2016-09-13 16:46:55 -04:00
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---
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layout: doc_page
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---
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# JavaScript Programming Guide
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This page discusses how to use JavaScript to extend Druid.
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## Examples
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JavaScript can be used to extend Druid in a variety of ways:
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- [Aggregators](../querying/aggregations.html#javascript-aggregator)
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- [Extraction functions](../querying/aggregations.html#javascript-extraction-function)
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- [Filters](../querying/aggregations.html#javascript-filter)
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- [Post-aggregators](../querying/aggregations.html#javascript-post-aggregator)
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- [Input parsers](../ingestion/data-formats.html#javascript)
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- [Router strategy](../development/router.html#javascript)
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- [Worker select strategy](../configuration/indexing-service.html#javascript)
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JavaScript can be injected dynamically at runtime, making it convenient to rapidly prototype new functionality
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without needing to write and deploy Druid extensions.
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Druid uses the Mozilla Rhino engine at optimization level 9 to compile and execute JavaScript.
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## Global variables
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Avoid using global variables. Druid may share the global scope between multiple threads, which can lead to
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unpredictable results if global variables are used.
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## Performance
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2016-09-13 18:26:44 -04:00
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Simple JavaScript functions typically have a slight performance penalty to native speed. More complex JavaScript
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functions can have steeper performance penalties. Druid compiles JavaScript functions once per node per query.
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You may need to pay special attention to garbage collection when making heavy use of JavaScript functions, especially
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garbage collection of the compiled classes themselves. Be sure to use a garbage collector configuration that supports
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timely collection of unused classes (this is generally easier on JDK8 with the Metaspace than it is on JDK7).
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## Security
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Druid does not execute JavaScript functions in a sandbox, so they have full access to the machine. If you are running
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a cluster where users that can submit queries should not be allowed to execute arbitrary code, we recommend disabling
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JavaScript functionality by setting the [configuration property](../configuration/index.html)
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`druid.javascript.disabled = true`.
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## JavaScript vs. Native Extensions
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Generally we recommend using JavaScript when security is not an issue, and when speed of development is more important
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than performance or memory use. If security is an issue, or if performance and memory use are of the utmost importance,
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we recommend developing a native Druid extension.
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In addition, native Druid extensions are more flexible than JavaScript functions. There are some kinds of extensions
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(like sketches) that must be written as native Druid extensions due to their need for custom data formats.
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