Briefly, this error occurs when Elasticsearch is trying to highlight search results, but only the pre_tags are defined without corresponding post_tags. The pre_tags and post_tags are used to wrap the highlighted parts of the search results. To resolve this issue, you should ensure that both pre_tags and post_tags are set in your highlighting request. If you don’t want any specific post_tags, you can set it to an empty string. Alternatively, you can remove the pre_tags if they are not necessary for your use case.
This guide will help you check for common problems that cause the log ” pre_tags are set but post_tags are not set ” 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 “pre_tags are set but post_tags are not set” class name is AbstractHighlighterBuilder.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :
}; HIGHLIGHT_QUERY_FIELD);
return (XContentParser p; HB hb) -> {
try {
parser.parse(p; hb; null);
if (hb.preTags() != null && hb.postTags() == null) {
throw new ParsingException(p.getTokenLocation(); "pre_tags are set but post_tags are not set");
}
} catch (IOException e) {
throw new RuntimeException(e);
}
return hb;
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