nigozi / flag4s   0.1.5


A feature flag library for scala

flag4s: A simple feature flag library for Scala

flag4s helps you manage feature flags in your application via scala functions and http apis.

flag4s consists of the following modules:

  • flag4s-core: default key/val stores, syntaxes and scala functions.
  • flag4s-api-http4s: http endpoints configuration for http4s.
  • flag4s-api-akka-http: http endpoints configuration for akka-http.


flag4s uses IO type from cats-effect for all operations and all return types are IO.

libraryDependencies += "org.typelevel" %% "cats-effect" % "version"



libraryDependencies += "io.nigo" %% "flag4s-core" % "0.1.5"

choose your key/val store:


implicit val store = ConsulStore("localhost", 8500)
//implicit val store = RedisStore("localhost", 6379)
//implicit val store = ConfigStore("path-to-config-file")
  • you can choose one of the existing stores or create your own by implementing the Store trait.
  • ConfigStore is not recommended as it does not support value modification. Also it only supports String type, you can define the flags as any type but all the value checks will be as String. To use the ConfigStore, put the flags in a .conf file in the following format:
features {
  featureA: true
  featureB: "on"

use core functions to manage the flags:

all return types are IO, you should execute or compose them yourself.

read a flag

import flag4s.core._

flag("featX") //returns the flag as type of Either[Throwable, Flag] 

fatalFlag("featX") //returns the flag or throws exception if flag doesn't exist

enabled(boolFlag) //checks if the flag's value is true

is(strFlag, "on") //checks if the flag's value is "on"

withFlag("boolFlag", true) { //executes the code block if flag's value is true
  //code block

withFlag("strFlag", "on") { //executes the code block if flag's value is "on"
  //code block


ifEnabled(boolFlag) { //executes the code block if the flag's value is true
    //code block

ifIs(strFlag, "on") { //executes the code block if the flag's value is "on"
    //code block

get[Double](dblFlag) //returns the flag's value as Double

create/set flag

newFlag("boolFlag", true) //creates a new flag with value true, Left if flag already exists

switchFlag("boolFlag", false) //sets the flag's value to false, creates the flag if doesn't exist

set(strFlag, "off") //sets the flag's value to "off", Left if flag doesn't exist


import flag4s.core._
import flag4s.syntax._


boolFlag.ifEnabled {
    //code block

strFlag.ifIs("on") {
    //code block

dblFlag.ifIs(1.1) {
    //code block




http Api


libraryDependencies += "io.nigo" %% "flag4s-api-http4s" % "0.1.5"
import flag4s.api.Http4sFlagApi

implicit val store = RedisStore("localhost", 6379)

    .mountService(Http4sFlagApi.service(), "/")


libraryDependencies += "io.nigo" %% "flag4s-api-akka-http" % "0.1.5"
import flag4s.api.AkkaFlagApi

implicit val store = RedisStore("localhost", 6379)

Http().bindAndHandle(AkkaFlagApi.route(), "localhost", 8080)


create/update a flag

http PUT localhost:8080/flags key=featureA value=on
http PUT localhost:8080/flags key=featureB value:=true

get a flag

http localhost:8080/flags/featureA

get all flags

http localhost:8080/flags

enable a boolean flag

http POST localhost:8080/flags/featureB/enable

disable a boolean flag

http POST localhost:8080/flags/featureB/disable

delete a flag

http DELETE localhost:8080/flags/featureA