Advanced scripts using script engines

A ScriptEngine is a backend for implementing a scripting language. It may also be used to write scripts that need to use advanced internals of scripting. For example, a script that wants to use term frequencies while scoring.

The plugin documentation has more information on how to write a plugin so that Elasticsearch will properly load it. To register the ScriptEngine, your plugin should implement the ScriptPlugin interface and override the getScriptEngine(Settings settings) method.

The following is an example of a custom ScriptEngine which uses the language name expert_scripts. It implements a single script called pure_df which may be used as a search script to override each document’s score as the document frequency of a provided term.

private static class MyExpertScriptEngine implements ScriptEngine {
    @Override
    public String getType() {
        return "expert_scripts";
    }

    @Override
    public <T> T compile(String scriptName, String scriptSource,
            ScriptContext<T> context, Map<String, String> params) {
        if (context.equals(ScoreScript.CONTEXT) == false) {
            throw new IllegalArgumentException(getType()
                    + " scripts cannot be used for context ["
                    + context.name + "]");
        }
        // we use the script "source" as the script identifier
        if ("pure_df".equals(scriptSource)) {
            ScoreScript.Factory factory = PureDfLeafFactory::new;
            return context.factoryClazz.cast(factory);
        }
        throw new IllegalArgumentException("Unknown script name "
                + scriptSource);
    }

    @Override
    public void close() {
        // optionally close resources
    }

    private static class PureDfLeafFactory implements LeafFactory {
        private final Map<String, Object> params;
        private final SearchLookup lookup;
        private final String field;
        private final String term;

        private PureDfLeafFactory(
                    Map<String, Object> params, SearchLookup lookup) {
            if (params.containsKey("field") == false) {
                throw new IllegalArgumentException(
                        "Missing parameter [field]");
            }
            if (params.containsKey("term") == false) {
                throw new IllegalArgumentException(
                        "Missing parameter [term]");
            }
            this.params = params;
            this.lookup = lookup;
            field = params.get("field").toString();
            term = params.get("term").toString();
        }

        @Override
        public boolean needs_score() {
            return false;  // Return true if the script needs the score
        }

        @Override
        public ScoreScript newInstance(LeafReaderContext context)
                throws IOException {
            PostingsEnum postings = context.reader().postings(
                    new Term(field, term));
            if (postings == null) {
                /*
                 * the field and/or term don't exist in this segment,
                 * so always return 0
                 */
                return new ScoreScript(params, lookup, context) {
                    @Override
                    public double execute(ExplanationHolder explanation) {
                        return 0.0d;
                    }
                };
            }
            return new ScoreScript(params, lookup, context) {
                int currentDocid = -1;
                @Override
                public void setDocument(int docid) {
                    /*
                     * advance has undefined behavior calling with
                     * a docid <= its current docid
                     */
                    if (postings.docID() < docid) {
                        try {
                            postings.advance(docid);
                        } catch (IOException e) {
                            throw new UncheckedIOException(e);
                        }
                    }
                    currentDocid = docid;
                }
                @Override
                public double execute(ExplanationHolder explanation) {
                    if (postings.docID() != currentDocid) {
                        /*
                         * advance moved past the current doc, so this doc
                         * has no occurrences of the term
                         */
                        return 0.0d;
                    }
                    try {
                        return postings.freq();
                    } catch (IOException e) {
                        throw new UncheckedIOException(e);
                    }
                }
            };
        }
    }
}

You can execute the script by specifying its lang as expert_scripts, and the name of the script as the script source:

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