druid/docs/configuration/logging.md

id	title
logging	Logging

Apache Druid processes will emit logs that are useful for debugging to the console. Druid processes 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. The default configuration file log4j2.xml ships with Druid under conf/druid/{config}/_common/log4j2.xml .

By default, Druid uses RollingRandomAccessFile for rollover daily, and keeps log files up to 7 days. If that's not suitable in your case, you could modify the log4j2.xml to meet your need.

An example log4j2.xml file is shown below:

<?xml version="1.0" encoding="UTF-8" ?>
<Configuration status="WARN">
  <Console name="Console" target="SYSTEM_OUT">
    <PatternLayout pattern="%d{ISO8601} %p [%t] %c - %m%n"/>
  </Console>
    
  <RollingRandomAccessFile name="FileAppender"
                           fileName="${sys:druid.log.path}/${sys:druid.node.type}.log"
                           filePattern="${sys:druid.log.path}/${sys:druid.node.type}.%d{yyyyMMdd}.log">
    <PatternLayout pattern="%d{ISO8601} %p [%t] %c - %m%n"/>
    <Policies>
        <TimeBasedTriggeringPolicy interval="1" modulate="true"/>
    </Policies>
    <DefaultRolloverStrategy>
      <Delete basePath="${sys:druid.log.path}/" maxDepth="1">
        <IfFileName glob="*.log" />
        <IfLastModified age="7d" />
      </Delete>
    </DefaultRolloverStrategy>
  </RollingRandomAccessFile>
  <Loggers>
    <Root level="info">
      <AppenderRef ref="FileAppender"/>
    </Root>

    <!-- Uncomment to enable logging of all HTTP requests
    <Logger name="org.apache.druid.jetty.RequestLog" additivity="false" level="DEBUG">
        <AppenderRef ref="FileAppender"/>
    </Logger>
    -->
  </Loggers>
</Configuration>

How to change log directory

By default, Druid outputs the logs to a directory log under the directory where Druid is launched from. For example, if Druid is started from its bin directory, there will be a subdirectory log generated under bin directory to hold the log files. If you want to change the log directory, set environment variable DRUID_LOG_DIR to the right directory before you start Druid.

My logs are really chatty, can I set them to asynchronously write?

Yes, using a log4j2.xml similar to the following causes some of the more chatty classes to write asynchronously:

<?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>
    <!-- AsyncLogger instead of Logger -->
    <AsyncLogger name="org.apache.druid.curator.inventory.CuratorInventoryManager" level="debug" additivity="false">
      <AppenderRef ref="Console"/>
    </AsyncLogger>
    <AsyncLogger name="org.apache.druid.client.BatchServerInventoryView" level="debug" additivity="false">
      <AppenderRef ref="Console"/>
    </AsyncLogger>
    <!-- Make extra sure nobody adds logs in a bad way that can hurt performance -->
    <AsyncLogger name="org.apache.druid.client.ServerInventoryView" level="debug" additivity="false">
      <AppenderRef ref="Console"/>
    </AsyncLogger>
    <AsyncLogger name ="org.apache.druid.java.util.http.client.pool.ChannelResourceFactory" level="info" additivity="false">
      <AppenderRef ref="Console"/>
    </AsyncLogger>
    <Root level="info">
      <AppenderRef ref="Console"/>
    </Root>
  </Loggers>
</Configuration>