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recon4s

Reloadable configs for Scala 3

Motivation

Imagine a legacy config that follows no strict naming rules, having a mixture of kebab, camel and snake case keys.

Being able to put mixed-case config into a case class was the primary goal.

Secondly, if the whole config or part of it is fetched from a file or network, it would be great to refresh it once in a while and not restart the whole app.

Main features

  • recon4s can read mixed-case configs
  • supports hot-reload

Details

  • Defines Configurable[T] type class. Instances for commonly used Scala and Java types provided
  • Supports enums, trait families, default values
  • Reloadable[T] trait marks hot-reloadable members
  • Adds no result wrappers. Throws exceptions (com.typesafe.config.ConfigException)
  • Assumes case class fields follow camelCase naming and looks for kebab-case, camelCase, CamelCaps and snake_case config keys by default. Convention is configurable via given override

Sbt

libraryDependencies += "io.github.ancane" %% "recon4s" % "0.4"

Import

import recon4s.{*, given}

Basic example

com.typesafe.config.Config extention methods .as[T] and .as[T]("path") read and return an instance of type T.

import com.typesafe.config.*
import recon4s.{*, given}

case class AppConf(
    appName: String,
    appVersion: String = "0.1",
    snakeBites: Boolean
) derives Configurable

val config = ConfigFactory.parseString(
    """|
       |    appName = recon4s
       |    app-version = "1.2.3"
       |    snake_bites = yes
       |
       |""".stripMargin
)

val appConfig: AppConf = config.as[AppConf]

Reloadable example

watchConfig[T] and watchConfigPath[T]("path") functions read freshConfig return instance of type T and update each Reloadable[T] encountered in T every reloadInterval and report success or failure to a corresponding callback.

import com.typesafe.config.*
import recon4s.{*, given}
import scala.concurrent.duration.*

case class AppConf(
    appName: String,
    featureFlag: Reloadable[Boolean]
) derives Configurable

val appConfig = watchConfig[AppConf](
    freshConfig = ConfigFactory.parseString("{ include file(...) }"),
    reloadInterval = 1.minute,
    onReloadFailure = (e => println(s"reload failure: ${e.getMessage()}")),
    onReloadSuccess = (path => println(s"$path reload success"))
)

// When `featureFlag` changes inside the included file,
// featureFlag.get() returns updated value
def flag = appConfig.featureFlag.get()

Naming

Conventions are:

  • CamelToCebab
  • CamelToCebabCamel
  • CamelToCebabCamelCaps
  • CamelToCebabCamelCapsSnake (default)

CamelToCebabCamelCapsSnake means, that given fieldName, recon4s will look for config key named field-name or fieldName or FieldName or field_name or as-is in that order.

Switching to stricter convention:

given Convention = recon4s.CamelToCebab

Config keys direct override:

given Convention = recon4s.CamelToCebab.substitute("one", "TWO")

case class One(one: String)

ConfigFactory
    .parseString("{ TWO = 1}")
    .as[One]

Custom Configurable[T] instances

given (using c: Configurable[String]): Configurable[LocalDate] =
    c.map(LocalDate.parse(_, DateTimeFormatter.ofPattern("yyyy/MM/dd")))

case class LocalDateConf(date: LocalDate)

val actual = ConfigFactory
    .parseString("date = 2007/12/03")
    .as[LocalDateConf]

trait/enum type information field override

type key is used by default to get the type information when reading trait families or enums (with parameters).

Changing default type key:

given Convention = recon4s.CamelToCebab.withDescriminator("name")