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@72328b32

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@30d82efe
val delta = Random.nextInt(1000)
// delta: Int = 771

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$ColoredConsoleSink$@8318048
val jsonSink = ConsoleSink(LogstageCirceRenderingPolicy(prettyPrint = true))
// jsonSink: ConsoleSink = izumi.logstage.sink.ConsoleSink@6e3cf167

val sinks = List(jsonSink, textSink)
// sinks: List[ConsoleSink] = List(izumi.logstage.sink.ConsoleSink@6e3cf167, izumi.logstage.sink.ConsoleSink$ColoredConsoleSink$@8318048)

val logger: IzLogger = IzLogger(Trace, sinks)
// logger: IzLogger = izumi.logstage.api.IzLogger@1470b1be
val contextLogger: IzLogger = logger("key" -> "value")
// contextLogger: IzLogger = izumi.logstage.api.IzLogger@10c740b8



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@19f92c0e

val log = LogIO.fromLogger[IO](logger)
// log: LogIO[IO] = logstage.LogIO$$anon$1@371724e2

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@31a108e4

val logger: LogZIO.Service = LogIO3.fromLogger(IzLogger())
// logger: LogZIO.Service = logstage.LogIO$$anon$1@61e8db82

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@44c8c625

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!

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@151d3b78

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@2c42e717

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@b511dd6

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