<!DOCTYPE html><html lang="en"><head><meta charSet="utf-8"/><meta http-equiv="X-UA-Compatible" content="IE=edge"/><title>JavaScript programming guide · Apache Druid</title><meta name="viewport" content="width=device-width"/><link rel="canonical" href="https://druid.apache.org/docs/latest/development/javascript.html"/><meta name="generator" content="Docusaurus"/><meta name="description" content="&lt;!--"/><meta name="docsearch:language" content="en"/><meta property="og:title" content="JavaScript programming guide · Apache Druid"/><meta property="og:type" content="website"/><meta property="og:url" content="https://druid.apache.org/index.html"/><meta property="og:description" content="&lt;!--"/><meta property="og:image" content="https://druid.apache.org/img/druid_nav.png"/><meta name="twitter:card" content="summary"/><meta name="twitter:image" content="https://druid.apache.org/img/druid_nav.png"/><link rel="shortcut icon" href="/img/favicon.png"/><link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/styles/default.min.css"/><script async="" src="https://www.googletagmanager.com/gtag/js?id=UA-131010415-1"></script><script>
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SQLServer</a></li><li class="navListItem"><a class="navItem" href="/docs/latest/development/extensions-contrib/statsd.html">StatsD Emitter</a></li><li class="navListItem"><a class="navItem" href="/docs/latest/development/extensions-contrib/tdigestsketch-quantiles.html">T-Digest Quantiles Sketch module</a></li><li class="navListItem"><a class="navItem" href="/docs/latest/development/extensions-contrib/thrift.html">Thrift</a></li><li class="navListItem"><a class="navItem" href="/docs/latest/development/extensions-contrib/time-min-max.html">Timestamp Min/Max aggregators</a></li><li class="navListItem"><a class="navItem" href="/docs/latest/ingestion/standalone-realtime.html">Realtime Process</a></li></ul></div></div></section></div><script>
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<p>This page discusses how to use JavaScript to extend Apache Druid (incubating).</p>
<h2><a class="anchor" aria-hidden="true" id="examples"></a><a href="#examples" 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>Examples</h2>
<p>JavaScript can be used to extend Druid in a variety of ways:</p>
<ul>
<li><a href="../querying/aggregations.html#javascript-aggregator">Aggregators</a></li>
<li><a href="../querying/dimensionspecs.html#javascript-extraction-function">Extraction functions</a></li>
<li><a href="../querying/filters.html#javascript-filter">Filters</a></li>
<li><a href="../querying/post-aggregations.html#javascript-post-aggregator">Post-aggregators</a></li>
<li><a href="../ingestion/data-formats.html#javascript">Input parsers</a></li>
<li><a href="../design/router.html#javascript">Router strategy</a></li>
<li><a href="../configuration/index.html#javascript-worker-select-strategy">Worker select strategy</a></li>
</ul>
<p>JavaScript can be injected dynamically at runtime, making it convenient to rapidly prototype new functionality
without needing to write and deploy Druid extensions.</p>
<p>Druid uses the Mozilla Rhino engine at optimization level 9 to compile and execute JavaScript.</p>
<h2><a class="anchor" aria-hidden="true" id="security"></a><a href="#security" 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>Security</h2>
<p>Druid does not execute JavaScript functions in a sandbox, so they have full access to the machine. So JavaScript
functions allow users to execute arbitrary code inside druid process. So, by default, JavaScript is disabled.
However, on dev/staging environments or secured production environments you can enable those by setting
the <a href="../configuration/index.html#javascript">configuration property</a>
<code>druid.javascript.enabled = true</code>.</p>
<h2><a class="anchor" aria-hidden="true" id="global-variables"></a><a href="#global-variables" 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>Global variables</h2>
<p>Avoid using global variables. Druid may share the global scope between multiple threads, which can lead to
unpredictable results if global variables are used.</p>
<h2><a class="anchor" aria-hidden="true" id="performance"></a><a href="#performance" 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>Performance</h2>
<p>Simple JavaScript functions typically have a slight performance penalty to native speed. More complex JavaScript
functions can have steeper performance penalties. Druid compiles JavaScript functions once on each data process per query.</p>
<p>You may need to pay special attention to garbage collection when making heavy use of JavaScript functions, especially
garbage collection of the compiled classes themselves. Be sure to use a garbage collector configuration that supports
timely collection of unused classes (this is generally easier on JDK8 with the Metaspace than it is on JDK7).</p>
<h2><a class="anchor" aria-hidden="true" id="javascript-vs-native-extensions"></a><a href="#javascript-vs-native-extensions" 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>JavaScript vs. Native Extensions</h2>
<p>Generally we recommend using JavaScript when security is not an issue, and when speed of development is more important
than performance or memory use. If security is an issue, or if performance and memory use are of the utmost importance,
we recommend developing a native Druid extension.</p>
<p>In addition, native Druid extensions are more flexible than JavaScript functions. There are some kinds of extensions
(like sketches) that must be written as native Druid extensions due to their need for custom data formats.</p>
</span></div></article></div><div class="docs-prevnext"><a class="docs-prev button" href="/docs/latest/development/modules.html"><span class="arrow-prev">← </span><span>Creating extensions</span></a><a class="docs-next button" href="/docs/latest/development/build.html"><span>Build from source</span><span class="arrow-next"> →</span></a></div></div></div><nav class="onPageNav"><ul class="toc-headings"><li><a href="#examples">Examples</a></li><li><a href="#security">Security</a></li><li><a href="#global-variables">Global variables</a></li><li><a href="#performance">Performance</a></li><li><a href="#javascript-vs-native-extensions">JavaScript vs. Native Extensions</a></li></ul></nav></div><footer class="nav-footer druid-footer" id="footer"><div class="container"><div class="text-center"><p><a href="/technology">Technology</a> · <a href="/use-cases">Use Cases</a> · <a href="/druid-powered">Powered by Druid</a> · <a href="/docs/latest/latest">Docs</a> · <a href="/community/">Community</a> · <a href="/downloads.html">Download</a> · <a href="/faq">FAQ</a></p></div><div class="text-center"><a title="Join the user group" href="https://groups.google.com/forum/#!forum/druid-user" target="_blank"><span class="fa fa-comments"></span></a> · <a title="Follow Druid" href="https://twitter.com/druidio" target="_blank"><span class="fab fa-twitter"></span></a> · <a title="Download via Apache" href="https://www.apache.org/dyn/closer.cgi?path=/incubator/druid/{{ site.druid_versions[0].versions[0].version }}/apache-druid-{{ site.druid_versions[0].versions[0].version }}-bin.tar.gz" target="_blank"><span class="fas fa-feather"></span></a> · <a title="GitHub" href="https://github.com/apache/incubator-druid" target="_blank"><span class="fab fa-github"></span></a></div><div class="text-center license">Copyright © 2019 <a href="https://www.apache.org/" target="_blank">Apache Software Foundation</a>.<br/>Except where otherwise noted, licensed under <a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA 4.0</a>.<br/>Apache Druid, Druid, and the Druid logo are either registered trademarks or trademarks of The Apache Software Foundation in the United States and other countries.</div></div></footer></div></body></html>