Using MDC for labels

[stefnats]

Hey there,

i want to migrate from ELK Stack to Loki.
Is it possible to send the MDC Parameters (set dynamically in ThreadContext.put(key, value)) as Labels to Loki? And how would i achieve this with log4j2?

Thanks in advance

[tkowalcz]

Hello,

yes, you can do that. In log4j you have variable ctx that can be used in Label definition, you can use mdc or X:

  <appenders>
        <Loki name="Loki">
...
            <Label name="tid1" pattern="%X{tid}"/>
            <Label name="tid2" value="${ctx:tid}"/>
            <Label name="tid3" pattern="%mdc{tid}"/>
       </Loki>
    </appenders>

The ${ctx} syntax also allows for specifying default values if context is empty:

            <Label name="default_value_test" value="${ctx:missing:-use_this_if_missing}"/>

You might find out this documentation helpful:

• https://github.com/tkowalcz/tjahzi/tree/master/log4j2-appender#lookups--variable-substitution
• https://logging.apache.org/log4j/2.x/manual/layouts.html
• https://github.com/tkowalcz/tjahzi/blob/9e07ef34ca5bb6cbb1a13f69151db35aade22bf3/log4j2-appender/src/test/resources/log4j-patterns-in-labels-configuration.xml

[tkowalcz]

@stefnats Please let me know or reopen the issue if you need more info.

[lucianocallero]

Is there any way to do the same but without knowing previously the name of the MDC vars? I have dynamic context variables depending on the code and i don't want to re-write my xml every time i add a new one.

By instance, i have 2 methods with 3 different value, can i do something at XML to show 3 without telling the name up front?

// Method 1..
ThreadContext.put("foo1", "value1");
ThreadContext.put("foo2", "value2");

// Method 2
ThreadContext.put("foo1", "value1");
ThreadContext.put("foo3", "OtherValue3");

[tkowalcz]

Is there any way to do the same but without knowing previously the name of the MDC vars? I have dynamic context variables depending on the code and i don't want to re-write my xml every time i add a new one.

By instance, i have 2 methods with 3 different value, can i do something at XML to show 3 without telling the name up front?

// Method 1.. ThreadContext.put("foo1", "value1"); ThreadContext.put("foo2", "value2");

// Method 2 ThreadContext.put("foo1", "value1"); ThreadContext.put("foo3", "OtherValue3");

@lucianocallero I don't think it is possible. Is it possible in other appenders?

I guess we could add a switch that will convert the whole thread context into loki labels.

[lucianocallero]

Thanks. That would be great. In me case more than one appender does not fit because labels are dynamic depending on the context of the appliances.