[[modules-scripting-engine]] === 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 {plugins}/plugin-authors.html[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. ["source","java",subs="attributes,callouts,macros"] -------------------------------------------------- include-tagged::{plugins-examples-dir}/script-expert-scoring/src/main/java/org/elasticsearch/example/expertscript/ExpertScriptPlugin.java[expert_engine] -------------------------------------------------- You can execute the script by specifying its `lang` as `expert_scripts`, and the name of the script as the the script source: [source,js] -------------------------------------------------- POST /_search { "query": { "function_score": { "query": { "match": { "body": "foo" } }, "functions": [ { "script_score": { "script": { "source": "pure_df", "lang" : "expert_scripts", "params": { "field": "body", "term": "foo" } } } } ] } } } -------------------------------------------------- // CONSOLE // TEST[skip:we don't have an expert script plugin installed to test this]