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Troubleshooting background
To troubleshoot Elasticsearch log “Clear SC failed on node” it’s important to know common problems related to Elasticsearch concepts: scroll, search. See below-detailed explanations complete with common problems, examples and useful tips.
Scroll in Elasticsearch
What it is
In Elasticsearch, the concept of scroll comes into play when you have a large set of search results. Large search results are exhaustive for both the Elasticsearch cluster and the requesting client in terms of memory and processing. The scroll API enables you to take a snapshot of a large number of results from a single search request.
Examples
To perform a scroll search, you need to add the scroll parameter to search query and specify how long Elasticsearch should keep the search context viable.
GET mydocs-2019/_search?scroll=40s
{
"size": 5000,
"query": {
"match_all": {}
},
"sort": [
{
"_doc": {
"order": "asc"
}
}
]
}
This query will return a maximum of 5000 hits and if the scroll is idle for more than 40 seconds, then it will be deleted. The response will return the first page of the results and a scroll id, you can use the scroll id to get additional documents from the scroll. You’re able to keep retrieving the documents until you have all of them.
Notes
- Changes made to documents after the scroll will not show up in your results.
- When you are done with the scroll, you can delete it manually using the scroll id.
DELETE _search/scroll/<sroll_id>
Search in Elasticsearch
What it is
Search refers to the searching of documents in an index or multiple indices. The simple search is just a GET request to _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 will mean the request completed successfully.
{
"took" : 1,
"timed_out" : false,
"_shards" : {
"total" : 2,
"successful" : 2,
"failed" : 0
},
"hits" : {
"total" : 2,
"max_score" : 1.0,
"hits" : [
...
]
}
}
Notes
- 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.
Nodes in Elasticsearch
What it is
Simply explained a node is a single server that is part of a cluster. Each node is assigned with one or more roles, which describes the node responsibility and operations – Data nodes stores the data, and participates in the cluster’s indexing and search capabilities, while master nodes are responsible for managing the cluster activities and storing the cluster state, including the metadata.
While it’s 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.
Coordinator Node (aka client node)
Coordinator Node – is a node that does not hold any configured role. It doesn’t hold data, not part of the master eligible group nor execute ingest pipelines. Coordinator node serves incoming search requests and is acting as the query coordinator – running the query and fetch phases, sending requests to every node which holds a shard being queried. The client node also distributes bulk indexing operations and route queries to shards copies based on the nodes responsiveness.
Related issues to this log
To help troubleshoot related issues we have gathered selected Q&A from the community and issues from Github , please review the following for further information :
1. elasticsearch – remove a second elasticsearch node and add an other node, get unassigned shards – Stats : ♥ 0.93 K Ι √ –
2. Remove bad node from ElasticSearch cluster – Stats : ♥ 0.47 K Ι √ 1
How to remove node from elasticsearch cluster on runtime without down time
Log Context
Log ”Clear SC failed on node” classname is ClearScrollController.java
We have extracted the following from Elasticsearch source code to get 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);
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