Ask HN: Do you use feature flags? What's the best tool for the job?
I'm currently evaluating tools to add feature flags to my application at work. Looking for open-source or SaaS tools you folks have used or are currently using.
Obviously I'd like you to use our service, but I'd rather that you find the one that best suits your business needs. There are plenty of good open source feature management systems out there. Just - whatever you do - don't fool yourself into building your own. Your time's worth more than that.
If you want a deeper argument, I wrote it up here, along with some examples of good open source systems: https://launchdarkly.com/blog/feature-management-platform-bu...
To this I'd say it totally depends on the usecase, I've seen 10s of tech teams build this badly and end up causing more harm than good, some even just static config files that require a build to change which totally defeats the point.
I think looking for an open source tool is great, to this I'd say consider the following :
- Before anything else, you'd definitely want the ability to turn the flags off per environment (dev/prod etc) without having to deploy code.
- Do you want to control who can manage features? Quite often you'd want to enable non-tech people to manage / test features.
- How scalable does it need to be? Some people might want to just not have to worry about serving thousands or millions of flags.
- Do you want to serve flags based on users or maybe even groups of users? I've seen attempts at this but then you start having to worry about setting traits and building some sort of rules engine.
Theirs was dynamic in that you could change the rules and at a given interval the connected user's app would refresh and react accordingly. It was pretty complex, I suppose, but I'd roll out my own and go to something like them if I had a lot more conditions or targets.
It's really not complicated.
Some webframeworks require an app restart for config file changes to be detected, or restart the app automatically when it detects changes to the file, so I've seen people use databases. But it tends to be an overcomplication and it's usually turned out to be a bit of a pita. It's only worth it if you're regulalrly turning the feature on and off.
Many languages also support build flags, which strips the extra code out completely, but it's an obscure feature, unless you write libraries, that can confuse other programmers.
There's a huge amount of value in being able to flip a flag value and have the app react without a restart. The most basic scenario: being able to separate deploy from release. Deploy code with each major code change turned off; ensure the deploy's gone well, then turn the changes on one at a time (or whenever the feature is meant to go live). Not only does it make problem diagnosis much easier, but it means you don't need to rollback if one of them is buggy - just turn it off again.
But these are only feature flags in the simplest sense. Modern feature flag systems also allow targeting rules, which allow the flag to evaluate differently based on contextual attributes (e.g. something about the user). They also stream these rules to a simple local rule engine within the code so that evaluation isn't blocked on I/O. Then there's access control, experiments, multivariate flags...
KISS, you most likely don't need to use an external feature flag "hadron collider".
[1] https://www.martinfowler.com/articles/feature-toggles.html
If it's a SaaS, how does your app function if the SaaS is unavailable?
* feature flags were generalised as certain class of functions that made an on/off decision based on certain inputs, and the definition of these functions could be sent from the server to client services that had integrated the feature flag library. Functions could be refined at runtime etc -- generalised notion of toggling a toggle.
* there appeared to be capability for services to maintain local cache of the feature flag / expression rules so it didn't introduce a hard failure point on external service. Functions uses to make toggling decisions could be evaluated with inputs locally.
Take this with several grains of salt, never used it, I might be misremembering and it might not actually work this way
Both of these are true. See also my reply to the parent comment - you don't even need a local cache, since all flag evaluation calls should include a fallback value.
There's the local cache feature mentioned by shoo below, but more simply, there's also a fallback in the flag eval function, which looks like:
flag_value = LDClient.variation("flag-identifier", context_object, fallback_value)
As for DIY-ing it, I wrote about the pros and cons of rolling your own here: https://launchdarkly.com/blog/feature-management-platform-bu...
(TL;DR: Sure, you could build your own super simple one in a few hours, but it wouldn't have most of the features that make feature flags useful, and why bother when there are open source implementations already?)
In Rails if you have Active Admin, you just define the migration for feature flags and you are basically done. AA gives you a UI which you can protect with roles. Put a couple methods on the model and then you can toggle features anywhere in your code.
Our use case is very small, feature flags get deleted after all the moving parts are shipped and working. I'm sure your product has many more use cases and capabilities.
https://github.com/BulletTrainHQ
We'd love feedback!