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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: node, persistent and task.
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.
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%"
}
}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.
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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