druid/docs/content/Logging.md

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Logging

Druid nodes will emit logs that are useful for debugging to the console. Druid nodes also emit periodic metrics about their state. For more about metrics, see Configuration. Metric logs are printed to the console by default, and can be disabled with -Ddruid.emitter.logging.logLevel=debug.

Druid uses log4j2 for logging. Logging can be configured with a log4j2.xml file. Add the path to the directory containing the log4j2.xml file (eg a config dir) to your classpath if you want to override default Druid log configuration. Note that this directory should be earlier in the classpath than the druid jars. The easiest way to do this is to prefix the classpath with the config dir. For example, if the log4j2.xml file is in config/_common:

java -Xmx512m -Duser.timezone=UTC -Dfile.encoding=UTF-8         \
     -Ddruid.realtime.specFile=examples/indexing/wikipedia.spec \
     -classpath "config/_common:config/realtime:lib/*"          \
     io.druid.cli.Main server realtime

Note the "-classpath" in this example has the config dir before the jars under lib/*.

An example log4j2.xml ships with Druid under config/_common/log4j2.xml, and a sample file is also shown below:

<?xml version="1.0" encoding="UTF-8" ?>
<Configuration status="WARN">
  <Appenders>
    <Console name="Console" target="SYSTEM_OUT">
      <PatternLayout pattern="%d{ISO8601} %p [%t] %c - %m%n"/>
    </Console>
  </Appenders>
  <Loggers>
    <Root level="info">
      <AppenderRef ref="Console"/>
    </Root>
  </Loggers>
</Configuration>