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| <h1 id="javascript-programming-guide">JavaScript Programming Guide</h1> |
| |
| <p>This page discusses how to use JavaScript to extend Apache Druid (incubating).</p> |
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| <h2 id="examples">Examples</h2> |
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| <p>JavaScript can be used to extend Druid in a variety of ways:</p> |
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| <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="../development/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 id="security">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 id="global-variables">Global variables</h2> |
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| <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 id="performance">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 id="javascript-vs-native-extensions">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> |
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