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experiment overrides (#1195)
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Task/Issue URL:
https://app.asana.com/0/1204186595873227/1209266907041599/f
iOS PR: duckduckgo/iOS#3892
macOS PR: duckduckgo/macos-browser#3798
What kind of version bump will this require?: Major

**Optional**:

Tech Design URL:
https://app.asana.com/0/1204186595873227/1209266907041602/f

**Description**:
- Experiment types will be defined as FeatureFlag enum values, with
their cohort types defined as per FeatureFlagDescribing protocol.
- Add support for overrides of experiment cohorts, Cohort type can be
used to populate local overrides menu
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@SabrinaTardio
SabrinaTardio authored Feb 3, 2025
1 parent b874310 commit 4376de0
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Showing 15 changed files with 535 additions and 250 deletions.
14 changes: 7 additions & 7 deletions Sources/BrowserServicesKit/FeatureFlagger/ExperimentCohortsManager.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -51,26 +51,26 @@ public protocol ExperimentCohortsManaging {
/// for the specified experiment. If the assigned cohort is valid (i.e., it matches
/// one of the experiment's defined cohorts), the method returns the assigned cohort.
/// Otherwise, the invalid cohort is removed, and a new cohort is assigned if
/// `allowCohortReassignment` is `true`.
/// `allowCohortAssignment` is `true`.
///
/// - Parameters:
/// - experiment: The `ExperimentSubfeature` representing the experiment and its associated cohorts.
/// - allowCohortReassignment: A Boolean value indicating whether cohort assignment is allowed
/// - allowCohortAssignment: A Boolean value indicating whether cohort assignment is allowed
/// if the user is not already assigned to a valid cohort.
///
/// - Returns: The valid `CohortID` assigned to the user for the experiment, or `nil`
/// if no valid cohort exists and `allowCohortReassignment` is `false`.
/// if no valid cohort exists and `allowCohortAssignment` is `false`.
///
/// - Behavior:
/// 1. Retrieves the currently assigned cohort for the experiment using the `subfeatureID`.
/// 2. Validates if the assigned cohort exists within the experiment's cohort list:
/// - If valid, the assigned cohort is returned.
/// - If invalid, the cohort is removed from storage.
/// 3. If cohort assignment is enabled (`allowCohortReassignment` is `true`), a new cohort
/// 3. If cohort assignment is enabled (`allowCohortAssignment` is `true`), a new cohort
/// is assigned based on the experiment's cohort weights and saved in storage.
/// - Cohort assignment is probabilistic, determined by the cohort weights.
///
func resolveCohort(for experiment: ExperimentSubfeature, allowCohortReassignment: Bool) -> CohortID?
func resolveCohort(for experiment: ExperimentSubfeature, allowCohortAssignment: Bool) -> CohortID?
}

public class ExperimentCohortsManager: ExperimentCohortsManaging {
Expand All @@ -95,14 +95,14 @@ public class ExperimentCohortsManager: ExperimentCohortsManaging {
self.fireCohortAssigned = fireCohortAssigned
}

public func resolveCohort(for experiment: ExperimentSubfeature, allowCohortReassignment: Bool) -> CohortID? {
public func resolveCohort(for experiment: ExperimentSubfeature, allowCohortAssignment: Bool) -> CohortID? {
queue.sync {
let assignedCohort = cohort(for: experiment.subfeatureID)
if experiment.cohorts.contains(where: { $0.name == assignedCohort }) {
return assignedCohort
}
removeCohort(from: experiment.subfeatureID)
return allowCohortReassignment ? assignCohort(to: experiment) : nil
return allowCohortAssignment ? assignCohort(to: experiment) : nil
}
}
}
Expand Down
104 changes: 97 additions & 7 deletions Sources/BrowserServicesKit/FeatureFlagger/FeatureFlagLocalOverrides.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -28,11 +28,23 @@ public protocol FeatureFlagLocalOverridesPersisting {
///
func value<Flag: FeatureFlagDescribing>(for flag: Flag) -> Bool?

/// Return value for the cohort override for the experiment flag.
///
/// If there's no override, this function should return `nil`.
///
func value<Flag: FeatureFlagDescribing>(for flag: Flag) -> CohortID?

/// Set new override for the feature flag.
///
/// Flag can be overridden to `true` or `false`. Setting `nil` clears the override.
///
func set<Flag: FeatureFlagDescribing>(_ value: Bool?, for flag: Flag)

/// Sets a new cohort override for the specified experiment feature flag.
///
/// Flag can be overridden to one of the cohort of the feature. Setting `nil` clears the override.
///
func setExperiment<Flag: FeatureFlagDescribing>(_ value: CohortID?, for flag: Flag)
}

public struct FeatureFlagLocalOverridesUserDefaultsPersistor: FeatureFlagLocalOverridesPersisting {
Expand All @@ -48,18 +60,36 @@ public struct FeatureFlagLocalOverridesUserDefaultsPersistor: FeatureFlagLocalOv
return keyValueStore.object(forKey: key) as? Bool
}

public func value<Flag: FeatureFlagDescribing>(for flag: Flag) -> CohortID? {
let key = experimentKey(for: flag)
return keyValueStore.object(forKey: key) as? CohortID
}

public func set<Flag: FeatureFlagDescribing>(_ value: Bool?, for flag: Flag) {
let key = key(for: flag)
keyValueStore.set(value, forKey: key)
}

public func setExperiment<Flag: FeatureFlagDescribing>(_ value: CohortID?, for flag: Flag) {
let key = experimentKey(for: flag)
keyValueStore.set(value, forKey: key)
}

/// This function returns the User Defaults key for a feature flag override.
///
/// It uses camel case to simplify inter-process User Defaults KVO.
///
private func key<Flag: FeatureFlagDescribing>(for flag: Flag) -> String {
return "localOverride\(flag.rawValue.capitalizedFirstLetter)"
}

/// This function returns the User Defaults key for a feature flag cohort override.
///
/// It uses camel case to simplify inter-process User Defaults KVO.
///
private func experimentKey<Flag: FeatureFlagDescribing>(for flag: Flag) -> String {
return "localOverride\(flag.rawValue.capitalizedFirstLetter)_cohort"
}
}

private extension String {
Expand All @@ -77,6 +107,13 @@ public protocol FeatureFlagLocalOverridesHandling {
/// It can be implemented by client apps to react to changes to feature flag
/// value in runtime, caused by adjusting its local override.
func flagDidChange<Flag: FeatureFlagDescribing>(_ featureFlag: Flag, isEnabled: Bool)

/// This function is called whenever the effective cohort of a feature flag changes due to a local override.
/// changes as a result of adding or removing a local override.
///
/// It can be implemented by client apps to react to changes to feature flag
/// value in runtime, caused by adjusting its local override.
func experimentFlagDidChange<Flag: FeatureFlagDescribing>(_ featureFlag: Flag, cohort: CohortID)
}

/// `FeatureFlagLocalOverridesHandling` implementation providing Combine publisher for flag changes.
Expand All @@ -87,15 +124,24 @@ public struct FeatureFlagOverridesPublishingHandler<F: FeatureFlagDescribing>: F

public let flagDidChangePublisher: AnyPublisher<(F, Bool), Never>
private let flagDidChangeSubject = PassthroughSubject<(F, Bool), Never>()
public let experimentFlagDidChangePublisher: AnyPublisher<(F, CohortID), Never>
private let experimentFlagDidChangeSubject = PassthroughSubject<(F, CohortID), Never>()

public init() {
flagDidChangePublisher = flagDidChangeSubject.eraseToAnyPublisher()
experimentFlagDidChangePublisher = experimentFlagDidChangeSubject.eraseToAnyPublisher()
}

public func flagDidChange<Flag: FeatureFlagDescribing>(_ featureFlag: Flag, isEnabled: Bool) {
guard let flag = featureFlag as? F else { return }
flagDidChangeSubject.send((flag, isEnabled))
}

public func experimentFlagDidChange<Flag: FeatureFlagDescribing>(_ featureFlag: Flag, cohort: CohortID) {
guard let flag = featureFlag as? F else { return }
experimentFlagDidChangeSubject.send((flag, cohort))
}

}

/// This protocol defines the interface for feature flag overriding mechanism.
Expand All @@ -117,12 +163,21 @@ public protocol FeatureFlagLocalOverriding: AnyObject {
/// Returns the current override for a feature flag, or `nil` if override is not set.
func override<Flag: FeatureFlagDescribing>(for featureFlag: Flag) -> Bool?

/// Returns the current cohort override for a feature flag, or `nil` if override is not set.
func experimentOverride<Flag: FeatureFlagDescribing>(for featureFlag: Flag) -> CohortID?

/// Toggles override for a feature flag.
///
/// If override is not currently present, it sets the override to the opposite of the current flag value.
///
func toggleOverride<Flag: FeatureFlagDescribing>(for featureFlag: Flag)

/// Sets the cohort override for a feature flag.
///
/// If override is not currently present, it sets the override to the opposite of the current flag value.
///
func setExperimentCohortOverride<Flag: FeatureFlagDescribing>(for featureFlag: Flag, cohort: CohortID)

/// Clears override for a feature flag.
///
/// Calls `FeatureFlagLocalOverridesHandling.flagDidChange` if the effective flag value
Expand All @@ -135,11 +190,17 @@ public protocol FeatureFlagLocalOverriding: AnyObject {
/// This function calls `clearOverride(for:)` for each flag.
///
func clearAllOverrides<Flag: FeatureFlagDescribing>(for flagType: Flag.Type)

/// Returns the effective value of a feature flag, considering the current state and overrides.
func currentValue<Flag: FeatureFlagDescribing>(for featureFlag: Flag) -> Bool?

/// Returns the effective cohort of a feature flag, considering the current state and overrides.
func currentExperimentCohort<Flag: FeatureFlagDescribing>(for featureFlag: Flag) -> (any FeatureFlagCohortDescribing)?
}

public final class FeatureFlagLocalOverrides: FeatureFlagLocalOverriding {

public let actionHandler: FeatureFlagLocalOverridesHandling
public var actionHandler: FeatureFlagLocalOverridesHandling
public weak var featureFlagger: FeatureFlagger?
private let persistor: FeatureFlagLocalOverridesPersisting

Expand Down Expand Up @@ -168,6 +229,13 @@ public final class FeatureFlagLocalOverrides: FeatureFlagLocalOverriding {
return persistor.value(for: featureFlag)
}

public func experimentOverride<Flag: FeatureFlagDescribing>(for featureFlag: Flag) -> CohortID? {
guard featureFlag.supportsLocalOverriding else {
return nil
}
return persistor.value(for: featureFlag)
}

public func toggleOverride<Flag: FeatureFlagDescribing>(for featureFlag: Flag) {
guard featureFlag.supportsLocalOverriding else {
return
Expand All @@ -178,13 +246,30 @@ public final class FeatureFlagLocalOverrides: FeatureFlagLocalOverriding {
actionHandler.flagDidChange(featureFlag, isEnabled: newValue)
}

public func clearOverride<Flag: FeatureFlagDescribing>(for featureFlag: Flag) {
guard let override = override(for: featureFlag) else {
public func setExperimentCohortOverride<Flag: FeatureFlagDescribing>(for featureFlag: Flag, cohort: CohortID) {
guard featureFlag.supportsLocalOverriding else {
return
}
persistor.set(nil, for: featureFlag)
if let defaultValue = currentValue(for: featureFlag), defaultValue != override {
actionHandler.flagDidChange(featureFlag, isEnabled: defaultValue)
let newValue = cohort
persistor.setExperiment(newValue, for: featureFlag)
actionHandler.experimentFlagDidChange(featureFlag, cohort: cohort)
}

public func clearOverride<Flag: FeatureFlagDescribing>(for featureFlag: Flag) {
// Clear the regular override
if let override = override(for: featureFlag) {
persistor.set(nil, for: featureFlag)
if let defaultValue = currentValue(for: featureFlag), defaultValue != override {
actionHandler.flagDidChange(featureFlag, isEnabled: defaultValue)
}
}

// Clear the experiment cohort override
if let experimentOverride = experimentOverride(for: featureFlag) {
persistor.setExperiment(nil, for: featureFlag)
if let defaultValue = currentExperimentCohort(for: featureFlag)?.rawValue, defaultValue != experimentOverride {
actionHandler.experimentFlagDidChange(featureFlag, cohort: defaultValue)
}
}
}

Expand All @@ -194,7 +279,12 @@ public final class FeatureFlagLocalOverrides: FeatureFlagLocalOverriding {
}
}

private func currentValue<Flag: FeatureFlagDescribing>(for featureFlag: Flag) -> Bool? {
public func currentValue<Flag: FeatureFlagDescribing>(for featureFlag: Flag) -> Bool? {
featureFlagger?.isFeatureOn(for: featureFlag, allowOverride: true)
}

public func currentExperimentCohort<Flag: FeatureFlagDescribing>(for featureFlag: Flag) -> (any FeatureFlagCohortDescribing)? {
guard let flagger = featureFlagger as? CurrentExperimentCohortProviding else { return nil }
return flagger.assignedCohort(for: featureFlag)
}
}
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