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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
- Extraction functions
- Filters
- Post-aggregators
- Input parsers
- Router strategy
- Worker select strategy
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.
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).
Security
Druid does not execute JavaScript functions in a sandbox, so they have full access to the machine. If you are running
a cluster where users that can submit queries should not be allowed to execute arbitrary code, we recommend disabling
JavaScript functionality by setting the configuration property
druid.javascript.disabled = true.
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.