Flipping Strategies¶

A FlippingStrategy decides whether an enabled feature is active for a given evaluation context:

fun interface FlippingStrategy {
    suspend fun evaluate(feature: Feature, context: Map<String, Any>): Boolean
}

The context is whatever the caller passes to ff4k.check(id, context). Strategies read the keys they need from it.

Built-in strategies¶

Strategy	JSON `type`	Active when
`ContextFilterStrategy(key, allowed)`	`contextFilter`	`context[key]` is one of `allowed`
`PercentageStrategy(percentage)`	`percentage`	a random `percentage` of evaluations
`UserPercentageStrategy(percentage, key = "userId")`	`userPercentage`	a stable `percentage` of users, bucketed by `context[key]`
`ReleaseDateStrategy(releaseDate)`	`releaseDate`	now ≥ `releaseDate`
`DateRangeStrategy(start, end)`	`dateRange`	`start` ≤ now < `end`
`DailyHoursStrategy(startHour, endHour, timezone)`	`dailyHours`	the local hour is in `startHour until endHour`
`WeekdayStrategy(days, timezone)`	`weekday`	the local day of week is in `days`

A missing context key evaluates to false.

Feature("eu-only", enabled = true, strategy = ContextFilterStrategy("region", setOf("eu")))
Feature("rollout", enabled = true, strategy = UserPercentageStrategy(25))
Feature("office-hours", enabled = true, strategy = DailyHoursStrategy(9, 17, TimeZone.of("Europe/Berlin")))

ff4k.check("eu-only", mapOf("region" to "eu"))
ff4k.check("rollout", mapOf("userId" to user.id))

Combining strategies¶

and, or and ! build AndStrategy, OrStrategy and NotStrategy, all serializable:

val strategy = ContextFilterStrategy("region", setOf("eu")) and
    !ContextFilterStrategy("userId", blockedUsers) and
    (UserPercentageStrategy(10) or ContextFilterStrategy("role", setOf("beta")))

A deny list is the negation of a filter: !ContextFilterStrategy("userId", setOf("alice")).

In JSON:

{ "type": "and", "strategies": [
  { "type": "contextFilter", "key": "region", "allowed": ["eu"] },
  { "type": "not", "strategy": { "type": "contextFilter", "key": "userId", "allowed": ["alice"] } }
] }

Custom strategies¶

Any lambda works at runtime:

Feature("f", enabled = true, strategy = FlippingStrategy { _, ctx -> ctx["tier"] == "gold" })

To persist a strategy, make it a @Serializable data class and register it; see Custom Flipping Strategy.