japgolly / clear-config   3.1.0

Apache License 2.0 GitHub

Scala FP configuration library with a focus on runtime clarity

Scala versions: 3.x 2.13
Scala.js versions: 1.x


Build Status Latest version

A type-safe, FP, Scala config library.

libraryDependencies += "com.github.japgolly.clearconfig" %%% "core" % "<ver>"

What's special about this?

There are plenty of config libraries out there, right? This library is pure FP, type-safe, super-simple to use, highly composable and powerful, yada yada yada... All true but it's biggest and most unique feature is actually:


Haven't we all had enough of crap like:

  • changing an environment variable setting, pushing all the way though to an environment, testing and then discovering that your expected change didn't occur. Was the new setting picked up? What setting did it use? Where did it come from?

  • after hours of frustration: "That setting isn't even used any more?! Why the hell is it still all over our deployment config?! Why didn't person X magically know to remove this specific piece of text in this big blob of text in this completely separate deployment repo at the same time they made their code change?"

This library endeavours to provide clarity. When you get an instance of your config, you also get a report that describes:

  • where config comes from
  • how config sources override other sources
  • what values each config source provided
  • what config keys are in use
  • what the total, resulting config is
  • which config is still hanging around but is actually stale and no longer in use

(sample report below)


Here's a pretty common scenario that will serve as a decent introduction.

We have an app which has the following config:

import java.net.URL

final case class DatabaseConfig(
  port     : Int,
  url      : URL,
  username : String,
  password : String,
  schema   : Option[String])

Let's define how we populate our config from the outside world...

import japgolly.clearconfig._
import cats.implicits._

object DatabaseConfig {

  def config: ConfigDef[DatabaseConfig] =
      ConfigDef.getOrUse("PORT", 8080),

Great, now let's define where we want to read config from. At this point you also need to decide which effect type to use. You'd typically use something like IO but for simplicity, we'll just use Id and opt-out of safe FP.

import cats.Id

def configSources: ConfigSources[Id] =
  ConfigSource.environment[Id] >                                             // Highest priority
  ConfigSource.propFileOnClasspath[Id]("/database.props", optional = true) > //
  ConfigSource.system[Id]                                                    // Lowest priority

Now we're ready to create a real instance based on the real environment.

val dbCfg: DatabaseConfig =
    .getOrDie() // Just throw an exception if necessary config is missing

Done! But so far we're not using the most important feature of the library: the report.

Let's get and print out a report at the end.

  1. We'll remove env & system from unused keys to keep the report small seeing as it's just a demo.
  2. We'll also prefix all keys by POSTGRES_ to make it look a bit more realistic.
val (dbCfg, report) =
    .getOrDie() // Just throw an exception if necessary config is missing


  // Remove env & system columns from the unused section of the report
  .mapUnused(_.withoutSources(ConfigSourceName.environment, ConfigSourceName.system))

  // Show the full report

Sample output:

4 sources (highest to lowest priority):
  - Env
  - cp:/database.properties
  - System
  - Default

Used keys (5):
| Key               | Env  | cp:/database.properties | Default |
| POSTGRES_PASSWORD |      | Obfuscated (1C02B9F6)   |         |
| POSTGRES_PORT     | 4000 |                         | 8080    |
| POSTGRES_SCHEMA   |      |                         |         |
| POSTGRES_URL      |      | http://localhost/blah   |         |
| POSTGRES_USERNAME |      | demo                    |         |

Unused keys (1):
| Key            | cp:/database.properties |
| POSTGRES_SCHMA | public                  |

From the above report we can immediately observe the following:

  • Which sources override other sources; the report columns (left-to-right) respect this
  • We'll be running at port 4000 and the reason for that is there's an override set by the environment
  • There's a typo in our database.properties; POSTGRES_SCHMA should be POSTGRES_SCHEMA
  • The password value has been hashed for the report. This still allows you to compare the hash between envs or time to determine change without compromising the value.


  • Simplest and most common methods:

    ConfigDef.get     [A](key: String)             // provides an Option[A] - optional config
    ConfigDef.getOrUse[A](key: String, default: A) // provides an A - optional config with default
    ConfigDef.need    [A](key: String)             // provides an A - mandatory config; error if not provided
  • To define your own type of config value, create an implicit ConfigValueParser. Example:

    sealed trait MyBool
    case object Yes extends MyBool
    case object No extends MyBool
    implicit val myBoolParser: ConfigValueParser[MyBool] =
      ConfigValueParser.oneOf[MyBool]("yes" -> Yes, "no" -> No)
        .preprocessValue(_.toLowerCase) // Make it case-insensitive
  • Call .secret on your ConfigDef to force it to be obfuscated in the report. The default (implicit) report settings already obfuscate keys that contain substrings like password, credential, and secret. Override configReportSettings if required.

  • Shell-style Comments (beginning with #) are automatically removed from config values. Create your own implicit ConfigValuePreprocessor to customise this behaviour.

  • Keys can be modified after the fact. Eg. ConfigDef.get("A").withPrefix("P_").withKeyMod(_.replace('_', '.')) is equivalent to ConfigDef.get("P.A"). This also works when a ConfigDef is a composition of more than one key, in which case they'll all be modified.

  • There is special DSL to create A => Unit functions to configure a mutable object (which you typically use when working with a Java library) ConfigDef.consumerFn[A](...). There is an example below:

  • More... (explore the source)

Larger Example

You typically compose using Applicative, give the composite a prefix, then use (nest) it in some higher-level config.

For example, this Scala code...

import cats.syntax.apply._
import japgolly.clearconfig._
import java.net.{URI, URL}
import redis.clients.jedis.JedisPoolConfig

case class AppConfig(postgres: PostgresConfig, redis: RedisConfig, logLevel: LogLevel)

object AppConfig {
  def config: ConfigDef[AppConfig] =
    ( PostgresConfig.config,
      ConfigDef.getOrUse("log_level", LogLevel.Info)

case class PostgresConfig(url: URL, credential: Credential, schema: Option[String])

object PostgresConfig {
  def config: ConfigDef[PostgresConfig] =
    ( ConfigDef.need[URL]("url"),

case class Credential(username: String, password: String)

object Credential {
  def config: ConfigDef[Credential] =
    ( ConfigDef.need[String]("username"),

case class RedisConfig(uri: URI, credential: Credential, configurePool: JedisPoolConfig => Unit)

object RedisConfig {

  def poolConfig: ConfigDef[JedisPoolConfig => Unit] =
      _.get("block_when_exhausted", _.setBlockWhenExhausted),
      _.get("eviction_policy_class_name", _.setEvictionPolicyClassName),
      _.getOrUse("fairness", _.setFairness)(true),
      _.get("jmx_enabled", _.setJmxEnabled),
      _.get("jmx_name_base", _.setJmxNameBase),
      _.get("jmx_name_prefix", _.setJmxNamePrefix),
      _.get("lifo", _.setLifo),
      _.get("max_idle", _.setMaxIdle),
      _.get("max_total", _.setMaxTotal),
      _.get("max_wait_millis", _.setMaxWaitMillis),
      _.get("min_evictable_idle_time_millis", _.setMinEvictableIdleTimeMillis),
      _.getOrUse("min_idle", _.setMinIdle)(2),
      _.get("num_tests_per_eviction_run", _.setNumTestsPerEvictionRun),
      _.get("soft_min_evictable_idle_time_millis", _.setSoftMinEvictableIdleTimeMillis),
      _.get("test_on_borrow", _.setTestOnBorrow),
      _.get("test_on_create", _.setTestOnCreate),
      _.get("test_on_return", _.setTestOnReturn),
      _.get("test_while_idle", _.setTestWhileIdle),
      _.get("time_between_eviction_runs_millis", _.setTimeBetweenEvictionRunsMillis)

  def config: ConfigDef[RedisConfig] =
    ( ConfigDef.need[URI]("uri"),

sealed trait LogLevel
object LogLevel {
  case object Debug extends LogLevel
  case object Info extends LogLevel
  case object Warn extends LogLevel

  implicit def configValueParser: ConfigValueParser[LogLevel] =
    ConfigValueParser.oneOf[LogLevel]("debug" -> Debug, "info" -> Info, "warn" -> Warn)

will read the following properties:





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