Zumba Swivel

Swivel is a fresh spin on an old idea: Feature Flags (toggles, bits, switches, etc.).

Typical Feature Flags are all or nothing: either the feature is on for everyone, or it is off for everyone.

// Old School Feature Flag

if ($flagIsOn) {
    // Do something new
} else {
    // Do something old
}

Typical Feature Flags are based on boolean conditionals with few abstractions (if this, then that). Although powerful in their simplicity, this typically leads to increased cyclomatic complexity and eventual technical debt.

Swivel is Different

Swivel is fundamentally different from Typical Feature Flags in two ways:

• Features can be enabled for a subset of an application's users.
• Features are not simple conditionals; a developer defines one or more strategies (behaviors) and Swivel takes care of determining which strategy to use.

Buckets

With Swivel, users are separated into one of ten "buckets," allowing a feature to be enabled for a subset of users. The advantages of this approach are clear:

• Deploying a new feature to 10% of users enables developers to catch unforeseen bugs/problems with new code without negatively affecting all users. These kind of deployments are called Canary Releases. As soon as it is determined that new code is safe, roll out the new feature to more users in increments (30%, 50%, etc.); eventually the feature can enabled for all users, safely.
• A/B testing becomes a breeze. Imagine running up to 9 versions of a new feature with one group kept in reserve as a control. Is feature "A" negatively affecting revenue metrics for 10% of your users? No problem: turn it off and go with version "B" instead. This is easy to do with Swivel.

Behaviors

Agile code needs to be simple and easy to change. Typical Feature Flags allow developers to quickly iterate when business rules change or new features are implemented, but this can often lead to complex, under engineered, brittle blocks of code.

Swivel encourages the developer to implement changes to business logic as independent, high level strategies rather than simple, low level deviations.

Example: Quick Look

$formula = $swivel->forFeature('AwesomeSauce')
    ->addBehavior('formulaSpicy', [$this, 'getNewSpicyFormula'])
    ->addBehavior('formulaSaucy', [$this, 'getNewSaucyFormula'])
    ->defaultBehavior([$this, 'getFormula'])
    ->execute();

Getting Started

The first thing you'll want to do is generate a random number between 1 and 10 for each user in your application. We call this the user's "bucket" index. This is what is used by Swivel to determine which features are enabled for that user.

Note: As a best practice, once a user is assigned to a bucket they should remain in that bucket forever. You'll want to store this value in a session or cookie like you would other basic user info.

Next, you'll need to create a map of features. This map indicates which buckets should have certain features enabled. Here is an example of a simple feature map:

$map = [
    // This is a parent feature slug, arbitrarily named "Payment."
    // The "Payment" feature is enabled for users in buckets 4, 5, and 6
    'Payment' => [4,5,6],

    // This is a behavior slug.  It is a subset of the parent slug,
    // and it is only enabled for users in buckets 4 and 5
    'Payment.Test' => [4, 5],

    // Behavior slugs can be nested.
    // This one is only enabled for users in bucket 5.
    'Payment.Test.VersionA' => [5]
];

When your application starts, configure Swivel and create a new manager instance:

// Get this value from the session or from persistent storage.
$userBucket = 5; // $_SESSION['bucket'];

// Get the feature map data from persistent storage.
$mapData = [ 'Feature' => [4,5,6], 'Feature.Test' => [4,5] ];

// Make a new configuration object
$config = new \Zumba\Swivel\Config($mapData, $userBucket);

// Make a new Swivel Manager.  This is your primary API to the Swivel library.
$swivel = new \Zumba\Swivel\Manager($config);

Way to go! Swivel is now ready to use.

Using Strategies

Now that you have a new Swivel manager created, you can start using it in your application. To use Swivel you need to define behaviors for features of your code; Swivel will decide which behavior to execute based on the current user's bucket and the feature map you loaded in the configuration step.

Strategy Example

Say you have coded a new search algorithm for your website. The Search feature of your site is integral to your business, so you only want to roll out the new algorithm to 10% of your users at first. You decide to only enable the algorithm for users in bucket 5. You configure Swivel and register it with your application:

$map = [ 'Search' => [5], 'Search.NewAlgorithm' => [5] ];
$config = new \Zumba\Swivel\Config($map, $_SESSION['bucketIndex']);
$swivel = new \Zumba\Swivel\Manager($config);

// ServiceLocator is fictional in this example.  Use your own framework or repository to store the
// swivel instance.
ServiceLocator::add('Swivel', $swivel);

In your code to search the site, you define two distinct strategies, and tell Swivel about them:

public function search($params = []) {
    $swivel = ServiceLocator::get('Swivel');
    return $swivel->forFeature('Search')
        ->addBehavior('NewAlgorithm', [$this, 'awesomeSearch'], [$params])
        ->defaultBehavior([$this, 'normalSearch'], [$params])
        ->execute();
}

protected function normalSearch($params) {
    // Tried and True method.
}

protected function awesomeSearch($params) {
    // Super cool new search method.
}

Now, when you call search, Swivel will execute awesomeSearch for users in bucket 5, and normalSearch for all other users.

Swivel API

Zumba\Swivel\Config

constructor($map, $index, $logger)

Used to configure your Swivel Manager instance.

Param	Type	Details
$map (optional)	mixed	Can be one of the following: array — an array of feature/behavior slugs as keys and enabled buckets as values. \Zumba\Swivel\MapInterface — An instance of a configured feature map. \Zumba\Swivel\DriverInterface — An instance of a Swivel driver that will build a MapInterface object.
$index (optional)	integer	The user's predefined bucket index. A number between 1 and 10.
$logger (optional)	LoggerInterface	An optional logger that implements \Psr\Log\LoggerInterface

Examples

$features = [ 'Feature' => [1,2,3] ];
$bucket = 3;

// array
$config = new \Zumba\Swivel\Config($features, $bucket);

// \Zumba\Swivel\Map
$config = new \Zumba\Swivel\Config(new \Zumba\Swivel\Map($features), $bucket);

 // $driver implements \Zumba\Swivel\DriverInterface
$config = new \Zumba\Swivel\Config($driver, $bucket);

Zumba\Swivel\Manager

constructor($config)

This is the primary Swivel object that you will use in your app.

Param	Type	Details
$config	Config	A \Zumba\Swivel\Config instance.

Examples

$config = new \Zumba\Swivel\Config($features, $bucket);
$swivel = new \Zumba\Swivel\Manager($config);

forFeature($slug)

Create a point of deviation in your code. Returns a new Zumba\Swivel\Builder that accepts multiple behaviors, default behaviors, and executes the appropriate code for the user's bucket.

Param	Type	Details
$slug	string	The first section of a feature map slug. i.e., for the feature slug "Test.Version.Two", the $slug would be "Test"

Examples

$builder = $swivel->forFeature('Test');

invoke($slug, $a, $b)

Shorthand syntactic sugar for invoking a simple feature behavior. Useful for ternary style code.

Param	Type	Details
$slug	string	The first section of a feature map slug. i.e., for the feature slug "Test.Version.Two", the $slug would be "Test"
$a	callable	The strategy to execute if the $slug is enabled for the user's bucket.
$b (optional)	callable	The strategy to execute if the $slug is not enabled for the user's bucket. If omitted, invoke will return null if the feature slug is not enabled.

Examples

// without Swivel
$result = $newSearch ? $this->search() : $this->noOp();

// Zumba\Swivel\Manager::invoke
$result = $swivel->invoke('Search.New', [$this, 'search'], [$this, 'noOp']);
$result = $swivel->invoke('Search.New', [$this, 'search']);

returnValue($slug, $a, $b)

Shorthand syntactic sugar for invoking a simple feature behavior using Builder::addValue. Useful for ternary style code.

Param	Type	Details
$slug	string	The first section of a feature map slug. i.e., for the feature slug "Test.Version.Two", the $slug would be "Test"
$a	mixed	The value to return if the $slug is enabled for the user's bucket.
$b (optional)	mixed	The value to return if the $slug is not enabled for the user's bucket. If omitted, returnValue will return null if the feature slug is not enabled.

Examples

// without Swivel
$result = $newSearch ? 'Everything' : null;

// Zumba\Swivel\Manager::returnValue
$result = $swivel->returnValue('Search.New', 'Everything', null);
$result = $swivel->returnValue('Search.New', 'Everything');

Zumba\Swivel\Builder

The Builder API is the primary way that you will write Swivel code. You get a new instance of the Builder when you call Manager::forFeature.

addBehavior($slug, $strategy, $args)

Lazily adds a behavior to this feature that will only be executed if the feature is enabled for the user's bucket.

Param	Type	Details
$slug	string	The second section of a feature map slug. i.e., for the feature slug "Test.Version.Two", the $slug here would be "Version.Two"
$strategy	callable	The strategy to execute if the $slug is enabled for the user's bucket. Since version 2.0.0 $strategy must be a callable. If you want to return a simple value, use Builder::addValue instead.
$args (optional)	array	Parameters to pass to the $strategy callable if it is executed.

Examples

$builder = $swivel->forFeature('Test');

$builder
    // Inline function.  This one will return 'ab'
    ->addBehavior('versionA', function($a, $b) { return $a . $b; }, ['a', 'b'])

    // Callable.  Will return the result of $obj->someMethod('c', 'd');
    ->addBehavior('versionB', [$obj, 'someMethod'], ['c', 'd'])

     // Since version 2.0.0, this will throw a \LogicException.  Use `addValue` instead.
    ->addBehavior('versionC', 'result');

addValue($slug, $value)

Lazily adds a behavior to this feature that will return the value provided. It will only be executed if the feature is enabled for the user's bucket.

Param	Type	Details
$slug	string	The second section of a feature map slug. i.e., for the feature slug "Test.Version.Two", the $slug here would be "Version.Two"
$value	mixed	The value to return if the $slug is enabled for the user's bucket. If $value is a callable it will not be executed. If you want Swivel to execute a callable, use Builder::addBehavior instead.

Examples

$builder = $swivel->forFeature('Test');

$builder
    // This will return `'result'`
    ->addValue('versionA', 'result')

    // Callable.  This will not be executed; Swivel will just return the unexecuted callable.
    ->addValue('versionB', [$obj, 'someMethod']);

defaultBehavior($strategy, $args)

Lazily adds a behavior to this feature that will only be executed if no other feature behaviors are enabled for the user's bucket.

Param	Type	Details
$strategy	callable	The strategy to execute if no other feature behaviors are enabled for the user's bucket. Since version 2.0.0 $strategy must be a callable. If you want to return a simple value, use Builder::defaultValue instead.
$args (optional)	array	Parameters to pass to the $strategy callable if it is executed.

Examples

$swivel
    ->forFeature('Test');
    ->addBehavior('New.Version', [$this, 'someMethod'], $args)
    ->defaultBehavior([$this, 'defaultMethod'], $args);

defaultValue($value)

Lazily adds a behavior to this feature that will return the provided value. It will only be executed if no other feature behaviors are enabled for the user's bucket.

Param	Type	Details
$value	mixed	The value to return if no other feature behaviors are enabled for the user's bucket. If you want Swivel to execute a callable, use Builder::defaultBehavior instead.

Examples

$swivel
    ->forFeature('Test');
    ->addBehavior('New.Version', [$this, 'someMethod'], $args)
    ->defaultValue('some default value');

execute()

Executes the appropriate behavior strategy based on the user's bucket.

Examples

// $result will contain either the result of
// $this->someMethod(1, 2, 3) or $this->defaultMethod('test')
// depending on the user's bucket
$result = $swivel
    ->forFeature('Test');
    ->addBehavior('New.Version', [$this, 'someMethod'], [1, 2, 3])
    ->defaultBehavior([$this, 'defaultMethod'], ['test'])
    ->execute();

noDefault()

If you do not need to define a default behavior to be executed when a feature is not enabled for a user's bucket, call noDefault on the Builder. Swivel will throw a \LogicException if you neglect to define a default behavior and do not call noDefault. Likewise, Swivel will throw a \LogicException if you call both noDefault and defaultBehavior on the same Builder instance.

Examples

$swivel
    ->forFeature('Test');
    ->addBehavior('A', [$obj, 'someMethod'])
    ->execute(); // throws \LogicException here.

$swivel
    ->forFeature('Test');
    ->addBehavior('A', [$obj, 'someMethod'])
    ->noDefault()
    ->execute(); // no exception thrown.

$swivel
    ->forFeature('Test');
    ->addBehavior('A', [$obj, 'someMethod'])
    ->defaultBehavior([$obj, 'anotherMethod'])
    ->noDefault() // throws \LogicException here.
    ->execute();