mirror of https://github.com/apache/druid.git
55 lines
2.6 KiB
Markdown
55 lines
2.6 KiB
Markdown
---
|
|
layout: doc_page
|
|
---
|
|
# JavaScript Programming Guide
|
|
|
|
This page discusses how to use JavaScript to extend Druid.
|
|
|
|
## Examples
|
|
|
|
JavaScript can be used to extend Druid in a variety of ways:
|
|
|
|
- [Aggregators](../querying/aggregations.html#javascript-aggregator)
|
|
- [Extraction functions](../querying/dimensionspecs.html#javascript-extraction-function)
|
|
- [Filters](../querying/filters.html#javascript-filter)
|
|
- [Post-aggregators](../querying/post-aggregations.html#javascript-post-aggregator)
|
|
- [Input parsers](../ingestion/data-formats.html#javascript)
|
|
- [Router strategy](../development/router.html#javascript)
|
|
- [Worker select strategy](../configuration/indexing-service.html#javascript)
|
|
|
|
JavaScript can be injected dynamically at runtime, making it convenient to rapidly prototype new functionality
|
|
without needing to write and deploy Druid extensions.
|
|
|
|
Druid uses the Mozilla Rhino engine at optimization level 9 to compile and execute JavaScript.
|
|
|
|
## Security
|
|
|
|
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 [configuration property](../configuration/index.html)
|
|
`druid.javascript.enabled = true`.
|
|
|
|
## Global variables
|
|
|
|
Avoid using global variables. Druid may share the global scope between multiple threads, which can lead to
|
|
unpredictable results if global variables are used.
|
|
|
|
## Performance
|
|
|
|
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 per node per query.
|
|
|
|
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).
|
|
|
|
## JavaScript vs. Native Extensions
|
|
|
|
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.
|
|
|
|
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.
|