kotlinx-coroutines-slf4j/kotlinx.coroutines.slf4j/MDCContext

MDCContext

class MDCContext(val contextMap: MDCContextMap = MDC.getCopyOfContextMap()) : AbstractCoroutineContextElement, ThreadContextElement<MDCContextMap> (source)

MDC context element for CoroutineContext.

Example:

MDC.put("kotlin", "rocks") // Put a value into the MDC context

launch(MDCContext()) {
    logger.info { "..." }   // The MDC context contains the mapping here
}

Note that you cannot update MDC context from inside the coroutine simply using MDC.put. These updates are going to be lost on the next suspension and reinstalled to the MDC context that was captured or explicitly specified in contextMap when this object was created on the next resumption.

For example, the following code will not work as expected:

launch(MDCContext()) {
    MDC.put("key", "value") // This update will be lost
    delay(100)
    println(MDC.get("key")) // This will print null
}

Instead, you should use withContext to capture the updated MDC context:

launch(MDCContext()) {
    MDC.put("key", "value") // This update will be captured
    withContext(MDCContext()) {
        delay(100)
        println(MDC.get("key")) // This will print "value"
    }
}

There is no way to implicitly propagate MDC context updates from inside the coroutine to the outer scope. You have to capture the updated MDC context and restore it explicitly. For example:

MDC.put("a", "b")
val contextMap = withContext(MDCContext()) {
    MDC.put("key", "value")
    withContext(MDCContext()) {
        MDC.put("key2", "value2")
        withContext(MDCContext()) {
            yield()
            MDC.getCopyOfContextMap()
        }
    }
}
// contextMap contains: {"a"="b", "key"="value", "key2"="value2"}
MDC.setContextMap(contextMap)