Is there a way to Route logs based on Marker with the RoutingAppender in Log4j2

Question

It is possible to filter messages using markers, such as :

      <MarkerFilter marker="FLOW" onMatch="ACCEPT" onMismatch="DENY"/>

However I'm trying to route a message based on the marker using the RoutingAppender . I don't want to filter the same arguments multiple times in multiple Appenders. Here's my configuration sample (yaml):

Routing:
  name: ROUTING_APPENDER
  Routes:
    pattern: "$${ctx:marker}" #<-- How to use Marker here?
    Route:
      - key: MyRoutingKey
        ref: MyCustomAppender

The documentation stipulates :

The pattern is evaluated against all the registered Lookups and the result is used to select a Route

However there seems to be no Lookup for Markers, same for LogLevel. It would be possible to add a custom MarkerValue or LogLevelValue in the ThreadContextMap but I don't find the solution really efficient, it duplicates known information.

Is it not documented or just impossible? Should there be a built-in way to have access to those values in Lookup?

Answer 1

The documentation for RoutingAppender shows the ThreadContext lookup, but routing can also work with other lookups. One idea is to create a custom lookup.

A custom lookup is implemented as a log4j2 plugin. To help log4j2 find your plugin you can enable packages="yourCustomPackage" in your configuration file. Your plugin class needs to be on the classpath so log4j can find it. Here's the plugin code for the custom lookup:

import org.apache.logging.log4j.Marker;
import org.apache.logging.log4j.core.LogEvent;
import org.apache.logging.log4j.core.config.plugins.Plugin;
import org.apache.logging.log4j.core.lookup.StrLookup;

@Plugin(name = "marker", category = "Lookup")
public class MarkerLookup implements StrLookup {

    public String lookup(String key) {
        return null
    }

    public String lookup(LogEvent event, String key) {
        final Marker marker = event.getMarker();
        return marker == null ? null : marker.getName();
    }
}

And in the configuration file :

Routing:
  name: ROUTING_APPENDER
  Routes:
    pattern: "$${marker:}"
    Route:
    - key: PERFORMANCE
      ref: PERFORMANCE_APPENDER
    - key: PAYLOAD
      ref: PAYLOAD_APPENDER
    - key: FATAL
      ref: FATAL_APPENDER
    - ref: APPLICATION_APPENDER #Default route

Credits to the Log4j2 developers ( https://issues.apache.org/jira/browse/LOG4J2-1015 ).

UPDATE : According to them, it should be built-in in the next version (2.4). So no needs to write custom plugin after that.