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Logging

Apache Druid services emit logs that to help you debug. The same services also emit periodic metrics about their state. To disable metric info logs set the following runtime property: -Ddruid.emitter.logging.logLevel=debug.

Druid uses log4j2 for logging. The default configuration file log4j2.xml ships with Druid at the following path: 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, modify the log4j2.xml accordingly.

The following example log4j2.xml is based upon the micro quickstart:

<?xml version="1.0" encoding="UTF-8" ?>
<Configuration status="WARN">
  <Properties>
    <!-- to change log directory, set DRUID_LOG_DIR environment variable to your directory before launching Druid -->
    <Property name="druid.log.path" value="log" />
  </Properties>

  <Appenders>
    <Console name="Console" target="SYSTEM_OUT">
      <PatternLayout pattern="%d{ISO8601} %p [%t] %c - %m%n"/>
    </Console>

    <!-- Rolling Files-->
    <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>

  </Appenders>

  <Loggers>
    <Root level="info">
      <AppenderRef ref="FileAppender"/>
    </Root>

    <!-- Set level="debug" to see stack traces for query errors -->
    <Logger name="org.apache.druid.server.QueryResource" level="info" additivity="false">
      <Appender-ref ref="FileAppender"/>
    </Logger>
    <Logger name="org.apache.druid.server.QueryLifecycle" level="info" additivity="false">
      <Appender-ref ref="FileAppender"/>
    </Logger>

    <!-- Set level="debug" or "trace" to see more Coordinator details (segment balancing, load/drop rules, etc) -->
    <Logger name="org.apache.druid.server.coordinator" level="info" additivity="false">
      <Appender-ref ref="FileAppender"/>
    </Logger>

    <!-- Set level="debug" to see low-level details about segments and ingestion -->
    <Logger name="org.apache.druid.segment" level="info" additivity="false">
      <Appender-ref ref="FileAppender"/>
    </Logger>

    <!-- Set level="debug" to see more information about extension initialization -->
    <Logger name="org.apache.druid.initialization" level="info" additivity="false">
      <Appender-ref ref="FileAppender"/>
    </Logger>

    <!-- Quieter logging at startup -->
    <Logger name="org.skife.config" level="warn" additivity="false">
      <Appender-ref ref="FileAppender"/>
    </Logger>
    <Logger name="com.sun.jersey.guice" level="warn" additivity="false">
      <Appender-ref ref="FileAppender"/>
    </Logger>
  </Loggers>
</Configuration>

NOTE: Although Druid shares the log4j configuration file task peon processes, the appenders in this file DO NOT take effect for peon processes. Peons always output logs to standard output. Middle Managers redirect task logs from standard output to long-term storage.

However, log level settings do take effect for these task peon processes. This means you can configure loggers at different logging level for task logs using log4j2.xml.

Log directory

The included log4j2.xml configuration for Druid and ZooKeeper writes logs to the log directory at the root of the distribution.

If you want to change the log directory, set the environment variable DRUID_LOG_DIR to the right directory before you start Druid.

All-in-one start commands

If you use one of the all-in-one start commands, such as bin/start-micro-quickstart, the default configuration for each service has two kinds of log files. Log4j2 writes the main log file and rotates it periodically. For example, log/historical.log.

The secondary log file contains anything that is written by the component directly to standard output or standard error without going through log4j2. For example, log/historical.stdout.log. This consists mainly of messages from the Java runtime itself. This file is not rotated, but it is generally small due to the low volume of messages. If necessary, you can truncate it using the Linux command truncate --size 0 log/historical.stdout.log.

Avoid reflective access warnings in logs

On Java 11, you may see warnings like the following in the logs:

WARNING: An illegal reflective access operation has occurred
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

To avoid these, add the --add-exports and --add-opens command line parameters described in the documentation section about Java strong encapsulation.

Set the logs to asynchronously write

If your logs are really chatty, you can set them to write asynchronously. The following example shows a log4j2.xml that configures 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>
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