How to add additional fields to log4j-submitted JSON?

[unitedmarsupial]

We're using Splunk's "javalogging" JAR to send events to Splunk from our Java-application directly. This works, but the event's JSON received by the server has only two fields: message and severity (plus the host, index, source, and sourcetype of course):

{   
    "event": {
      "message": "This is a test 20:41:34",
      "severity": "ERROR"
    },
    "host" = "myhost",
    "source" = "mysource",
    "sourcetype" = "logj4"
}

We'd like to add some others -- to help us identify the application and the application instance, et cætera. Can this be done via configuration file -- without recompiling the code?

I realize, we can use a pattern-layout to prepend the additional data to the message:

  <param name="ConversionPattern" value="FIELD1=VALUE1 FIELD2=VALUE2 %m"/>

but then the extra fields would simply be part of the message -- not separately-indexed fields of their own...

[unitedmarsupial]

Ok, there are two options here, which can also be combined:

1. Set the includeMDC parameter to true and hope (or provide for), the MDC in your case contains all of the fields you need — they will be in the properties sub-dictionary of every logged event.

2. Set the messageFormat parameter to json -- and format your message to be in proper JSON itself:

   
   
   The message field of the submitted event will then itself be a dictionary. In the above example, that sub-dictionary will contain two fields: cat and message. These can be searched for on Splunk-server as message.cat="meow".

I'd still like to be able to add additional fields next to the message and the severity, though -- not inside a sub-dictionary...