[[modules-scripting-painless-debugging]]
=== Painless Debugging

experimental[The Painless scripting language is new and is still marked as experimental. The syntax or API may be changed in the future in non-backwards compatible ways if required.]

==== Debug.Explain

Painless doesn't have a
https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop[REPL]
and while it'd be nice for it to have one one day, it wouldn't tell you the
whole story around debugging painless scripts embedded in Elasticsearch because
the data that the scripts have access to or "context" is so important. For now
the best way to debug embedded scripts is by throwing exceptions at choice
places. While you can throw your own exceptions
(`throw new Exception('whatever')`), Painless's sandbox prevents you from
accessing useful information like the type of an object. So Painless has a
utility method, `Debug.explain` which throws the exception for you. For
example, you can use the <<search-explain>> to explore the context available to
a <<query-dsl-script-query>>.

[source,js]
---------------------------------------------------------
PUT /hockey/player/1?refresh
{"first":"johnny","last":"gaudreau","goals":[9,27,1],"assists":[17,46,0],"gp":[26,82,1]}

POST /hockey/player/1/_explain
{
  "query": {
    "script": {
      "script": "Debug.explain(doc.goals)"
    }
  }
}
---------------------------------------------------------
// CONSOLE
// TEST[s/_explain/_explain?error_trace=false/ catch:/painless_explain_error/]
// The test system sends error_trace=true by default for easier debugging so
// we have to override it to get a normal shaped response

Which shows that the class of `doc.first` is
`org.elasticsearch.index.fielddata.ScriptDocValues$Longs` by responding with:

[source,js]
---------------------------------------------------------
{
   "error": {
      "type": "script_exception",
      "class": "org.elasticsearch.index.fielddata.ScriptDocValues$Longs",
      "to_string": "[1, 9, 27]",
      ...
   },
   "status": 500
}
---------------------------------------------------------
// TESTRESPONSE[s/\.\.\./"script_stack": $body.error.script_stack, "script": $body.error.script, "lang": $body.error.lang, "caused_by": $body.error.caused_by, "root_cause": $body.error.root_cause, "reason": $body.error.reason/]

You can use the same trick to see that `_source` is a `java.util.LinkedHashMap`
in the `_update` API:

[source,js]
---------------------------------------------------------
POST /hockey/player/1/_update
{
  "script": "Debug.explain(ctx._source)"
}
---------------------------------------------------------
// CONSOLE
// TEST[continued s/_update/_update?error_trace=false/ catch:/painless_explain_error/]

The response looks like:

[source,js]
---------------------------------------------------------
{
  "error" : {
    "root_cause": ...,
    "type": "illegal_argument_exception",
    "reason": "failed to execute script",
    "caused_by": {
      "type": "script_exception",
      "class": "java.util.LinkedHashMap",
      "to_string": "{gp=[26, 82, 1], last=gaudreau, assists=[17, 46, 0], first=johnny, goals=[9, 27, 1]}",
      ...
    }
  },
  "status": 400
}
---------------------------------------------------------
// TESTRESPONSE[s/"root_cause": \.\.\./"root_cause": $body.error.root_cause/]
// TESTRESPONSE[s/\.\.\./"script_stack": $body.error.caused_by.script_stack, "script": $body.error.caused_by.script, "lang": $body.error.caused_by.lang, "caused_by": $body.error.caused_by.caused_by, "reason": $body.error.caused_by.reason/]
// TESTRESPONSE[s/"to_string": ".+"/"to_string": $body.error.caused_by.to_string/]

// TODO we should build some javadoc like mashup so people don't have to jump through these hoops.

Once you have the class of an object you can go
https://github.com/elastic/elasticsearch/tree/{branch}/modules/lang-painless/src/main/resources/org/elasticsearch/painless[here]
and check the available methods. Painless uses a strict whitelist to prevent
scripts that don't work well with Elasticsearch and all whitelisted methods
are listed in a file named after the package of the object (everything before
the last `.`). So `java.util.Map` is listed in a file named `java.util.txt`
starting on the line that looks like `class Map -> java.util.Map {`.

With the list of whitelisted methods in hand you can turn to either
https://docs.oracle.com/javase/8/docs/api/[Javadoc],
https://github.com/elastic/elasticsearch/tree/{branch}[Elasticsearch's source tree]
or, for whitelisted methods ending in `*`, the
https://github.com/elastic/elasticsearch/blob/{branch}/modules/lang-painless/src/main/java/org/elasticsearch/painless/Augmentation.java[Augmentation]
class.