Painless Debugging
editPainless Debugging
editDebug.Explain
editPainless doesn’t have a
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 _explain
to explore the
context available to a script query.
PUT /hockey/_doc/1?refresh {"first":"johnny","last":"gaudreau","goals":[9,27,1],"assists":[17,46,0],"gp":[26,82,1]} POST /hockey/_explain/1 { "query": { "script": { "script": "Debug.explain(doc.goals)" } } }
Which shows that the class of doc.first
is
org.elasticsearch.index.fielddata.ScriptDocValues.Longs
by responding with:
{ "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 }
You can use the same trick to see that _source
is a LinkedHashMap
in the _update
API:
POST /hockey/_update/1 { "script": "Debug.explain(ctx._source)" }
The response looks like:
{ "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 }
Once you have a class you can go to Painless API Reference to see a list of available methods.