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Briefly, this error occurs when OpenSearch fails to clear the search context on a specific node. This could be due to a network issue, a node failure, or a problem with the search context itself. To resolve this issue, you can try restarting the node, checking the network connection, or increasing the search context lifetime. If the problem persists, consider checking the node’s logs for more detailed error information or reindexing the data.
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This guide will help you check for common problems that cause the log ” Clear SC failed on node[{}] ” to appear. To understand the issues related to this log, read the explanation below about the following OpenSearch concepts: search.
Quick links
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 “Clear SC failed on node[{}]” classname is ClearScrollController.java.
We extracted the following from OpenSearch source code for those seeking an in-depth context :
listener.onResponse(new ClearScrollResponse(succeeded; freedSearchContexts.get()));
}
}
private void onFailedFreedContext(Throwable e; DiscoveryNode node) {
logger.warn(() -> new ParameterizedMessage("Clear SC failed on node[{}]"; node); e);
/*
* We have to set the failure marker before we count down otherwise we can expose the failure marker before we have set it to a
* racing thread successfully freeing a context. This would lead to that thread responding that the clear scroll succeeded.
*/
hasFailed.set(true);
