mirror of https://github.com/apache/druid.git
76 lines
3.5 KiB
Markdown
76 lines
3.5 KiB
Markdown
---
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id: javascript
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title: "JavaScript programming guide"
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sidebar_label: "JavaScript functionality"
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---
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<!--
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~ Licensed to the Apache Software Foundation (ASF) under one
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~ or more contributor license agreements. See the NOTICE file
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~ distributed with this work for additional information
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~ regarding copyright ownership. The ASF licenses this file
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~ to you under the Apache License, Version 2.0 (the
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~ "License"); you may not use this file except in compliance
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~ with the License. You may obtain a copy of the License at
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~
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~ http://www.apache.org/licenses/LICENSE-2.0
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~
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~ Unless required by applicable law or agreed to in writing,
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~ software distributed under the License is distributed on an
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~ "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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~ KIND, either express or implied. See the License for the
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~ specific language governing permissions and limitations
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~ under the License.
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-->
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This page discusses how to use JavaScript to extend Apache Druid (incubating).
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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/dimensionspecs.html#javascript-extraction-function)
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- [Filters](../querying/filters.html#javascript-filter)
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- [Post-aggregators](../querying/post-aggregations.html#javascript-post-aggregator)
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- [Input parsers](../ingestion/data-formats.html#javascript)
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- [Router strategy](../design/router.html#javascript)
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- [Worker select strategy](../configuration/index.html#javascript-worker-select-strategy)
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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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## Security
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Druid does not execute JavaScript functions in a sandbox, so they have full access to the machine. So Javascript
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functions allow users to execute arbitrary code inside druid process. So, by default, Javascript is disabled.
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However, on dev/staging environments or secured production environments you can enable those by setting
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the [configuration property](../configuration/index.html#javascript)
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`druid.javascript.enabled = true`.
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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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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 on each data process 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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## 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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