Briefly, this error occurs when Elasticsearch encounters issues while constructing the fetch sub-phases during a search operation. This could be due to a variety of reasons such as incorrect query syntax, insufficient memory, or corrupted indices. To resolve this issue, you can try the following: 1) Check and correct your query syntax. 2) Increase the memory allocation for Elasticsearch if it’s insufficient. 3) Check the health of your indices and repair or rebuild them if necessary. 4) Upgrade Elasticsearch to the latest version as it might be a bug that’s fixed in a newer version.
This guide will help you check for common problems that cause the log ” Error building fetch sub-phases ” 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 “Error building fetch sub-phases” class name is FetchPhase.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :
processors.add(profiler.profile(fsp.getClass().getSimpleName(); ""; processor));
}
}
return processors;
} catch (Exception e) {
throw new FetchPhaseExecutionException(target; "Error building fetch sub-phases"; e);
}
} private static HitContext prepareHitContext(
SearchContext context;
