logging4s / logging4s   0.2.4

Apache License 2.0 GitHub

Structured logging for Scala 3

Scala versions: 3.x


Logging4s logo - Beaver logging

Logging4s is small logging library for structured (json) logs build on top of logback and logstash-encoder.

Quick start


  • logging4s-core - type classes for abstract encoding, base Logging support work with Try, Either and unsafe variants.
  • logging4s-cats - implementation for cats and cats-effect
    • logging4s-cats-core - implementation for PlainEncoder via cats.Show
    • logging4s-ce-2 - implementation for cats-effect 2 Sync
    • logging4s-ce-3 - implementation for cats-effect 3 Sync
  • logging4s-zio - implementation on top of zio.Task for runtime and zio.prelude.Debug for plain logs.
  • logging4s-json - implementation json logs for different libs
    • logging4s-circe - implementation for circe.Encoder
    • logging4s-jsoniter - implementation for jsoneter-scala JsonValueCodec
    • logging4s-argonaut - implementation for argonaut EncodeJson
    • logging4s-play-json - implementation for play-json Writes
    • logging4s-json4s - implementation for json4s Formats
    • logging4s-spray-json - implementation for spray-json JsonWriter


Let's say you are using cats-effect 3 and circe.

Plug the library in for sbt

libraryDependencies ++= Seq(
  "org.logging4s" %% "logging4s-ce-3" % version,
  "org.logging4s" %% "logging4s-circe" % version

Create Loggable implementation for your domain objects, create Logging instance and log your objects.

// Your domain
import java.util.UUID
import cats.Show
import io.circe.Encoder
import io.circe.generic.semiauto.deriveEncoder
import logging4s.core.Loggable

import logging4s.cats.instances.given
import logging4s.json.circe.instances.given

final case class User(id: UUID, name: String, age: Int)

object User:
  given Show[User]     = user => s"id=${user.id}, name=${user.name}, age=${user.age}"
  given Encoder[User]  = deriveEncoder
  given Loggable[User] = Loggable.make("user")

// Your program
import java.util.UUID
import cats.effect.std.UUIDGen
import cats.effect.{ExitCode, IO, IOApp}
import logging4s.cats.Logging

import logging4s.core.syntax.withKey

import logging4s.cats.instances.given
import logging4s.json.circe.instances.given

object CatsEffect3Example extends IOApp:

  private def createUser(name: String, age: Int): IO[User] =
    for id <- UUIDGen.randomUUID[IO]
    yield User(id, name, age)

  override def run(args: List[String]): IO[ExitCode] =
      context <- IO.randomUUID.map(uuid => LoggingContext(uuid.withKey("session_id")))
      logging <- Logging.create[IO]("CatsEffect3Example", context)

      johnShow <- createUser("John Show", 22)
      _        <- logging.info("User created", johnShow)
      daenerys <- createUser("Daenerys Targaryen", 22)
      _        <- logging.info("User created", daenerys)
      _ <- logging.info("All users created", Seq(johnShow, daenerys))
    yield ExitCode.Success

This will output:

{"@timestamp":"2023-01-30T13:42:13.249+03:00","message":"User created: session_id -> (9602ed80-e54b-4e0a-8b9c-64762d28d05e), user -> (id=5db8c5e2-6275-437a-bca8-1ad8cd84fbd8, name=John Show, age=22)","name":"CatsEffect3Example","level":"INFO","user":{"id":"5db8c5e2-6275-437a-bca8-1ad8cd84fbd8","name":"John Show","age":22}}
{"@timestamp":"2023-01-30T13:42:13.249+03:00","message":"User created: session_id -> (9602ed80-e54b-4e0a-8b9c-64762d28d05e), user -> (id=c5e4bd53-abd8-4922-bcd2-5e40322e6b9b, name=Daenerys Targaryen, age=22)","name":"CatsEffect3Example","level":"INFO","user":{"id":"c5e4bd53-abd8-4922-bcd2-5e40322e6b9b","name":"Daenerys Targaryen","age":22}}
{"@timestamp":"2023-01-30T13:42:13.249+03:00","message":"All users created: session_id -> (9602ed80-e54b-4e0a-8b9c-64762d28d05e), users -> ([id=5db8c5e2-6275-437a-bca8-1ad8cd84fbd8, name=John Show, age=22,id=c5e4bd53-abd8-4922-bcd2-5e40322e6b9b, name=Daenerys Targaryen, age=22])","name":"CatsEffect3Example","level":"INFO","users":[{"id":"5db8c5e2-6275-437a-bca8-1ad8cd84fbd8","name":"John Show","age":22},{"id":"c5e4bd53-abd8-4922-bcd2-5e40322e6b9b","name":"Daenerys Targaryen","age":22}]}

In the logback.xml file, you can configure the output of logs as you need.

See ./examples for more examples.


Have a library for structured logging that supports Scala 3 and various implementations of effects and json libraries cause izumi logstage and tofu-logging still not ported for new scala.