Briefly, this error occurs when Elasticsearch attempts to clear the search context on a specific node but fails. This could be due to a variety of reasons such as network issues, node unavailability, or insufficient resources. To resolve this issue, you can try the following: 1) Check the health and availability of the node, 2) Ensure the node has sufficient resources (CPU, memory, disk space), 3) Check network connectivity between the nodes, and 4) Review Elasticsearch logs for more detailed error information.
This guide will help you check for common problems that cause the log ” Clear SC failed on node[” + node + “] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: search, node.
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
To put it simply, a node is a single server that is part of a cluster. Each node is assigned one or more roles, which describe the node’s responsibility and operations. Data nodes store the data, and participate in the cluster’s indexing and search capabilities, while master nodes are responsible for managing the cluster’s activities and storing the cluster state, including the metadata.
While it is possible to run several node instances of Elasticsearch on the same hardware, it’s considered a best practice to limit a server to a single running instance of Elasticsearch.
Nodes connect to each other and form a cluster by using a discovery method.
Roles
Master node
Master nodes are in charge of cluster-wide settings and changes – deleting or creating indices and fields, adding or removing nodes and allocating shards to nodes. Each cluster has a single master node that is elected from the master eligible nodes using a distributed consensus algorithm and is reelected if the current master node fails.
Coordinating (client) node
There is some confusion in the use of coordinating node terminology. Client nodes were removed from Elasticsearch after version 2.4 and became coordinating nodes.
Coordinating nodes are nodes that do not hold any configured role. They don’t hold data and are not part of the master eligible group nor execute ingest pipelines. Coordinating nodes serve incoming search requests and act as the query coordinator running query and fetch phases, sending requests to every node that holds a shard being queried. The coordinating node also distributes bulk indexing operations and route queries to shards based on the node’s responsiveness.
Log Context
Log “Clear SC failed on node[” + node + “]” classname is ClearScrollController.java.
We extracted the following from Elasticsearch source code for those seeking an in-depth context :
freedSearchContexts.incrementAndGet();
}
}
private void onFailedFreedContext(Throwable e; DiscoveryNode node) {
logger.warn(() -> "Clear SC failed on node[" + node + "]"; e);
hasFailed.set(true);
}
private void finish() {
listener.onResponse(new ClearScrollResponse(hasFailed.get() == false; freedSearchContexts.get()));
