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Elasticsearch Error Guide In Page Navigation :
Troubleshooting Background – start here to get the full picture
Related Issues – selected resources on related issues
Log Context – usefull for experts
About Opster – offering a diffrent approach to troubleshoot Elasticsearch
Troubleshooting background
To troubleshoot Elasticsearch log “Failed to reassign persistent tasks” it’s important to know common problems related to Elasticsearch concepts: cluster, persistent. See below-detailed explanations complete with common problems, examples and useful tips.
Cluster in Elasticsearch
What is it
In Elasticsearch a cluster is a collection of one or more nodes (servers / VMs). A cluster can consist of an unlimited number of nodes. The cluster provides interface for indexing and storing data and search capability across all of the data which is stored in the data nodes
Each cluster has a single master node that is elected by the master eligible nodes. In cases where the master is not available the other connected master eligible nodes elect a new master. Clusters are identified by a unique name, which defaults to “Elasticsearch”.
Persistent in Elasticsearch
What it is
In Elasticsearch, Persistent refers to cluster settings that persist across cluster restarts. This setting is used in Cluster Update API calls. Persistent settings can also be configured in elasticsearch.yml file.
Examples
## enable shard routing
PUT /_cluster/settings
{
"persistent" : {
"cluster.routing.allocation.enable" : "all"
}
}
## enable rebalancing of shards
PUT /_cluster/settings
{
"persistent" : {
"cluster.routing.rebalance.enable" : "all"
}
}
## limit the heap size for fielddata
PUT /_cluster/settings
{
"persistent" : {
“indices.breaker.fielddata.limit”: "30%"
}
}
Task in Elasticsearch
What it is
A task is equivalent to an Elasticsearch operation, which can be any request performed on an Elasticsearch cluster. For example, a delete by query request, a search request and so on. Elasticsearch provides a dedicated Task API for the task management which includes various actions, from retrieving the status of current running tasks to canceling any long running task.
Examples
Get all currently running tasks on all nodes of the cluster
Apart from other information, the response of the below request contains task IDs of all the tasks which can be used to get detailed information about the particular task in question.
GET _tasks
GET detailed information of a particular task
clQFAL_VRrmnlRyPsu_p8A:1132678759 is the ID of the task in below request.
GET _tasks/clQFAL_VRrmnlRyPsu_p8A:1132678759
Get all the current tasks running on particular nodes
GET _tasks?nodes=nodeId1,nodeId2
Cancel a long-running task
clQFAL_VRrmnlRyPsu_p8A:1132678759 is the ID of the task in the below request.
POST /_tasks/clQFAL_VRrmnlRyPsu_p8A:1132678759/_cancel?pretty
Notes
- The Task API will be most useful when you want to investigate the spike of resource utilization in the cluster or want to cancel an operation.
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 :
Github Issue Number 35792
github.com/elastic/elasticsearch/issues/35792
Github Issue Number 37545
github.com/elastic/elasticsearch/issues/37545
Log Context
Log ”Failed to reassign persistent tasks” classname is PersistentTasksClusterService.java
We have extracted the following from Elasticsearch source code to get an in-depth context :
return reassignTasks(currentState);
}
Override
public void onFailure(String source; Exception e) {
logger.warn("failed to reassign persistent tasks"; e);
// There must be a task that's worth rechecking because there was one
// that caused this method to be called and the method failed to assign it
periodicRechecker.rescheduleIfNecessary();
}
About Opster
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We are constantly updating our analysis of Elasticsearch logs, errors, and exceptions. Sharing best practices and providing troubleshooting guides.
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