Briefly, this error occurs when there’s an exception in the Elasticsearch operation. The error message is not specific, it’s a placeholder for the actual error message that the exception e.getMessage() returns. The issue could be due to various reasons such as incorrect query syntax, connection issues, or insufficient resources. To resolve this, you should first check the actual error message returned by e.getMessage(). Then, depending on the actual error, you may need to correct your query syntax, ensure Elasticsearch is running and accessible, or allocate more resources to Elasticsearch if it’s a resource-related issue.
This guide will help you check for common problems that cause the log ” :[” + e.getMessage() + “] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: search, lucene.
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.
Overview
Lucene or Apache Lucene is an open-source Java library used as a search engine. Elasticsearch is built on top of Lucene.
Elasticsearch converts Lucene into a distributed system/search engine for scaling horizontally. Elasticsearch also provides other features like thread-pool, queues, node/cluster monitoring API, data monitoring API, Cluster management, etc. In short, Elasticsearch extends Lucene and provides additional features beyond it.
Elasticsearch hosts data on data nodes. Each data node hosts one or more indices, and each index is divided into shards with each shard holding part of the index’s data. Each shard created in Elasticsearch is a separate Lucene instance or process.
Notes and good things to know
When an index is created in Elasticsearch, it is divided into one or more primary shards for scaling the data and splitting it into multiple nodes/instances.
- As each shard is a separate instance of Lucene, creating too many shards will consume unnecessary resources and damage performance.
It takes proper planning to decide the number of primary shards for your index, taking into account the index size, max growth, and the number of data nodes.
- Previous versions of Elasticsearch defaulted to creating five shards per index. Starting with 7.0.0, the default is now one shard per index.
Log Context
Log “:[” + e.getMessage() + “]” class name is Queries.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :
public static Query newUnmappedFieldsQuery(Collectionfields) { return Queries.newMatchNoDocsQuery("unmapped fields " + fields); } public static Query newLenientFieldQuery(String field; RuntimeException e) { String message = ElasticsearchException.getExceptionName(e) + ":[" + e.getMessage() + "]"; return Queries.newMatchNoDocsQuery("failed [" + field + "] query; caused by " + message); } private static final IndexVersion NESTED_DOCS_IDENTIFIED_VIA_PRIMARY_TERMS_VERSION = IndexVersion.fromId(6010099);
