugsnag.add_feature_flag('New checkout flow')

On my team, we disabled Flipper's preload due to performance implications, so we would like to call Bugsnag.add_feature_flag every time we interact with Flipper to retrieve the status of a particular feature flag. Can I do that already?

One way to achieve this would be to make the Feature class observable and register observers in the library like the example below:

class FeatureCheckObserverForBugsnag < Flipper::FeatureCheckObserver
  def call(feature, enabled)
    if enabled
      Bugsnag.add_feature_flag(feature.name)
    else
      Bugsnag.clear_feature_flag(feature.name)
    end
  end
end

Flipper.add_feature_check_observer(FeatureCheckObserverForBugsnag.new)

# Flipper will now include the new observer on every new instance of Feature that it instantiates

Let me know if there is already an approach to get the outcome I'm looking for and if you have any comments about my proposal. I will happily implement it if you think it is worth it.

Cheers!

[bkeepers]

@nicolasiensen This should be possible with Flipper's existing support for Instrumentation (assuming you're using Rails, it everything will already be configured for you). I'll update the docs based on this discussion because that link doesn't show you how to do custom stuff, but try adding this to an initializer in your app:

ActiveSupport::Notifications.subscribe('feature_operation.flipper') do |name, start, finish, id, payload|
  if payload[:operation] == :enabled?
    puts "#{payload[:feature_name]} enabled? #{payload[:result]}"
  end
end

Now when I run this code…

Flipper.enabled? :instrument
Flipper.enable :instrument
Flipper.enabled? :instrument

…I see this:

my_feature enabled? false
my_feature enabled? true

Other operations besides :enabled? are :enable, :disable, :add, :remove, :exist?, and :clear.

Let me know if this doesn't solve your problem and I'll reopen the issue.