LogStage is a zero-cost structural logging framework for Scala & Scala.js

Key features:

  1. LogStage extracts structure from ordinary string interpolations in your log messages with zero changes to code.
  2. LogStage uses macros to extract log structure, its faster at runtime than a typical reflective structural logging frameworks,
  3. Log contexts
  4. Console, File and SLF4J sinks included, File sink supports log rotation,
  5. Human-readable output and JSON output included,
  6. Method-level logging granularity. Can configure methods com.example.Service.start and com.example.Service.doSomething independently,
  7. Slf4J adapters: route legacy Slf4J logs into LogStage router


libraryDependencies ++= Seq(
  // LogStage core library
  "io.7mind.izumi" %% "logstage-core" % "1.0.9-SNAPSHOT",
  // Json output
  "io.7mind.izumi" %% "logstage-rendering-circe" % "1.0.9-SNAPSHOT",
  // Router from Slf4j to LogStage
  "io.7mind.izumi" %% "logstage-adapter-slf4j" % "1.0.9-SNAPSHOT",
  // LogStage integration with DIStage
  "io.7mind.izumi" %% "distage-extension-logstage" % "1.0.9-SNAPSHOT",
  // Router from LogStage to Slf4J
  "io.7mind.izumi" %% "logstage-sink-slf4j " % "1.0.9-SNAPSHOT",


The following snippet:

import logstage.IzLogger
import scala.util.Random

val logger = IzLogger()
// logger: IzLogger = izumi.logstage.api.IzLogger@7489b5ea

val justAnArg = "example"
// justAnArg: String = example
val justAList = List[Any](10, "green", "bottles")
// justAList: List[Any] = List(10, green, bottles)

logger.trace(s"Argument: $justAnArg, another arg: $justAList")

// custom name, not based on `val` name

logger.info(s"Named expression: ${Random.nextInt() -> "random number"}")

// print result without a name

logger.warn(s"Invisible argument: ${Random.nextInt() -> "random number" -> null}")

// add following fields to all messages printed by a new logger value

val ctxLogger = logger("userId" -> "user@google.com", "company" -> "acme")
// ctxLogger: IzLogger = izumi.logstage.api.IzLogger@315d8ffb
val delta = Random.nextInt(1000)
// delta: Int = 349

ctxLogger.info(s"Processing time: $delta")

Will look like this in string form:


And like this in JSON:



  1. JSON formatter is type aware!
  2. Each JSON message contains @class field with holds a unique event class identifier. All events produced by the same source code line will share the same event class.

Syntax Reference

1) Simple variable:

logger.info(s"My message: $argument")

2) Chain:

logger.info(s"My message: ${call.method} ${access.value}")

3) Named expression:

logger.info(s"My message: ${Some.expression -> "argname"}")

4) Invisible named expression:

logger.info(s"My message: ${Some.expression -> "argname" -> null}")

5) De-camelcased name:

logger.info(${camelCaseName -> ' '})

Basic setup

import logstage.{ConsoleSink, IzLogger, Trace}
import logstage.circe.LogstageCirceRenderingPolicy

val textSink = ConsoleSink.text(colored = true)
// textSink: ConsoleSink = izumi.logstage.sink.ConsoleSink$ColoredConsoleSink$@4d0c25a
val jsonSink = ConsoleSink(LogstageCirceRenderingPolicy(prettyPrint = true))
// jsonSink: ConsoleSink = izumi.logstage.sink.ConsoleSink@3d4f01cf

val sinks = List(jsonSink, textSink)
// sinks: List[ConsoleSink] = List(izumi.logstage.sink.ConsoleSink@3d4f01cf, izumi.logstage.sink.ConsoleSink$ColoredConsoleSink$@4d0c25a)

val logger: IzLogger = IzLogger(Trace, sinks)
// logger: IzLogger = izumi.logstage.api.IzLogger@1b691f55
val contextLogger: IzLogger = logger("key" -> "value")
// contextLogger: IzLogger = izumi.logstage.api.IzLogger@1929cc70



Log algebras

LogIO, LogIO2 & LogIO3 algebras provide a purely-functional API for one-, two-, and three-parameter effect types respectively:

import logstage.{IzLogger, LogIO}
import cats.effect.IO

val logger = IzLogger()
// logger: IzLogger = izumi.logstage.api.IzLogger@5cade7e3

val log = LogIO.fromLogger[IO](logger)
// log: LogIO[IO] = logstage.LogIO$$anon$1@7a87b54d

log.info(s"Hey! I'm logging with ${log}stage!").unsafeRunSync()
I 2019-03-29T23:21:48.693Z[Europe/Dublin] r.S.App7.res8 ...main-12:5384  (00_logstage.md:92) Hey! I'm logging with log=logstage.LogIO$$anon$1@72736f25stage!

LogIO.log/LogIO2.log/LogIO3.log/IzLogger.log let you refer to an implicit logger’s methods without having to name a logger variable

import logstage.LogIO
import logstage.LogIO.log

def logfun[F[_]: LogIO]: F[Unit] = {
  log.info(s"I'm logging with ${log}stage!")

Adding Custom Context to Loggers

LogIO* algebras can be extended with custom context using their .withCustomContext method, same as IzLogger:

import cats.effect.IO
import cats.syntax.all._
import logstage.LogIO
import io.circe.Printer
import io.circe.syntax._

def importEntity(entity: Entity)(implicit log: LogIO[IO]): IO[Unit] = {
  val logWithContext: LogIO[IO] = {
      "ID" -> entity.id,
      "entityAsJSON" -> entity.asJson.printWith(Printer.spaces2),

  load(entity).handleErrorWith {
    case error =>
      logWithContext.error(s"Failed to import entity: $error.").void
      // JSON message includes `ID` and `entityAsJSON` fields

ZIO environment support

LogZIO.log lets you carry LogZIO capability in environment.


import logstage.{IzLogger, LogIO3, LogZIO}
import logstage.LogZIO.log
import zio.{Has, URIO}

val fn: URIO[LogZIO, Unit] = {
  log.info(s"I'm logging with ${log}stage!")
// fn: URIO[LogZIO, Unit] = zio.ZIO$Read@2d027951

val logger: LogZIO.Service = LogIO3.fromLogger(IzLogger())
// logger: LogZIO.Service = logstage.LogIO$$anon$1@1ea311ed

zio.Runtime.default.unsafeRun {

Adding FiberId to log messages

LogZIO.withFiberId provides a LogIO instance that logs the current ZIO FiberId in addition to the JVM thread id:

val logger: LogZIO.Service = LogZIO.withFiberId(IzLogger())
// logger: LogZIO.Service = logstage.LogZIO$$anon$1@4d202b27

zio.Runtime.default.unsafeRun {
I 2019-03-29T23:21:48.760Z[Europe/Dublin] r.S.App9.res10 ...main-12:5384  (00_logstage.md:123) {fiberId=0} Hey! I'm logging with log=logstage.LogZIO$$anon$1@c39104astage!

Adding custom logging context to log messages

LogZIO.withCustomContext allows to append to the custom log context carried in ZIO environment when LogZIO.log is used for logging:

import zio._

def databaseCall(): ZIO[LogZIO, Throwable, String] = ZIO.succeed("stubbed")

def dbLayerFunction(arg: Int): ZIO[LogZIO, Throwable, String] = {
  LogZIO.withCustomContext("arg" -> arg) {
    for {
      result <- databaseCall
      _      <- log.info(s"Database call $result")
    } yield result

def serviceLayerFunction1(): ZIO[LogZIO, Throwable, String] = {
  for {
    _      <- log.info("Going to call dbLayerFunction")
    result <- dbLayerFunction(1)
  } yield result

def serviceLayerFunction2(): ZIO[LogZIO, Throwable, String] = {
  log.info("Called serviceLayerFunction2").as("stubbed")

def controllerFunction(correlationId: String): ZIO[LogZIO, Throwable, String] = {
  LogZIO.withCustomContext("correlation_id" -> correlationId) {
    for {
      x     <- serviceLayerFunction1()
      y     <- serviceLayerFunction2()
      result = x + y
      _     <- log.info(s"Controller produced $result")
    } yield result
  } <* log.info("Some log after controller function (without correlation_id)")

// at the end of the world
zio.Runtime.default.unsafeRun {
// res15: String = "stubbedstubbed"
I 2021-08-17T15:07:54.244 (00_logstage.md:220)  …App12.serviceLayerFunction1 [2280:Thread-60           ] correlation_id=123 Going to call dbLayerFunction
I 2021-08-17T15:07:54.342 (00_logstage.md:213)  …n.App12.dbLayerFunction.212 [2280:Thread-60           ] correlation_id=123, arg=1 Database call result=stubbed
I 2021-08-17T15:07:54.358 (00_logstage.md:226)  …App12.serviceLayerFunction2 [2280:Thread-60           ] correlation_id=123 Called serviceLayerFunction2
I 2021-08-17T15:07:54.367 (00_logstage.md:235)  ….controllerFunction.232.233 [2280:Thread-60           ] correlation_id=123 Controller produced result=stubbedstubbed
I 2021-08-17T15:07:54.371 (00_logstage.md:237)  …on.App12.controllerFunction [2280:Thread-60           ] Some log after controller function (without correlation_id)

Tagless trifunctor support

LogIO3Ask.log adds environment support for all trifunctor effect types with an instance of MonadAsk3[F] typeclass from BIO hierarchy.


import logstage.{LogIO3, LogIO3Ask, IzLogger}
import logstage.LogIO3Ask.log
import zio.{Has, ZIO}

def fn[F[-_, +_, +_]: LogIO3Ask]: F[Has[LogIO3[F]], Nothing, Unit] = {
 log.info(s"I'm logging with ${log}stage!")

val logger = LogIO3.fromLogger(IzLogger())
// logger: LogIO3[ZIO] = logstage.LogIO$$anon$1@4cbf2c2d

zio.Runtime.default.unsafeRun {

Custom JSON rendering with LogstageCodec

If you define an instance of LogstageCodec for your type, it will be used when rendering your logs to JSON instead of the default renderer.

You can derive instances of LogstageCodec from Circe codecs with logstage-rendering-circe or write them manually.


import io.circe.Codec
import io.circe.derivation
import logstage.LogstageCodec
import logstage.circe.LogstageCirceCodec

final case class KV(key: String, value: Int)

object KV {
  implicit val circeCodec: Codec[KV] = derivation.deriveCodec[KV]
  implicit val logstageCodec: LogstageCodec[KV] = LogstageCirceCodec.derived[KV]

If you want to make sure that all values in your interpolations are rendered using LogstageCodec instead of default renderer, use IzStrictLogger/LogIOStrict types from logstage.strict package:

import logstage.strict.IzStrictLogger
import logstage.ConsoleSink
import logstage.circe.LogstageCirceRenderingPolicy

val logger = IzStrictLogger(sink = ConsoleSink(LogstageCirceRenderingPolicy()))
// logger: IzStrictLogger = izumi.logstage.api.strict.IzStrictLogger@72491c1a

logger.info(s"Got ${KV("key", 7) -> "KeyValue"}")

SLF4J Router

When not configured, logstage-adapter-slf4j will log messages with level >= Info to stdout.

Due to the global mutable nature of slf4j, to configure slf4j logging you’ll have to mutate a global singleton StaticLogRouter. Replace its LogRouter with the same one you use elsewhere in your application to use the same configuration for Slf4j.

import logstage.IzLogger
import izumi.logstage.api.routing.StaticLogRouter

val myLogger = IzLogger()
// myLogger: IzLogger = izumi.logstage.api.IzLogger@746347c7

// configure SLF4j to use the same router that `myLogger` uses