Retrieve MDC values in logback - logstash

Question

I am trying to log the MDC values in stdout and in logstash.
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<encoder>
<pattern>%-5level [%thread] %logger{0}: %msg -someName=%X{mdcFieldKey} %n</pattern>
<charset>utf8</charset>
</encoder>
</appender>
<appender name="logstash" class="net.logstash.logback.appender.LogstashTcpSocketAppender">
<destination>kibanaurl:port</destination>
<encoder class="net.logstash.logback.encoder.LogstashEncoder">
<customFields>
{
"application-name":"${spring.application.name}",
"someName":"${mdcFieldKey}",
"dummyName":"dummyValue"
}
</customFields>
<provider class="net.logstash.logback.composite.loggingevent.ArgumentsJsonProvider"/>
</encoder>
</appender>
Java code:
MDC.put("mdcFieldKey", "value to be displayed in kibana");
The STDOUT appender is displaying the mdc value correctly on kibana but for the logstash appender I see someName_IS_UNDEFINED. The hardcoded dummyname is also displayed correctly.
Why is the mdc value not populated in logstash custom field?

Answer 1

check here https://github.com/logstash/logstash-logback-encoder#mdc-fields
Add the line <includeMdcKeyName>mdcFieldKey</includeMdcKeyName> as below is one option. The field name will be same as mdc key in this case
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<encoder>
<pattern>%-5level [%thread] %logger{0}: %msg -someName=%X{mdcFieldKey} %n</pattern>
<charset>utf8</charset>
</encoder>
</appender>
<appender name="logstash" class="net.logstash.logback.appender.LogstashTcpSocketAppender">
<destination>kibanaurl:port</destination>
<encoder class="net.logstash.logback.encoder.LogstashEncoder">
<includeMdcKeyName>mdcFieldKey</includeMdcKeyName>
<customFields>
{
"application-name":"${spring.application.name}",
"dummyName":"dummyValue"
}
</customFields>
<provider class="net.logstash.logback.composite.loggingevent.ArgumentsJsonProvider"/>
</encoder>
</appender>