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# Rollout, feature flag, experiment, operational toggle

@Different use cases for backend, frontend and mobile 

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# "Feature flags" tend to come up when talking about continuous deployment

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@CI: continuous integration
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@CD: continuous delivery
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@CD: continuous deployment

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## Types
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1. rollout
2. feature flag
3. experiment
4. operational toggle

% {favicon.svg}
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## Rollout

# For *rolling out* a new version of software

Short-lived using percentages

% FIXME: links
- a new deployment of kubernetes
- new APK released to the Play Store

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## Feature flag

# For turning a feature *on* or *off* 

Medium-lived using allow list, A/B test, percentage, app version, etc.

- :new-chargeback-flow
- :new-debit-card-activation-screen

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## Experiment

# For analysing behaviour

Medium-lived using allow list and A/B test

- :debit-withdrawal-test

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## Operational toggle

# For disabling features in #crash-like situations

Long-lived using percentage

- :bank-barcode-payment
- :savings-bank-barcode-query-provider

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@We know know about the types
# But they have different relevance for backend, frontend and mobile

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## backend
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1. rollout: k8s blue/green, canary and ~common-rollout~ common-xp
2. feature flag: ~common-rollout~ common-xp and datasets
3. experiment: common-xp
4. operational toggle: ~common-rollout~ common-xp 

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## frontend
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1. rollout: CDN and page refreshes
2. feature flag: percentages and maybe IPs (no :customer/id on the website)
3. experiment: via dynamic backend control
4. operational toggle: via dynamic backend control 

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## backend
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1. rollout: app stores
2. feature flag: via dynamic backend control
3. experiment: via dynamic backend control
4. operational toggle: via dynamic backend control 

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@Key differentiator is
# how much *control* we have over the environment

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## backend
# full control
% FIXME: emoji
% 🎉

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## frontend
# partial control
We choose when to make a new version available 

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## mobile
# very limited control
- app stores can restrict updates (worse for iOS)
- customers still have to download new versions 

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# Costs
- more complex code
- compatibility with old app versions
- nesting is exponential

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# Benefits
- dynamicity

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## weighting costs × benefits
# The less control we have, the more we value dynamicity 

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## weighting costs × benefits
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- backend: sometimes worth the cost
- frontend: almost always worth the cost
- mobile: *always* worth the cost

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# Best practices

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# dynamic content > feature flag
Always true for mobile, almost always for frontend 

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# Use :include-list for named groups 
Always true for backend, frontend and mobile 

	{:rules
	 #{{:types   :include-list
	    :content {:filename "debit-team-members.txt"}}}}

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# Always use :app-version
Only for mobile

	{:rules
	 #{{:types   :app-version
	    :content {:min-version #{{:platform :android
	                              :code     1000000}
	                             {:platform :ios
	                              :code     2000000}}}}}}

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# Extend ~common-rollout~ common-xp if required

That's how :include-list, :app-version, etc. were born

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# Beware of many nested feature flags
True for backend, frontend and mobile

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# Don't delete app-facing feature flags
True for mobile

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# Include a feature flag on the whiteboarding phase

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# Include deleting/retiring the feature flag at the end

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# Avoid renaming a feature flag
Use :app-version with :min-version instead

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# And most importantly...

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# *Always* rely on a feature flag on the app
Never do a hotfix, avoid expedited releases at all costs

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## References
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% FIXME: links
1. "Feature Toggles (aka Feature Flags)", by Pete Hodgson
2. "Continuous integration vs. delivery vs. deployment", by Sten Pittet
3. Accelerate, by N. Forsgren, J. Humble and G. Kim
4. these slides: euandre.org/slide/
5. prose version of this presentation
6. view source
