Simple FeatureFlag component

Introduction

Co-authored with @Neirda24 who is also at the origin of this component.

Based on the initial pull request #51649:

Currently, there is no straightforward method to conditionally execute parts of the source code based on specific
contexts. This PR tries to solve this issue by integrating some easy way to check is this or that feature should be
enabled.

First check out Martin Fowler's article about different use cases for feature flag (a.k.a. feature toggling).
He categorizes feature flag like this:

• Experiment: show a beta version of your website to users who subscribed to.
• Release: deploy a new version of your code but keep the old one to compare them easily and rollback quickly if needed
  or control when a feature is released.
• Permission: grant access to a feature for paid accounts using the Security component.
• Ops: remove access to a consuming feature if server ressources are low (a k.a. kill switch). During Black Friday for
  example it is common to deactivate certain features for Ops because the load will be on other pages.

There are already some libraries / bundles out there but they either lack extensibility or are locked in to a SAAS
tool (Unleash, Gitlab (which uses Unleash), ...).

• https://github.com/akeneo/pim-community-dev/tree/master/src/Akeneo/Platform/Bundle/FeatureFlagBundle
• https://medium.com/devwarlocks/implementing-feature-toggles-in-symfony-920993afd2b4
• https://smaine-milianni.medium.com/feature-flag-and-strategy-pattern-with-the-symfony-framework-7863ecc9556a

Proposal

The FeatureFlag component provides a service that checks if a feature is enabled. A feature is a callable which returns
a value compared to the expected value to determine is the feature is enabled (mostly a boolean but not limited to).

Since every declared feature is a callable, they are only resolved when needed.

The component

A service implementing the ProviderInterface is responsible for giving a callable from a feature name. This callable,
which does not accept any argument (for now), can be called to get the feature value.

The FeatureChecker is responsible for checking if a feature is enabled from its name. It uses a ProviderInterface to
retrieve the corresponding feature, resolves it and compare it to the expected value (true by default).

For now, two providers are currently implemented:

• InMemoryProvider which contains the features passed in its constructor
• ChainProvider which aggregates multiple providers and iterates through them to retrieve a feature. It allows
  combining different sources of feature definitions, such as in-memory, database, or external services.

Usage

Declare your features using a provider and pass it to the checker:

use Symfony\Component\FeatureFlag\FeatureChecker;
use Symfony\Component\FeatureFlag\Provider\InMemoryProvider;

$provider = new InMemoryProvider([
    'weekend_feature' => fn() => date('N') >= 6, // will only be activated on weekends
    'feature_version' => fn() => random_int(1, 3), // will return a random version to use
]);
$featureChecker = new FeatureChecker($provider);

Of course, invokable classes can be used:

use Symfony\Component\FeatureFlag\FeatureChecker;
use Symfony\Component\FeatureFlag\Provider\InMemoryProvider;

final class XmasFeature
{
    public function __invoke(): bool
    {
        return date('m-d') === '12-25';
    }
}

$features = new InMemoryProvider([
    'xmas-feature' => new XmasFeature(),
]);
$featureChecker = new FeatureChecker($features);

Check the feature's value:

$featureChecker->isEnabled('weekend_feature'); // returns true on weekend
$featureChecker->isEnabled('not_a_feature'); // returns false

Once the feature's value is resolved, it is stored to make it immutable:

$featureChecker->getValue('feature_version'); // returns a random version between 1 and 3
$featureChecker->getValue('feature_version'); // returns the previous value

FrameworkBundle integration

In addition, the #[AsFeature] attribute can be used to make a feature from a service:

// Will create a feature named "UserFeature"
use App\Security\User;
use Symfony\Bundle\SecurityBundle\Security;

#[AsFeature]
final class UserFeature
{
    public function __construct(private readonly Security $security) {}

    public function __invoke(?User $user): bool
    {
        if (!($user = $this->security->getUser()) instanceof User) {
            return false;
        } 
    
        return $user->hasGroup(1);
    }
}

In fact, any callable can be declared as a feature using this attribute.

// Will create three features named "AppFeatures::alpha", "AppFeatures::beta", "gamma" and "delta"
#[AsFeature(name: 'delta', method: 'delta')]
final class AppFeatures
{
    #[AsFeature]
    public function alpha(): bool
    {
        // ...
    }
    
    #[AsFeature]
    public function beta(): bool
    {
        // ...
    }
    
    #[AsFeature(name: 'gamma')]
    public function gamma(): bool
    {
        // ...
    }
    
    public function delta(): bool
    {
        // ...
    }
}

Thanks to the ExpressionLanguage component, feature flags can be used in Route definitions.

class MyController
{
    #[Route(path: '/', name: 'homepage_new', condition: 'feature_is_enabled("beta")', priority: 50)]
    public function homepageNew(FeatureCheckerInterface $featureChecker): Response
    {
        // Will be called if the "beta" feature flag is enabled
    }
   
    #[Route(path: '/', name: 'homepage')]
    public function homepage(FeatureCheckerInterface $featureChecker): Response
    {
        // Will be called if the "beta" feature flag is not enabled
    }

WebProfilerBundle integration

The Feature Flag component integrates with the WebProfilerBundle to provide insights into feature evaluations:

Resolved features in the toolbar:

Resolved features in the panel:

Unresolved features in the panel:

Disabled FeatureFlag menu in the panel:

TwigBridge integration

Feature flags can be checked in Twig template if needed.

Beta features enabled: {{ feature_is_enabled('beta')  ? '✅' : '❌' }}

App version used: {{ feature_get_value('app_version')  ? '✅' : '❌' }}

Going further (to discuss in dedicated PRs)

• Add documentation
• Integrate popular SaaS tool as bridges (Gitlab,
  LaunchDarkly, etc.) by adding a dedicated provider
• Propose pre-implemented strategies like opening a feature to a percentage of visitors
• Resolve feature's arguments like it is done for controllers
• Add a #[Feature] to restrict access to a controller
• Declare features from the semantic configuration using the ExpressionLanguage component
• Add a debug:feature to make feature debugging easier

About caching

Immutability

The resolved value of a feature must be immutable during the same request to avoid invalid behaviors. That's why the
FeatureChecker keeps values internally.

Heavy computing

Even if features are lazy loaded, they can be resolved in every request to determine if a controller can be accessed. It
could be problematic when the provider retrieves this value from a database or an external service. Depending on the
solution chosen to store features, the logic to cache value may differ. That's why the value caching between requests
should be handled by providers.

For instance, Unleashed can define some strategy configuration that can be cached and used to resolve feature values.
See https://github.com/Unleash/unleash-client-php/tree/main/src/Strategy.