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Version: ZIO 2.x

ZIO Logging

ZIO Logging is simple logging for ZIO apps, with correlation, context, and pluggable backends out of the box.


When we are writing our applications using ZIO effects, to log easy way we need a ZIO native solution for logging. ZIO Logging is an environmental effect for adding logging into our ZIO applications.

Key features of ZIO Logging:

  • ZIO Native — Other than it is a type-safe and purely functional solution, it leverages ZIO's features.
  • Multi-Platform - It supports both JVM and JS platforms.
  • Composable — Loggers are composable together via contraMap.
  • Pluggable Backends — Support multiple backends like ZIO Console, SLF4j, JS Console, JS HTTP endpoint.
  • Logger Context — It has a first citizen Logger Context implemented on top of FiberRef. The Logger Context maintains information like logger name, filters, correlation id, and so forth across different fibers. It supports Mapped Diagnostic Context (MDC) which manages contextual information across fibers in a concurrent environment.


In order to use this library, we need to add the following line in our build.sbt file:

libraryDependencies += "dev.zio" %% "zio-logging" % "2.1.0" 

There are also some optional dependencies:

  • zio-logging-slf4j — SLF4j integration
  • zio-logging-slf4j-bridge — Using ZIO Logging for SLF4j loggers, usually third-party non-ZIO libraries
  • zio-logging-jsconsole — Scala.js console integration
  • zio-logging-jshttp — Scala.js HTTP Logger which sends logs to a backend via Ajax POST


Let's try an example of ZIO Logging which demonstrates a simple application of ZIO logging along with its Logger Context feature:

import zio.clock.Clock
import zio.duration.durationInt
import zio.logging._
import zio.random.Random
import zio.{ExitCode, NonEmptyChunk, ZIO}

object ZIOLoggingExample extends zio.App {

val myApp: ZIO[Logging with Clock with Random, Nothing, Unit] =
for {
_ <-"Hello from ZIO logger")
_ <-
ZIO.foreachPar(NonEmptyChunk("UserA", "UserB", "UserC")) { user =>
log.locally(UserId(Some(user))) {
for {
_ <-"User validation")
_ <- zio.random
.flatMap(t => ZIO.sleep(t.millis))
_ <-"Connecting to the database")
_ <- zio.random
.flatMap(t => ZIO.sleep(t.millis))
_ <-"Releasing resources.")
} yield ()

} yield ()

type UserId = String
def UserId: LogAnnotation[Option[UserId]] = LogAnnotation[Option[UserId]](
name = "user-id",
initialValue = None,
combine = (_, r) => r,
render = => s"[user-id: $userId]")

val env =
logLevel = LogLevel.Info,
format =
LogFormat.ColoredLogFormat((ctx, line) => s"${ctx(UserId)} $line")
) >>> Logging.withRootLoggerName("MyZIOApp")

override def run(args: List[String]) =

The output should be something like this:

2021-07-09 00:14:47.457+0000  info [MyZIOApp] undefined-user-id Hello from ZIO logger
2021-07-09 00:14:47.807+0000 info [MyZIOApp] [user-id: UserA] User validation
2021-07-09 00:14:47.808+0000 info [MyZIOApp] [user-id: UserC] User validation
2021-07-09 00:14:47.818+0000 info [MyZIOApp] [user-id: UserB] User validation
2021-07-09 00:14:48.290+0000 info [MyZIOApp] [user-id: UserC] Connecting to the database
2021-07-09 00:14:48.299+0000 info [MyZIOApp] [user-id: UserA] Connecting to the database
2021-07-09 00:14:48.321+0000 info [MyZIOApp] [user-id: UserA] Releasing resources.
2021-07-09 00:14:48.352+0000 info [MyZIOApp] [user-id: UserC] Releasing resources.
2021-07-09 00:14:48.820+0000 info [MyZIOApp] [user-id: UserB] Connecting to the database
2021-07-09 00:14:48.882+0000 info [MyZIOApp] [user-id: UserB] Releasing resources.