Example of contextual logging with IOLocal and Scribe in Cats Effect

README.md

Run with scala-cli run context-logging.scala

context-logging.scala

//> using dep org.typelevel::cats-effect:3.5.0
//> using dep com.outr::scribe-cats:3.11.5

import cats.effect.*
import scribe.data.MDC
import scribe.Scribe
import scala.util.Random

class MyLogger private (orig: Scribe[IO], loc: IOLocal[Map[String, String]]):
  inline def annotate[A](key: String, value: String)(io: IO[A]) =
    loc.set(Map(key -> value)).flatMap(_ => io).guarantee(loc.set(Map.empty))

  inline def info(msg: String): IO[Unit] =
    loc.get.flatMap { context =>
      MDC { implicit mdc =>
        context.foreach((k, v) => mdc.update(k, v))
        orig.info(msg)
      }
    }

object MyLogger:
  def create(orig: Scribe[IO]) = IOLocal(Map.empty).map(new MyLogger(orig, _))

object My extends IOApp.Simple:
  import cats.syntax.all.*
  import concurrent.duration.*
  val randomDelay = cats.effect.std.Random
    .scalaUtilRandom[IO]
    .flatMap(_.nextIntBounded(1000))
    .flatMap(t => IO.sleep(t.millis))

  val run =
    MyLogger
      .create(scribe.cats.io)
      .flatMap { log =>
        List("UserA", "UserB", "UserC").parTraverse { userId =>
          log.annotate("user-id", userId) {
            for
              _ <- randomDelay
              _ <- log.info("fetching user from database")
              _ <- randomDelay
              _ <- log.info("downloading user's profile picture")
            yield ()
          }
        }
      }
      .void