How To Solve Issues Related to Log – Un-assigning persistent tasks :

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Updated: Jan-20

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Troubleshooting background

To troubleshoot Elasticsearch log “Un-assigning persistent tasks :” it’s important to understand common problems related to Elasticsearch concepts: persistent, plugin, upgrade. See detailed explanations below complete with common problems, examples and useful tips.

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%"
    }
}

Plugin in Elasticsearch

What it is

A plugin is used to enhance the core functionalities of Elasticsearch. Elasticsearch provides some core plugins as a part of their release installation. In addition to those core plugins, it is possible to write your own custom plugins as well. There are several community plugins available on GitHub for various use cases.

Examples:
  • Get all the instructions for the plugin usage
sudo bin/elasticsearch-plugin -h
  • Installing S3 plugin using URL for storing Elasticsearch snapshots on S3
sudo bin/elasticsearch-plugin install repository-s3
  • Removing a plugin
sudo bin/elasticsearch-plugin remove repository-s3
  • Installing a plugin using the file path
sudo bin/elasticsearch-plugin install file:///path/to/plugin.zip

Notes:
  • Plugins are installed and removed using the elasticsearch-plugin script, which ships as a part of Elasticsearch installation and can be found inside the bin/ directory of the Elasticsearch installation path.
  • A plugin has to be installed on every node of the cluster and each of the nodes has to be restarted to make the plugin visible.
  • You can also download the plugin manually and then install it using the elasticsearch-plugin install command, providing the file name/path of the plugin’s source file.
  • When a plugin is removed, you will need to restart every elasticsearch node in order to complete the removal process.

Common Problems:
  • Managing permission issues during and after plugin installation is the most common problem. If Elasticsearch was installed using the deb or rpm package then the plugin has to be installed using the root user, or else you can install the plugin as the user that owns all of the Elasticsearch files.
  • In case of deb or rpm package installation, it is important to check the permission of the plugins directory after plugin installation and update the permission if it has been modified using the following command:
chown -R elasticsearch:elasticsearch path_to_plugin_directory 
  • If your Elasticsearch nodes are running in a private subnet without internet access, you cannot install a plugin directly. In this case, you can simply download the plugins at once and copy the files inside the plugins directory of the Elasticsearch installation path on every node. The node has to be restarted in this case as well.

Upgrade in Elasticsearch

What is it

Upgrade refers to migrating your Elasticsearch version to a newer version. An upgrade of an existing cluster can be done in two ways: through a rolling upgrade and through a  full cluster restart. The benefit of a rolling upgrade is having zero downtime.

Commos problems and important points
  • The major problem with upgrades is version incompatibility between upgrades. Elasticsearch supports rolling upgrades only between minor versions. You need to make sure to go through the official documentation to see if your cluster can support a rolling upgrade, otherwise a complete reindexing is required.
    Once you upgrade an Elasticsearch node, a rollback can not be done. You need to make sure to take data backups before an upgrade.
  • Elasticsearch continuously removes or deprecates some of the features with every release, so  keep an eye on the change logs of each version before planning an upgrade. 
  • While doing a rolling upgrade, it is important to disable shard allocation before stopping a node and enable the shard allocation when node is upgraded and restarted. This process helps in avoiding unnecessary IO load in the cluster.

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.

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: Unassigned Shards, how to fix? 152 203.11 K 

2 How to stop   75 91.96 K 


Log Context

Log ”Un-assigning persistent tasks :” classname is TransportSetUpgradeModeAction.java
We have extracted the following from Elasticsearch source code to get an in-depth context :

<pre class="wp-block-syntaxhighlighter-code">             // We want to always have the same ordering of which tasks we un-allocate first.
            // However; the order in which the distributed tasks handle the un-allocation event is not guaranteed.
            .sorted(Comparator.comparing(PersistentTask::getTaskName))
            .collect(Collectors.toList());

        logger.info("Un-assigning persistent tasks : " +
            datafeedAndJobTasks.stream().map(PersistentTask::getId).collect(Collectors.joining("; "; "[ "; " ]")));

        TypedChainTaskExecutor<PersistentTask<?>> chainTaskExecutor =
            new TypedChainTaskExecutor<>(client.threadPool().executor(executor());
                r -> true;




</pre>

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