MDC async

This project contains code that was shamelessly copied from here so that it can easily be reused as a dependency.

Some slight alterations have been done (i.e. the signatures for MDC context are java.util.Map[String, String] instead of java.util.Map[_, _] due to recent changes in slf4j and MDCHttpExecutionContext has been renamed to MDCExecutionContext) but otherwise its exactly the same


The library can be found here

"org.mdedetrich" %% "mdc-async" % "0.1"

Currently its published for both Scala 2.11.8


The main idea of the library is to be able to use slf4j MDC in asynchronous environments. This library contains two different approaches to solving this problem, one is by using a Akka Dispatcher and the other is by using custom execution context (which you need to use when you are doing your logging)

Akka Dispatcher

To use the Akka dispatcher variant, you need to provide a global dispatcher. If you are using Play, then you need to do something like the following

play {
  akka {
    actor {
      default-dispatcher = {
        type = "monitoring.MDCPropagatingDispatcherConfigurator"

If you are not using Play, then you simple need to provide this dispatcher in context for your Akka application. Note that as stated here this can have performance impliciations, quoting directly from @jroper

I don't recommend doing this. It has performance implications, and in an asynchronous application, it can be very difficult to track what is done on what thread - you don't want to have to worry about that, you want the framework to worry about that for you, but transferring state on thread locals forces you to always have this implicit requirement in your mind as you code. In my opinion, this just isn't worth the pain. Pass state/context explicitly, then everything becomes very easy to understand, test, and reason about.

MDC Execution Context

Another solution is to use a custom MDC execution context which propagates a specified ExecutionContext. You can find the execution context in scala.concurrent.MDCExecutionContext. This execution context delegates from an already existing execution context, so if you are using Play (for example) you would do

package scala.concurrent

import scala.concurrent.ExecutionContext

 * The standard [[play.api.libs.concurrent.Execution.defaultContext]] loses the MDC context.
 * This custom [[ExecutionContext]] propagates the MDC context, so that the request
 * and the correlation IDs can be logged.
object Execution {

  object Implicits {
    implicit def defaultContext: ExecutionContext = Execution.defaultContext

  def defaultContext: ExecutionContext = MDCExecutionContext.fromThread(play.api.libs.concurrent.Execution.defaultContext)


Otherwise you could just the default ExecutionContext, i.e.


Currently all of the dependencies are set as provided, so you need to provide the actual dependency yourself. The dependencies