Does this library allow for structured MDC data?

[namero999]

I couldn't find any reference to MDC in the docs of the HTTP client, and I cannot see the MDC of my threads being logged. So I wonder if I need to configure it somehow or if it's not supported yet. Thanks!

[rfoltyns]

Yes, it does. Have a look at JacksonJsonLayout documentation.

MDC values are resolved with Log4j2 lookups (actually StrSubstitutor under the hood). VirtualProperty - optional and configurable within JacksonJsonLayout component - wraps StrSubstitutor and supports variable resolution just like Log4j2 KeyPair. It's quite limited though:

• ${ctx:variableName} can be used only with synchronous loggers
• log event properties e.g. ${event:Marker} are not supported

If you'd like to resolve MDC values, try this:

<JacksonJsonLayout>
    <VirtualProperty name="myMdcField" value="$${ctx:propertyName:-undefined}"/>
</JacksonJsonLayout>

[namero999]

Thanks, I managed to have it work by adding dynamic="true" to VirtualProperty

[namero999]

Is there any specific reason why this is limited to sync loggers?

[rfoltyns]

Is there any specific reason why this is limited to sync loggers?

StrSubstitutor gets the ctx values from ContextDataInjector. Log4j2 AsyncLogger doesn't copy ThreadContext data to this injector, so values are not available when VirtualPropertyWriter.valueResolver tries to resolve it.

[namero999]

Interesting, thanks. I've done some research and I've read that starting from recent versions of log4j2 it is recommended to use ContextDataProvider instead of ContextDataInjector https://logging.apache.org/log4j/2.x/manual/extending.html#Custom_ContextDataProvider

I'd look into it but navigating the codebase of this repo is a bit overwhelming right now. Any pointer?

[rfoltyns]

It's out of scope of this repo. This library handles Log4j2 LogEvent(s) on the appender level only.

Setting ThreadContext values in ContextDataInjector happens on logger level, before the event reaches any appenders, then it's passed as event.contextData only.

Have a look at variable resolution at VirtualPropertyWriter:138 all the way down to relevant lookup in log4j2-core ContextMapLookup and Interpolator.

Once you'll dig into data available in AsyncLogger's disruptor consumer thread, you'll notice that event context data is available only at event level, ContextMapLookup.currentContextData() is empty and ValueResolver (by design) doesn't have access to it.

Fortunately, you can still serialize event.contextData as described here. In that case VirtualProperty is obsolete.