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slf4j1
SLF4J v1

The Simple Logging Facade for Java (SLF4J v1 - working with JDK8) serves as a simple facade or abstraction for various logging frameworks (e.g. java.util.logging, logback, log4j).

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

libraryDependencies += "dev.zio" %% "zio-logging-slf4j" % "@VERSION@"

NOTE: SLF4J v1 implementation is similar to v2, however there are some differences, v1 using MDC context, working with JDK8, v2 using key-value pairs, working with JDK9+. It is recommended to use v2, as it is the latest version and also there may be MDC (using ThreadLocal) issues, where in the case of parallel executions it can happen that MDC will be reset by different Fiber before the message is logged.

Logger layer:

import zio.logging.backend.SLF4J

val logger = Runtime.removeDefaultLoggers >>> SLF4J.slf4j

Default SLF4J logger setup:

  • logger name (by default) is extracted from zio.Trace
    • for example, trace zio.logging.example.Slf4jSimpleApp.run(Slf4jSimpleApp.scala:17) will have zio.logging.example.Slf4jSimpleApp as logger name
    • NOTE: custom logger name may be set by zio.logging.loggerName aspect
  • all annotations (logger name and log marker name annotations are excluded) are placed into MDC context
  • cause is logged as throwable

See also LogFormat and LogAppender

Custom logger name set by aspect:

ZIO.logInfo("Starting user operation") @@ zio.logging.loggerName("zio.logging.example.UserOperation")

Log marker name set by aspect:

ZIO.logInfo("Confidential user operation") @@ SLF4J.logMarkerName("CONFIDENTIAL")

Examples

You can find the source code here

SLF4J logger name and annotations

package zio.logging.example

import zio.logging.LogAnnotation
import zio.logging.backend.SLF4J
import zio.{ ExitCode, Runtime, Scope, ZIO, ZIOAppDefault, _ }

import java.util.UUID

object Slf4jSimpleApp extends ZIOAppDefault {

  override val bootstrap: ZLayer[ZIOAppArgs, Any, Any] = Runtime.removeDefaultLoggers >>> SLF4J.slf4j

  private val users = List.fill(2)(UUID.randomUUID())

  override def run: ZIO[Scope, Any, ExitCode] =
    for {
      _       <- ZIO.logInfo("Start")
      traceId <- ZIO.succeed(UUID.randomUUID())
      _       <- ZIO.foreachPar(users) { uId =>
        {
          ZIO.logInfo("Starting user operation") *>
            ZIO.logInfo("Confidential user operation") @@ SLF4J.logMarkerName("CONFIDENTIAL") *>
            ZIO.sleep(500.millis) *>
            ZIO.logInfo("Stopping user operation")
        } @@ ZIOAspect.annotated("user", uId.toString)
      } @@ LogAnnotation.TraceId(traceId) @@ zio.logging.loggerName("zio.logging.example.UserOperation")
      _       <- ZIO.logInfo("Done")
    } yield ExitCode.success

}

Logback configuration:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
            <Pattern>%d{HH:mm:ss.SSS} [%thread] [%mdc] %-5level %logger{36} %msg%n</Pattern>
        </layout>
    </appender>
    <turboFilter class="ch.qos.logback.classic.turbo.MarkerFilter">
        <Name>CONFIDENTIAL_FILTER</Name>
        <Marker>CONFIDENTIAL</Marker>
        <OnMatch>DENY</OnMatch>
    </turboFilter>
    <root level="debug">
        <appender-ref ref="STDOUT" />
    </root>
</configuration>

Expected Console Output:

12:16:21.951 [ZScheduler-Worker-8] [] INFO  zio.logging.example.Slf4jSimpleApp Start
12:16:22.024 [ZScheduler-Worker-12] [user=0e3bd69c-ee62-4096-82b2-593760d3fb19, trace_id=6e956bcf-d534-4c16-9402-fb6bca13c9ab] INFO  zio.logging.example.UserOperation Starting user operation
12:16:22.024 [ZScheduler-Worker-10] [user=869ed4c7-924d-4c02-ab5c-c30c1996a139, trace_id=6e956bcf-d534-4c16-9402-fb6bca13c9ab] INFO  zio.logging.example.UserOperation Starting user operation
12:16:22.592 [ZScheduler-Worker-14] [user=869ed4c7-924d-4c02-ab5c-c30c1996a139, trace_id=6e956bcf-d534-4c16-9402-fb6bca13c9ab] INFO  zio.logging.example.UserOperation Stopping user operation
12:16:22.592 [ZScheduler-Worker-1] [user=0e3bd69c-ee62-4096-82b2-593760d3fb19, trace_id=6e956bcf-d534-4c16-9402-fb6bca13c9ab] INFO  zio.logging.example.UserOperation Stopping user operation
12:16:22.598 [ZScheduler-Worker-14] [] INFO  zio.logging.example.Slf4jSimpleApp Done

Custom tracing annotation

Following application has custom aspect currentTracingSpanAspect implementation which taking current span from Tracing service which then is added to logs by log annotation.

package zio.logging.example

import zio.logging.backend.SLF4J
import zio.{ ExitCode, Runtime, Scope, ZIO, ZIOAppDefault, _ }

import java.util.UUID

trait Tracing {
  def getCurrentSpan(): UIO[String]
}

final class LiveTracing extends Tracing {
  override def getCurrentSpan(): UIO[String] = ZIO.succeed(UUID.randomUUID().toString)
}

object LiveTracing {
  val layer: ULayer[Tracing] = ZLayer.succeed(new LiveTracing)
}

object CustomTracingAnnotationApp extends ZIOAppDefault {

  private def currentTracingSpanAspect(key: String): ZIOAspect[Nothing, Tracing, Nothing, Any, Nothing, Any] =
    new ZIOAspect[Nothing, Tracing, Nothing, Any, Nothing, Any] {
      def apply[R <: Tracing, E, A](zio: ZIO[R, E, A])(implicit trace: Trace): ZIO[R, E, A] =
        ZIO.serviceWithZIO[Tracing] { tracing =>
          tracing.getCurrentSpan().flatMap { span =>
            ZIO.logAnnotate(key, span)(zio)
          }
        }
    }

  override val bootstrap: ZLayer[ZIOAppArgs, Any, Any] = Runtime.removeDefaultLoggers >>> SLF4J.slf4j

  private val users = List.fill(2)(UUID.randomUUID())

  override def run: ZIO[Scope, Any, ExitCode] =
    (for {
      _ <- ZIO.foreachPar(users) { uId =>
        {
          ZIO.logInfo("Starting operation") *>
            ZIO.sleep(500.millis) *>
            ZIO.logInfo("Stopping operation")
        } @@ ZIOAspect.annotated("user", uId.toString)
      } @@ currentTracingSpanAspect("trace_id")
      _ <- ZIO.logInfo("Done")
    } yield ExitCode.success).provide(LiveTracing.layer)

}

Expected Console Output:

15:53:20.145 [ZScheduler-Worker-9] [user=1abd8458-aefd-4780-88ec-cccd1310d4c8, trace_id=71436dd4-22d5-4e06-aaa7-f3ff7b108037] INFO  z.l.e.CustomTracingAnnotationApp Starting operation
15:53:20.145 [ZScheduler-Worker-13] [user=878689e0-da30-49f8-8923-ed915c00db9c, trace_id=71436dd4-22d5-4e06-aaa7-f3ff7b108037] INFO  z.l.e.CustomTracingAnnotationApp Starting operation
15:53:20.688 [ZScheduler-Worker-15] [user=1abd8458-aefd-4780-88ec-cccd1310d4c8, trace_id=71436dd4-22d5-4e06-aaa7-f3ff7b108037] INFO  z.l.e.CustomTracingAnnotationApp Stopping operation
15:53:20.688 [ZScheduler-Worker-11] [user=878689e0-da30-49f8-8923-ed915c00db9c, trace_id=71436dd4-22d5-4e06-aaa7-f3ff7b108037] INFO  z.l.e.CustomTracingAnnotationApp Stopping operation
15:53:20.691 [ZScheduler-Worker-15] [] INFO  z.l.e.CustomTracingAnnotationApp Done

Feature changes

Version 2.2.0

Deprecated log annotation with key slf4j_logger_name (SLF4J.loggerNameAnnotationKey) removed, only common log annotation with key logger_name (zio.logging.loggerNameAnnotationKey) for logger name is supported now.