Inheritable MDC using Logback

Dependency:

"com.softwaremill.ox" %% "mdc-logback" % "1.0.4"

Ox provides support for setting inheritable MDC (mapped diagnostic context) values, when using the Logback logging library. Normally, value set using MDC.put aren’t inherited across (virtual) threads, which includes forks created in concurrency contexts.

Inheritable values are especially useful e.g. when setting a correlation id in an HTTP request interceptor, or at any entrypoint to the application. Such correlation id values can be then added automatically to each log message, provided the appropriate log encoder pattern is used.

To enable using inheritable MDC values, the application’s code should call InheritableMDC.init as soon as possible. The best place would be the application’s entrypoint (the main method).

Once this is done, inheritable MDC values can be set in a scoped & structured manner using InheritableMDC.supervisedWhere and variants.

As inheritable MDC values use a ForkLocal under the hood, their usage restrictions apply: outer concurrency scopes should not be used to create forks within the scopes. Only newly created scopes, or the provided scope can be used to create forks. That’s why supervisedWhere, unsupervisedWhere and supervisedErrorWhere methods are provided.

“Normal” MDC usage is not affected. That is, values set using MDC.put are not inherited, and are only available in the thread where they are set.

For example:

import org.slf4j.MDC

import ox.fork
import ox.logback.InheritableMDC

InheritableMDC.supervisedWhere("a" -> "1", "b" -> "2") {
  MDC.put("c", "3") // not inherited

  fork {
    MDC.get("a") // "1"
    MDC.get("b") // "2"
    MDC.get("c") // null
  }.join()

  MDC.get("a") // "1"
  MDC.get("b") // "2"
  MDC.get("c") // "3"
}