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<p>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 <a href="../configuration/index.html#enabling-metrics">Configuration</a>. Metric logs are printed to the console by default, and can be disabled with <code>-Ddruid.emitter.logging.logLevel=debug</code>.</p>
<p>Druid uses <a href="http://logging.apache.org/log4j/2.x/">log4j2</a> for logging. Logging can be configured with a log4j2.xml file. Add the path to the directory containing the log4j2.xml file (e.g. the _common/ 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.</p>
<p>To enable java logging to go through log4j2, set the <code>-Djava.util.logging.manager=org.apache.logging.log4j.jul.LogManager</code> server parameter.</p>
<p>An example log4j2.xml ships with Druid under config/_common/log4j2.xml, and a sample file is also shown below:</p>
<pre><code class="hljs"><span class="xml"><span class="hljs-meta">&lt;?xml version="1.0" encoding="UTF-8" ?&gt;</span>
<span class="hljs-tag">&lt;<span class="hljs-name">Configuration</span> <span class="hljs-attr">status</span>=<span class="hljs-string">"WARN"</span>&gt;</span>
  <span class="hljs-tag">&lt;<span class="hljs-name">Appenders</span>&gt;</span>
    <span class="hljs-tag">&lt;<span class="hljs-name">Console</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"Console"</span> <span class="hljs-attr">target</span>=<span class="hljs-string">"SYSTEM_OUT"</span>&gt;</span>
      <span class="hljs-tag">&lt;<span class="hljs-name">PatternLayout</span> <span class="hljs-attr">pattern</span>=<span class="hljs-string">"%d</span></span></span><span class="hljs-template-variable">{ISO8601}</span><span class="xml"><span class="hljs-tag"><span class="hljs-string"> %p [%t] %c - %m%n"</span>/&gt;</span>
    <span class="hljs-tag">&lt;/<span class="hljs-name">Console</span>&gt;</span>
  <span class="hljs-tag">&lt;/<span class="hljs-name">Appenders</span>&gt;</span>
  <span class="hljs-tag">&lt;<span class="hljs-name">Loggers</span>&gt;</span>
    <span class="hljs-tag">&lt;<span class="hljs-name">Root</span> <span class="hljs-attr">level</span>=<span class="hljs-string">"info"</span>&gt;</span>
      <span class="hljs-tag">&lt;<span class="hljs-name">AppenderRef</span> <span class="hljs-attr">ref</span>=<span class="hljs-string">"Console"</span>/&gt;</span>
    <span class="hljs-tag">&lt;/<span class="hljs-name">Root</span>&gt;</span>

    <span class="hljs-comment">&lt;!-- Uncomment to enable logging of all HTTP requests
    &lt;Logger name="org.apache.druid.jetty.RequestLog" additivity="false" level="DEBUG"&gt;
        &lt;AppenderRef ref="Console"/&gt;
    &lt;/Logger&gt;
    --&gt;</span>
  <span class="hljs-tag">&lt;/<span class="hljs-name">Loggers</span>&gt;</span>
<span class="hljs-tag">&lt;/<span class="hljs-name">Configuration</span>&gt;</span>
</span></code></pre>
<h2><a class="anchor" aria-hidden="true" id="my-logs-are-really-chatty-can-i-set-them-to-asynchronously-write"></a><a href="#my-logs-are-really-chatty-can-i-set-them-to-asynchronously-write" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>My logs are really chatty, can I set them to asynchronously write?</h2>
<p>Yes, using a <code>log4j2.xml</code> similar to the following causes some of the more chatty classes to write asynchronously:</p>
<pre><code class="hljs"><span class="xml"><span class="hljs-meta">&lt;?xml version="1.0" encoding="UTF-8" ?&gt;</span>
<span class="hljs-tag">&lt;<span class="hljs-name">Configuration</span> <span class="hljs-attr">status</span>=<span class="hljs-string">"WARN"</span>&gt;</span>
  <span class="hljs-tag">&lt;<span class="hljs-name">Appenders</span>&gt;</span>
    <span class="hljs-tag">&lt;<span class="hljs-name">Console</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"Console"</span> <span class="hljs-attr">target</span>=<span class="hljs-string">"SYSTEM_OUT"</span>&gt;</span>
      <span class="hljs-tag">&lt;<span class="hljs-name">PatternLayout</span> <span class="hljs-attr">pattern</span>=<span class="hljs-string">"%d</span></span></span><span class="hljs-template-variable">{ISO8601}</span><span class="xml"><span class="hljs-tag"><span class="hljs-string"> %p [%t] %c - %m%n"</span>/&gt;</span>
    <span class="hljs-tag">&lt;/<span class="hljs-name">Console</span>&gt;</span>
  <span class="hljs-tag">&lt;/<span class="hljs-name">Appenders</span>&gt;</span>
  <span class="hljs-tag">&lt;<span class="hljs-name">Loggers</span>&gt;</span>
    <span class="hljs-tag">&lt;<span class="hljs-name">AsyncLogger</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"org.apache.druid.curator.inventory.CuratorInventoryManager"</span> <span class="hljs-attr">level</span>=<span class="hljs-string">"debug"</span> <span class="hljs-attr">additivity</span>=<span class="hljs-string">"false"</span>&gt;</span>
      <span class="hljs-tag">&lt;<span class="hljs-name">AppenderRef</span> <span class="hljs-attr">ref</span>=<span class="hljs-string">"Console"</span>/&gt;</span>
    <span class="hljs-tag">&lt;/<span class="hljs-name">AsyncLogger</span>&gt;</span>
    <span class="hljs-tag">&lt;<span class="hljs-name">AsyncLogger</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"org.apache.druid.client.BatchServerInventoryView"</span> <span class="hljs-attr">level</span>=<span class="hljs-string">"debug"</span> <span class="hljs-attr">additivity</span>=<span class="hljs-string">"false"</span>&gt;</span>
      <span class="hljs-tag">&lt;<span class="hljs-name">AppenderRef</span> <span class="hljs-attr">ref</span>=<span class="hljs-string">"Console"</span>/&gt;</span>
    <span class="hljs-tag">&lt;/<span class="hljs-name">AsyncLogger</span>&gt;</span>
    <span class="hljs-comment">&lt;!-- Make extra sure nobody adds logs in a bad way that can hurt performance --&gt;</span>
    <span class="hljs-tag">&lt;<span class="hljs-name">AsyncLogger</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"org.apache.druid.client.ServerInventoryView"</span> <span class="hljs-attr">level</span>=<span class="hljs-string">"debug"</span> <span class="hljs-attr">additivity</span>=<span class="hljs-string">"false"</span>&gt;</span>
      <span class="hljs-tag">&lt;<span class="hljs-name">AppenderRef</span> <span class="hljs-attr">ref</span>=<span class="hljs-string">"Console"</span>/&gt;</span>
    <span class="hljs-tag">&lt;/<span class="hljs-name">AsyncLogger</span>&gt;</span>
    <span class="hljs-tag">&lt;<span class="hljs-name">AsyncLogger</span> <span class="hljs-attr">name</span> =<span class="hljs-string">"org.apache.druid.java.util.http.client.pool.ChannelResourceFactory"</span> <span class="hljs-attr">level</span>=<span class="hljs-string">"info"</span> <span class="hljs-attr">additivity</span>=<span class="hljs-string">"false"</span>&gt;</span>
      <span class="hljs-tag">&lt;<span class="hljs-name">AppenderRef</span> <span class="hljs-attr">ref</span>=<span class="hljs-string">"Console"</span>/&gt;</span>
    <span class="hljs-tag">&lt;/<span class="hljs-name">AsyncLogger</span>&gt;</span>
    <span class="hljs-tag">&lt;<span class="hljs-name">Root</span> <span class="hljs-attr">level</span>=<span class="hljs-string">"info"</span>&gt;</span>
      <span class="hljs-tag">&lt;<span class="hljs-name">AppenderRef</span> <span class="hljs-attr">ref</span>=<span class="hljs-string">"Console"</span>/&gt;</span>
    <span class="hljs-tag">&lt;/<span class="hljs-name">Root</span>&gt;</span>
  <span class="hljs-tag">&lt;/<span class="hljs-name">Loggers</span>&gt;</span>
<span class="hljs-tag">&lt;/<span class="hljs-name">Configuration</span>&gt;</span>
</span></code></pre>
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