Briefly, this error occurs when Elasticsearch fails to highlight a specific field during a search query. This could be due to incorrect field name, non-textual field, or a large field size. To resolve this, ensure the field name is correct and the field is of a textual data type. If the field is large, consider increasing the highlight.max_analyzed_offset setting or use the ignore_above parameter to limit the size of the field.
This guide will help you check for common problems that cause the log ” Failed to highlight field [” + highlighterContext.fieldName + “] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: search.
Overview
Search refers to the searching of documents in an index or multiple indices. The simple search is just a GET API request to the _search endpoint. The search query can either be provided in query string or through a request body.
Examples
When looking for any documents in this index, if search parameters are not provided, every document is a hit and by default 10 hits will be returned.
GET my_documents/_search
A JSON object is returned in response to a search query. A 200 response code means the request was completed successfully.
{
"took" : 1,
"timed_out" : false,
"_shards" : {
"total" : 2,
"successful" : 2,
"failed" : 0
},
"hits" : {
"total" : 2,
"max_score" : 1.0,
"hits" : [
...
]
}
}Notes and good things to know
- Distributed search is challenging and every shard of the index needs to be searched for hits, and then those hits are combined into a single sorted list as a final result.
- There are two phases of search: the query phase and the fetch phase.
- In the query phase, the query is executed on each shard locally and top hits are returned to the coordinating node. The coordinating node merges the results and creates a global sorted list.
- In the fetch phase, the coordinating node brings the actual documents for those hit IDs and returns them to the requesting client.
- A coordinating node needs enough memory and CPU in order to handle the fetch phase.
Log Context
Log “Failed to highlight field [” + highlighterContext.fieldName + “]” class name is PlainHighlighter.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :
// this can happen if for example a field is not_analyzed and ignore_above option is set.
// the field will be ignored when indexing but the huge term is still in the source and
// the plain highlighter will parse the source and try to analyze it.
return null;
} else {
throw new FetchPhaseExecutionException(context; "Failed to highlight field [" + highlighterContext.fieldName + "]"; e);
}
}
if (field.fieldOptions().scoreOrdered()) {
CollectionUtil.introSort(fragsList; new Comparator() {
@Override
