Implementing custom scripting language in Elasticsearch

A ScriptEngine is a backend for implementing a scripting language in Elasticsearch.

Custom script engines integrate with Elasticsearch scripting framework through the ScriptEngine interface. To register the ScriptEngine, your plugin should implement the ScriptPlugin interface and override the getScriptEngine(Settings settings) method during plugin initialization.

Consider implementing a custom script engine when you need to use advanced internals of scripting, such as scripts that require term frequencies while scoring, or when implementing specialized scripting languages with custom syntax beyond standard Painless capabilities.

The plugin documentation has more information on how to write a plugin so Elasticsearch will properly load it. For the complete ScriptEngine interface reference, refer to the official implementation.

This code creates a custom script engine that allows you to use expert_scripts as the language name and pure_df as the script source in your Elasticsearch queries. The script calculates document scores using term frequency data instead of Elasticsearch standard scoring algorithm.

The following example shows the essential parts of implementing a custom ScriptEngine:

private static class MyExpertScriptEngine implements ScriptEngine {

    // 1. Define your custom language name
    @Override
    public String getType() {
        return "expert_scripts";  // This becomes your "lang" value
    }

    // 2. Define your script source and compilation
    @Override
    public <T> T compile(String scriptName, String scriptSource,
                         ScriptContext<T> context, Map<String, String> params) {
        // This recognizes "pure_df" as your script source
        if ("pure_df".equals(scriptSource)) {
            ScoreScript.Factory factory = new PureDfFactory();
            return context.factoryClazz.cast(factory);
        }
        throw new IllegalArgumentException("Unknown script: " + scriptSource);
    }

    // ... (additional required methods)
}

// 3. Where the actual score calculation happens
private static class ScoreScriptImpl extends ScoreScript {
    @Override
    public double execute(ExplanationHolder explanation) {
        // This is where you define your custom scoring logic
        // In this example: return term frequency as the score
        try {
            return postings.freq();
        } catch (IOException e) {
            return 0.0d;
        }
    }
}

1. Custom score calculation

• Language Definition: The getType() method returns expert_scripts, which becomes the value you use for the lang parameter in your scripts.
• Script Recognition: The compile() method identifies pure_df as a valid script source, which becomes the value you use for the source parameter.
• Custom Scoring: The execute() method replaces Elasticsearch standard scoring with your custom logic. In this case, using term frequency as the document score.

For the complete implementation, refer to the official script engine example.

This example shows how to use your custom script engine in a search query:

POST /_search
{
  "query": {
    "function_score": {
      "query": {
        "match": {
          "body": "foo"
        }
      },
      "functions": [
        {
          "script_score": {
            "script": {
                "source": "pure_df",
                "lang" : "expert_scripts",
                "params": {
                    "field": "body",
                    "term": "foo"
                }
            }
          }
        }
      ]
    }
  }
}