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Feature Toggles

For an overview of feature toggles, see Feature Toggles (aka Feature Flags). The design presented here is partly inspired by that article.

Requirements

  • It should be possible to turn features on and off by:

  • changing a setting in application.conf

  • sending a particular HTTP header value with an API request

  • (in the future) using a web-based user interface to configure a feature toggle service that multiple subsystems can access

  • Feature implementations should be produced by factory classes, so that the code using a feature does not need to know about the toggling decision.

  • Feature factories should use toggle configuration taken from different sources, without knowing where the configuration came from.

  • An HTTP response should indicate which features are turned on.

  • A feature toggle should have metadata such as a description, an expiration date, developer contact information, etc.

  • A feature toggle should have a version number, so you can get different versions of the same feature.

  • It should be possible to configure a toggle in application.conf so that its setting cannot be overridden per request.

  • The design of feature toggles should avoid ambiguity and try to prevent situations where clients might be surprised by unexpected functionality. It should be clear what will change when a client requests a particular toggle setting. Therefore, per-request settings should require the client to be explicit about what is being requested.

Design

Configuration

Base Configuration

The base configuration of feature toggles is in application.conf under app.feature-toggles. Example:

app {
    feature-toggles {
        new-foo {
            description = "Replace the old foo routes with new ones."

            available-versions = [ 1, 2 ]
            default-version = 1
            enabled-by-default = yes
            override-allowed = yes

            expiration-date = "2021-12-01T00:00:00Z"

            developer-emails = [
                "A developer <a.developer@example.org>"
            ]
        }

        new-bar {
            description = "Replace the old bar routes with new ones."

            available-versions = [ 1, 2, 3 ]
            default-version = 3
            enabled-by-default = yes
            override-allowed = yes

            expiration-date = "2021-12-01T00:00:00Z"

            developer-emails = [
                "A developer <a.developer@example.org>"
            ]
        }

        fast-baz {
            description = "Replace the slower, more accurate baz route with a faster, less accurate one."

            available-versions = [ 1 ]
            default-version = 1
            enabled-by-default = no
            override-allowed = yes

            developer-emails = [
                "A developer <a.developer@example.org>"
            ]
        }
    }
}

All fields are required except expiration-date.

Since it may not be possible to predict which toggles will need versions, all toggles must have at least one version. (If a toggle could be created without versions, and then get versions later, it would not be obvious what should happen if a client then requested the toggle without specifying a version number.) Version numbers must be an ascending sequence of consecutive integers starting from 1.

If expiration-date is provided, it must be an xsd:dateTimeStamp. All feature toggles should have expiration dates except for long-lived ops toggles like fast-baz above.

KnoraSettingsFeatureFactoryConfig reads this base configuration on startup. If a feature toggle has an expiration date in the past, a warning is logged on startup.

Per-Request Configuration

A client can override the base configuration by submitting the HTTP header X-Knora-Feature-Toggles. Its value is a comma-separated list of toggles. Each toggle consists of:

  1. its name
  2. a colon
  3. the version number
  4. an equals sign
  5. a boolean value, which can be on/off, yes/no, or true/false

Using on/off is recommended for clarity. For example:

X-Knora-Feature-Toggles: new-foo:2=on,new-bar=off,fast-baz:1=on

A version number must be given when enabling a toggle. Only one version of each toggle can be enabled at a time. If a toggle is enabled by default, and you want a version other than the default version, simply enable the toggle, specifying the desired version number. The version number you specify overrides the default.

Disabling a toggle means disabling all its versions. When a toggle is disabled, you will get the functionality that you would have got before the toggle existed. A version number cannot be given when disabling a toggle, because it would not be obvious what this would mean (disable all versions or only the specified version).

Response Header

DSP-API v2 and admin API responses contain the header X-Knora-Feature-Toggles. It lists all configured toggles, in the same format as the corresponding request header.

Implementation Framework

A FeatureFactoryConfig reads feature toggles from some configuration source, and optionally delegates to a parent FeatureFactoryConfig.

KnoraRoute constructs a KnoraSettingsFeatureFactoryConfig to read the base configuration. For each request, it constructs a RequestContextFeatureFactoryConfig, which reads the per-request configuration and has the KnoraSettingsFeatureFactoryConfig as its parent. It then passes the per-request configuration object to the makeRoute method, which can in turn pass it to a feature factory, or send it in a request message to allow a responder to use it.

Feature Factories

The traits FeatureFactory and Feature are just tagging traits, to make code clearer. The factory methods in a feature factory will depend on the feature, and need only be known by the code that uses the feature. The only requirement is that each factory method must take a FeatureFactoryConfig parameter.

To get a FeatureToggle, a feature factory calls featureFactoryConfig.getToggle, passing the name of the toggle. If a feature toggle has only one version, it is enough to test whether test if the toggle is enabled, by calling isEnabled on the toggle.

If the feature toggle has more than one version, call its getMatchableState method. To allow the compiler to check that matches on version numbers are exhaustive, this method is designed to be used with a sealed trait (extending Version) that is implemented by case objects representing the feature's version numbers. The method returns an instance of MatchableState, which is analogous to Option: it is either Off or On, and an instance of On contains one of the version objects. For example:

// A trait for version numbers of the new 'foo' feature.
sealed trait NewFooVersion extends Version

// Represents version 1 of the new 'foo' feature.
case object NEW_FOO_1 extends NewFooVersion

// Represents version 2 of the new 'foo' feature.
case object NEW_FOO_2 extends NewFooVersion

// The old 'foo' feature implementation.
private val oldFoo = new OldFooFeature

// The new 'foo' feature implementation, version 1.
private val newFoo1 = new NewFooVersion1Feature

// The new 'foo' feature implementation, version 2.
private val newFoo2 = new NewFooVersion2Feature

def makeFoo(featureFactoryConfig: FeatureFactoryConfig): Foo = {
    // Get the 'new-foo' feature toggle.
    val fooToggle: FeatureToggle = featureFactoryConfig.getToggle("new-foo")

    // Choose an implementation according to the toggle state.
    fooToggle.getMatchableState(NEW_FOO_1, NEW_FOO_2) match {
        case Off => oldFoo
        case On(NEW_FOO_1) => newFoo1
        case On(NEW_FOO_2) => newFoo2
    }
}

Routes as Features

To select different routes according to a feature toggle:

  • Make a feature factory that extends KnoraRouteFactory and FeatureFactory, and has a makeRoute method that returns different implementations, each of which extends KnoraRoute and Feature.

  • Make a façade route that extends KnoraRoute, is used in ApplicationActor.apiRoutes, and has a makeRoute method that delegates to the feature factory.

To avoid constructing redundant route instances, each façade route needs its own feature factory class.

Documenting a Feature Toggle

The behaviour of each possible setting of each feature toggle should be documented. Feature toggles that are configurable per request should be described in the release notes.

Removing a Feature Toggle

To facilitate removing a feature toggle, each implementation should have:

  • a separate file for its source code

  • a separate file for its documentation

When the toggle is removed, the files that are no longer needed can be deleted.

Last update: 2022-06-23