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When feature flags do and don’t make sense (2019)

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Over the previous years, I’ve worked in a couple of teams adopting very varied options as regards to characteristic flags. I’ve viewed the execs and cons of both, and over time, I stumbled on myself disagreeing with any fundamentalist region on their use. There might be rather heaps of nuance to this matter, and I gain it’s miles price obsessed on more carefully the many eventualities the save characteristic flags style and charm now no longer save sense.

The Reasons For

There are a couple of most indispensable eventualities the save characteristic flags save rather heaps of sense. The fundamental is when it’s feeble for A/B trying out, the prevent absolutely style prefer varied behaviors for diverse customers, fixed with their randomly assigned remedy. I’ve viewed this strategy employed extremely properly at Amazon the save fresh capabilities are gated by a “characteristic flag” that is de facto controlled by an internal A/B trying out framework. The framework randomly exposes some Amazon possibilities to the fresh characteristic, after which shows their subsequent habits in negate to estimate the change affect of launching the characteristic. 

I used to be within the foundation skeptical, nevertheless used to be soon won over by how easy the framework used to be to use, and the treasured insights it supplied on the advantages (or drawbacks) of determined capabilities. “Taste of the month” decisions had been replaced with exact files. And none of here’s imaginable with out the usage of “characteristic flags” to dynamically toggle fresh capabilities.


One other immense use case for characteristic flags, is whenever you happen to’re working on a essentially complex legend that require many varied sub-tasks to be done in varied elements of the system. Sub-tasks that are too rather heaps of and invasive to be performed in a single pull-seek files from.

In such conditions, attempting to back all these disparate adjustments in facet-branches and coordinating a simultaneous merge and deployment, is a recipe for danger. It’s rather more manageable to gate any disruptive adjustments within the support of a master flag, merge and deploy the total sub-commits incrementally, and charm a flag-flip once the total items are in save of dwelling.


One closing use case for characteristic flags, is whenever you happen to style now no longer delight in adjust over your deployments. For instance, purchase into sage the Facebook Android app, which contains code contributed by hundreds of rather heaps of teams, all mixed and deployed as a single binary. In such eventualities, performing rollbacks might perhaps additionally be infeasible. For life like, political, bureaucratic or even marketing reasons. In such conditions, characteristic flags enable your crew to toggle fresh efficiency or mitigate unhealthy adjustments, with out having to rollback or deploy any fresh binaries.

Somebody on Reddit identified a identical use-case for characteristic flags: focusing on a essentially particular launch date for marketing reasons, whereas tranquil deploying your code indispensable earlier, in negate to make particular steadiness. That you might then delight in a “dynamic” characteristic flag that automatically permits itself at a particular time. Here is additionally a immense use-case for identical reasons – altering efficiency in eventualities the save deploying a brand fresh binary is impractical.

Threat Aversion

The above are all astonishing use conditions for characteristic flags, nevertheless I’ve additionally viewed teams collect bogged down by policies that overreach in their use. For instance, mandating that each code swap ought to be within the support of a characteristic flag, “swish in case we made a mistake”.

Threat management must tranquil certainly be a priority for all teams. Nonetheless there are higher ways of doing this than relying on characteristic flags, notably if your crew has adjust over its gain deployments. The immense majority of your bugs ought to be caught by your automatic take a look at suite and/or QA activity. And the closing few stragglers ought to be handled the usage of incremental deployments, manufacturing alarms and rollbacks.

Moreover, as soon as any predicament is detected, the advice at locations delight in Google is to rollback first and analysis the predicament later:

We now delight in viewed this at Google any quantity of events, the save a hasty deployed roll-ahead repair either fails to repair the customary predicament, or certainly makes issues worse. Even supposing it fixes the predicament it might perhaps perhaps then negate other latent bugs within the system; you’re taking yourself extra from a acknowledged-correct bid, into the wilds of a launch that hasn’t been enviornment to the humble strenuous QA trying out. At Google, our philosophy is that “rollbacks are traditional.” When an error is stumbled on or moderately suspected in a brand fresh launch, the releasing crew rolls support first and investigates the predicament 2d

When issues are on fireplace, the closing thing you ought to style is root-trigger the malicious program and figure out which flag-flip will safely repair the predicament. And that would now no longer even style issues – there’s no guarantee that even though your teammate tried to save diverse his adjustments within the support of a characteristic flag, he didn’t inadvertently introduce a malicious program that can now no longer be solved by a flag-flip.

Characteristic flags are a miserable man’s different to binary rollbacks, and they surely aren’t a change for having a immense automatic take a look at suite and an most indispensable QA activity. If you happen to’re relying on characteristic flags to unravel manufacturing bugs, you must tranquil cease and purchase into sage your crew’s practices. Threat aversion is incessantly a smell of your crew entering correct into a doom loop which is in a region to easiest collect worse and worse with time.

Death By Characteristic Flags

That you might very properly be questioning at this level why we shouldn’t use characteristic flags anyway. Finally, “defense in depth” … and it under no circumstances hurts to thrill in more vivid-grain flexibility correct?

While characteristic flags are immense in some conditions, we must tranquil additionally purchase into sage their fees. Software engineering is primarily an exercise in managing complexity. And every characteristic flag immediately doubles the universe of corner conditions that your programmers want to comprehend, and your code is required to tackle. “Nonetheless what would happen if Foo is enabled, Bar is disabled, and we style just A/B tests on Baz and Kaz on the same day?” In my ride, this combinatorial explosion in complexity can and will result in bugs. No longer to level out slowing down the tempo at which your crew can save any adjustments.

“Nonetheless these characteristic flags are easiest transient. Try and be removing them as soon as imaginable!”

Sure, and we must tranquil additionally now no longer enable our tech debt to make your mind up up and we must tranquil observe each ultimate-observe religiously. Sadly, this under no circumstances happens in any corporate atmosphere. Even in immense teams, tech debt customarily will get de-prioritized within the face of newest requests. Novices to the crew or these on their approach out, aren’t continually disciplined sufficient to tidy up their flags after a generous rollout. And in most cases, these tasks merely spin by the cracks and collect forgotten.

There’ll not be any longer any such thing as a higher illustration of this than the KCG debacle the save a monetary firm lost half one billion bucks and almost went bankrupt in 30 minutes, partly due to the ineffective code that used to be within the support of a characteristic flag.

The trigger of the failure used to be due to the a couple of issues. On the opposite hand, a few of the biggest used to be that a flag which had previously been feeble to enable Vitality Peg… Vitality Peg had been primitive since 2003, but tranquil remained within the codebase some eight years later.

In 2005, an alteration used to be made to the Vitality Peg code which inadvertently disabled security-tests which would delight in shunned this kind of predicament. On the opposite hand, this change used to be deployed to a producing system on the time, regardless of no effort having been made to substantiate that the Vitality Peg efficiency tranquil worked


Characteristic flags are a extremely advantageous instrument that enable you experiment with fresh capabilities, save of dwelling up the rollout of complex epics, and mitigate the issues connected to now no longer controlling your crew’s deployments.

Nonetheless they approach at a most indispensable charge, within the save of code complexity, tech debt, slower pattern speeds, and inevitably, bugs.

As tempting as it might perhaps perhaps very properly be, there is no such thing as a silver bullet here. Weigh the execs against the cons, and use this instrument judiciously when it makes sense to style so.


Dialogue thread on /r/programming

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