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-= Feature flags: differences between backend, frontend and mobile
-:categories: presentation
-:updatedat: 2020-11-03
-
-:empty:
-:slides: link:../../../../slides/2020/10/19/feature-flags.html FIXME
-:fowler-article: https://martinfowler.com/articles/feature-toggles.html
-
-_This article is derived from a {slides}[presentation] on the same subject._
-
-When discussing about feature flags, I find that their costs and benefits are
-often well exposed and addressed.  Online articles like
-"{fowler-article}[Feature Toggle (aka Feature Flags)]" do a great job of
-explaining them in detail, giving great general guidance of how to apply
-techniques to adopt it.
-
-However the weight of those costs and benefits apply differently on backend,
-frontend or mobile, and those differences aren't covered.  In fact, many of them
-stop making sense, or the decision of adopting a feature flag or not may change
-depending on the environment.
-
-In this article I try to make the distinction between environments and how
-feature flags apply to them, with some final best practices I've acquired when
-using them in production.
-
-== Why feature flags
-
-:atlassian-cicd: https://www.atlassian.com/continuous-delivery/principles/continuous-integration-vs-delivery-vs-deployment
-
-Feature flags in general tend to be cited on the context of
-{atlassian-cicd}[continuous deployment]:
-
-____
-A: With continuous deployment, you deploy to production automatically
-
-B: But how do I handle deployment failures, partial features, _etc._?
-
-A: With techniques like canary, monitoring and alarms, feature flags, _etc._
-____
-
-Though adopting continuous deployment doesn't force you to use feature flags, it
-creates a demand for it.  The inverse is also true: using feature flags on the
-code points you more obviously to continuous deployment.  Take the following
-code sample for example, that we will reference later on the article:
-
-[source,javascript]
-----
-function processTransaction() {
-	validate();
-	persist();
-	// TODO: add call to notifyListeners()
-}
-----
-
-While being developed, being tested for suitability or something similar,
-`notifyListeners()` may not be included in the code at once.  So instead of
-keeping it on a separate, long-lived branch, a feature flag can decide when the
-new, partially implemented function will be called:
-
-[source,javascript]
-----
-function processTransaction() {
-	validate();
-	persist();
-	if (featureIsEnabled("activate-notify-listeners")) {
-		notifyListeners();
-	}
-}
-----
-
-This allows your code to include `notifyListeners()`, and decide when to call it
-at runtime.  For the price of extra things around the code, you get more
-dynamicity.
-
-So the fundamental question to ask yourself when considering adding a feature
-flag should be:
-
-____
-Am I willing to pay with code complexity to get dynamicity?
-____
-
-It is true that you can make the management of feature flags as straightforward
-as possible, but having no feature flags is simpler than having any.  What you
-get in return is the ability to parameterize the behaviour of the application at
-runtime, without doing any code changes.
-
-Sometimes this added complexity may tilt the balance towards not using a feature
-flag, and sometimes the flexibility of changing behaviour at runtime is
-absolutely worth the added complexity.  This can vary a lot by code base,
-feature, but fundamentally by environment: its much cheaper to deploy a new
-version of a service than to release a new version of an app.
-
-So the question of which environment is being targeted is key when reasoning
-about costs and benefits of feature flags.
-
-== Control over the environment
-
-:fdroid: https://f-droid.org/
-:bad-apple: https://www.paulgraham.com/apple.html
-
-The key differentiator that makes the trade-offs apply differently is how much
-control you have over the environment.
-
-When running a *backend* service, you usually are paying for the servers
-themselves, and can tweak them as you wish.  This means you have full control do
-to code changes as you wish.  Not only that, you decide when to do it, and for
-how long the transition will last.
-
-On the *frontend* you have less control: even though you can choose to make a
-new version available any time you wish, you can't
-force{empy}footnote:force[
-  Technically you could force a reload with JavaScript using
-  `window.location.reload()`, but that not only is invasive and impolite, but
-  also gives you the illusion that you have control over the client when you
-  actually don't: clients with disabled JavaScript would be immune to such
-  tactics.
-] clients to immediately switch to the new version.  That means that a) clients
-could skip upgrades at any time and b) you always have to keep backward and
-forward compatibility in mind.
-
-Even though I'm mentioning frontend directly, it applies to other environment
-with similar characteristics: desktop applications, command-line programs,
-_etc_.
-
-On *mobile* you have even less control: app stores need to allow your app to be
-updated, which could bite you when least desired.  Theoretically you could make
-you APK available on third party stores like {fdroid}[F-Droid], or even make the
-APK itself available for direct download, which would give you the same
-characteristics of a frontend application, but that happens less often.
-
-On iOS you can't even do that.  You have to get Apple's blessing on every single
-update.  Even though we already know that is a {bad-apple}[bad idea] for over a
-decade now, there isn't a way around it.  This is where you have the least
-control.
-
-In practice, the amount of control you have will change how much you value
-dynamicity: the less control you have, the more valuable it is.  In other words,
-having a dynamic flag on the backend may or may not be worth it since you could
-always update the code immediately after, but on iOS it is basically always
-worth it.
-
-== Rollout
-
-:kubernetes-deployment: https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#creating-a-deployment
-:play-store-rollout: https://support.google.com/googleplay/android-developer/answer/6346149?hl=en
-:app-store-rolllout: https://help.apple.com/app-store-connect/#/dev3d65fcee1
-
-A rollout is used to _roll out_ a new version of software.
-
-They are usually short-lived, being relevant as long as the new code is being
-deployed.  The most common rule is percentages.
-
-On the *backend*, it is common to find it on the deployment infrastructure
-itself, like canary servers, blue/green deployments, {kubernetes-deployment}[a
-kubernetes deployment rollout], _etc_.  You could do those manually, by having a
-dynamic control on the code itself, but rollbacks are cheap enough that people
-usually do a normal deployment and just give some extra attention to the metrics
-dashboard.
-
-Any time you see a blue/green deployment, there is a rollout happening: most
-likely a load balancer is starting to direct traffic to the new server, until
-reaching 100% of the traffic.  Effectively, that is a rollout.
-
-On the *frontend*, you can selectively pick which user's will be able to
-download the new version of a page.  You could use geographical region, IP,
-cookie or something similar to make this decision.
-
-CDN propagation delays and people not refreshing their web pages are also
-rollouts by themselves, since old and new versions of the software will coexist.
-
-On *mobile*, the Play Store allows you to perform fine-grained
-{play-store-rollout}[staged rollouts], and the App Store allows you to perform
-limited {app-store-rollout}[phased releases].
-
-Both for Android and iOS, the user plays the role of making the download.
-
-In summary: since you control the servers on the backend, you can do rollouts at
-will, and those are often found automated away in base infrastructure.  On the
-frontend and on mobile, there are ways to make new versions available, but users
-may not download them immediately, and many different versions of the software
-end up coexisting.
-
-== Feature flag
-
-A feature flag is a _flag_ that tells the application on runtime to turn on or
-off a given _feature_.  That means that the actual production code will have
-more than one possible code paths to go through, and that a new version of a
-feature coexists with the old version.  The feature flag tells which part of the
-code to go through.
-
-They are usually medium-lived, being relevant as long as the new code is being
-developed.  The most common rules are percentages, allow/deny lists, A/B groups
-and client version.
-
-On the *backend*, those are useful for things that have a long development
-cycle, or that needs to done by steps.  Consider loading the feature flag rules
-in memory when the application starts, so that you avoid querying a database or
-an external service for applying a feature flag rule and avoid flakiness on the
-result due to intermittent network failures.
-
-Since on the *frontend* you don't control when to update the client software,
-you're left with applying the feature flag rule on the server, and exposing the
-value through an API for maximum dynamicity.  This could be in the frontend code
-itself, and fallback to a "just refresh the page"/"just update to the latest
-version" strategy for less dynamic scenarios.
-
-On *mobile* you can't even rely on a "just update to the latest version"
-strategy, since the code for the app could be updated to a new feature and be
-blocked on the store.  Those cases aren't recurrent, but you should always
-assume the store will deny updates on critical moments so you don't find
-yourself with no cards to play.  That means the only control you actually have
-is via the backend, by parameterizing the runtime of the application using the
-API.  In practice, you should always have a feature flag to control any relevant
-piece of code.  There is no such thing as "too small code change for a feature
-flag".  What you should ask yourself is:
-
-____
-If the code I'm writing breaks and stays broken for around a month, do I care?
-____
-
-If you're doing an experimental screen, or something that will have a very small
-impact you might answer "no" to the above question.  For everything else, the
-answer will be "yes": bug fixes, layout changes, refactoring, new screen,
-filesystem/database changes, _etc_.
-
-== Experiment
-
-An experiment is a feature flag where you care about analytical value of the
-flag, and how it might impact user's behaviour.  A feature flag with analytics.
-
-They are also usually medium-lived, being relevant as long as the new code is
-being developed.  The most common rule is A/B test.
-
-On the *backend*, an experiment rely on an analytical environment that will pick
-the A/B test groups and distributions, which means those can't be held in memory
-easily.  That also means that you'll need a fallback value in case fetching the
-group for a given customer fails.
-
-On the *frontend* and on *mobile* they are no different from feature flags.
-
-== Operational toggle
-
-An operational toggle is like a system-level manual circuit breaker, where you
-turn on/off a feature, fail over the load to a different server, _etc_.  They
-are useful switches to have during an incident.
-
-They are usually long-lived, being relevant as long as the code is in
-production.  The most common rule is percentages.
-
-They can be feature flags that are promoted to operational toggles on the
-*backend*, or may be purposefully put in place preventively or after a
-postmortem analysis.
-
-On the *frontend* and on *mobile* they are similar to feature flags, where the
-"feature" is being turned on and off, and the client interprets this value to
-show if the "feature" is available or unavailable.
-
-== Best practices
-
-=== Prefer dynamic content
-
-Even though feature flags give you more dynamicity, they're still somewhat
-manual: you have to create one for a specific feature and change it by hand.
-
-If you find yourself manually updating a feature flags every other day, or
-tweaking the percentages frequently, consider making it fully dynamic.  Try
-using a dataset that is generated automatically, or computing the content on the
-fly.
-
-Say you have a configuration screen with a list of options and sub-options, and
-you're trying to find how to better structure this list.  Instead of using a
-feature flag for switching between 3 and 5 options, make it fully dynamic.  This
-way you'll be able to perform other tests that you didn't plan, and get more
-flexibility out of it.
-
-=== Use the client version to negotiate feature flags
-
-After effectively finishing a feature, the old code that coexisted with the new
-one will be deleted, and all traces of the transition will vanish from the code
-base.  However if you just remove the feature flags from the API, all of the old
-versions of clients that relied on that value to show the new feature will go
-downgrade to the old feature.
-
-This means that you should avoid deleting client-facing feature flags, and
-retire them instead: use the client version to decide when the feature is
-stable, and return `true` for every client with a version greater or equal to
-that.  This way you can stop thinking about the feature flag, and you don't
-break or downgrade clients that didn't upgrade past the transition.
-
-=== Beware of many nested feature flags
-
-Nested flags combine exponentially.
-
-Pick strategic entry points or transitions eligible for feature flags, and
-beware of their nesting.
-
-=== Include feature flags in the development workflow
-
-Add feature flags to the list of things to think about during whiteboarding, and
-deleting/retiring a feature flags at the end of the development.
-
-=== Always rely on a feature flag on the app
-
-Again, there is no such thing "too small for a feature flag".  Too many feature
-flags is a good problem to have, not the opposite.  Automate the process of
-creating a feature flag to lower its cost.