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Feature Flags

Turn features on and off, and change their settings, without redeploying your Worker. The Flagship binding returns each flag's current value, or the default that your code passes with the call.

  • Libraries Runtime.Workers Runtime.Flagship
  • Binding Flagship in Runtime.Workers
  • npm @cloudflare/workers-types 5.20260906.1, @cloudflare/flagship 0.5.0

Dark Mode

The site's script requests this JSON and applies the dark theme when darkMode is true. getBooleanValue takes the flag key and a default. The binding returns that default when the flag is missing or evaluation fails, and also for a flag of another type.

open Fable.Core

module Workers = FSharp.CloudEdge.Runtime.Workers

type Env =
    abstract FLAGS: Workers.Flagship

[<ExportDefault>]
let worker: Workers.ExportedHandler<Env, obj, obj, obj> =
    Workers.ExportedHandler.Create(
        fetch = fun _ env _ ->
            async {
                let! darkMode = env.FLAGS.getBooleanValue("dark-mode", false) |> Async.AwaitPromise
                return Workers.Exports.Response.json {| darkMode = darkMode |}
            }
            |> Async.StartAsPromise
            |> U2.Case1
    )

Needs a flagship binding named FLAGS, with your Flagship app's ID as app_id. Worker Upload shows how to declare bindings.

Emitted JavaScript 
import { awaitPromise, startAsPromise } from "./fable_modules/fable-library-js.5.13.0/Async.js";
import { singleton } from "./fable_modules/fable-library-js.5.13.0/AsyncBuilder.js";

export const worker = {
    fetch: (_arg, env, _arg_1) => startAsPromise(singleton.Delay(() => singleton.Bind(awaitPromise(env.FLAGS.getBooleanValue("dark-mode", false)), (_arg_2) => singleton.Return(globalThis.Response.json({
        darkMode: _arg_2,
    }))))),
};

export default worker;

Storefront Settings

Each flag type has its own method. The promises from getStringValue and getNumberValue resolve to an F# string and a float. The result of getObjectValue has the F# type of its default, so the flag's JSON value should have the fields of standardPrices.

open Fable.Core

module Workers = FSharp.CloudEdge.Runtime.Workers

type Env =
    abstract FLAGS: Workers.Flagship

let standardPrices = {| monthly = 12.; yearly = 120. |}

[<ExportDefault>]
let worker: Workers.ExportedHandler<Env, obj, obj, obj> =
    Workers.ExportedHandler.Create(
        fetch = fun _ env _ ->
            async {
                let! banner = env.FLAGS.getStringValue("banner-text", "Welcome back!") |> Async.AwaitPromise
                let! trialDays = env.FLAGS.getNumberValue("trial-days", 14.) |> Async.AwaitPromise
                let! prices = env.FLAGS.getObjectValue("prices", standardPrices) |> Async.AwaitPromise
                let settings = {| banner = banner; trialDays = trialDays; prices = prices |}
                return Workers.Exports.Response.json settings
            }
            |> Async.StartAsPromise
            |> U2.Case1
    )

Flag Inspector

An admin endpoint shows why new-checkout has its current value. getBooleanDetails reports the value with its variant and reason. Both are F# options, and Option.defaultValue supplies "none" for the JSON.

open Fable.Core

module Workers = FSharp.CloudEdge.Runtime.Workers

type Env =
    abstract FLAGS: Workers.Flagship

[<ExportDefault>]
let worker: Workers.ExportedHandler<Env, obj, obj, obj> =
    Workers.ExportedHandler.Create(
        fetch = fun _ env _ ->
            async {
                let! details = env.FLAGS.getBooleanDetails("new-checkout", false) |> Async.AwaitPromise
                return
                    Workers.Exports.Response.json
                        {| newCheckout = details.value
                           variant = Option.defaultValue "none" details.variant
                           reason = Option.defaultValue "none" details.reason |}
            }
            |> Async.StartAsPromise
            |> U2.Case1
    )

Library Table

Runtime.Flagship binds Flagship's own SDK. Its FlagshipClient evaluates flags over HTTP, and its FlagshipServerProvider is an OpenFeature provider for server code. From .NET, Management.Compute creates Flagship apps and flags with FlagshipCreateApp and FlagshipCreateFlag.

Library npm package What it covers
Runtime.Workers @cloudflare/workers-types 5.20260906.1 Flagship binding
Runtime.Flagship @cloudflare/flagship 0.5.0 HTTP client, OpenFeature provider
WorkersRequest handlers Worker UploadDeclare bindings Client CatalogFlagship apps from .NET

NuGet packages

Management.Compute 0.1.0, Runtime.Flagship 0.1.0, Runtime.Workers 0.1.0.

See installation and release availability.

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