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

==== 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 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 {ref}/search-explain.html[`_explain`] to explore the
context available to a {ref}/query-dsl-script-query.html[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",
      "to_string": "[1, 9, 27]",
      "painless_class": "org.elasticsearch.index.fielddata.ScriptDocValues.Longs",
      "java_class": "org.elasticsearch.index.fielddata.ScriptDocValues$Longs",
      ...
   },
   "status": 400
}
---------------------------------------------------------
// 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 `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",
      "to_string": "{gp=[26, 82, 1], last=gaudreau, assists=[17, 46, 0], first=johnny, goals=[9, 27, 1]}",
      "painless_class": "java.util.LinkedHashMap",
      "java_class": "java.util.LinkedHashMap",
      ...
    }
  },
  "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/]

Once you have a class you can go to <<painless-api-reference>> to see a list of
available methods.