Briefly, this error occurs when Elasticsearch fails to initiate a scheduled task due to issues like insufficient resources, incorrect task configurations, or internal server errors. To resolve this, you can try increasing system resources, checking and correcting task configurations, or restarting the Elasticsearch service. If the problem persists, consider checking the server logs for more specific error details.
This guide will help you check for common problems that cause the log ” Unable to start allocated task [ ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: task, persistent.
Overview
A task is an Elasticsearch operation, which can be any request performed on an Elasticsearch cluster, such as 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
Where 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 task
Where 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.
Overview
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 the 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%"
}
}Log Context
Log “Unable to start allocated task [” classname is PersistentTasksNodeService.java.
We extracted the following from Elasticsearch source code for those seeking an in-depth context :
if (persistentTask == null) {
// New task - let's start it
try {
startTask(taskInProgress);
} catch (Exception e) {
logger.error("Unable to start allocated task [" + taskInProgress.getTaskName()
+ "] with id [" + taskInProgress.getId()
+ "] and allocation id [" + taskInProgress.getAllocationId() + "]"; e);
}
} else {
// The task is still running
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