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This guide will help you check for common problems that cause the log ” Un-assigning persistent tasks : ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: persistent, plugin and upgrade.
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 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 of the instructions for the plugin:
sudo bin/elasticsearch-plugin -h
Installing the S3 plugin 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’s path:
sudo bin/elasticsearch-plugin install file:///path/to/plugin.zip
Notes and good things to know
- Plugins are installed and removed using the elasticsearch-plugin script, which ships as a part of the 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 issues
- Managing permission issues during and after plugin installation is the most common problem. If Elasticsearch was installed using the DEB or RPM packages then the plugin has to be installed using the root user. Otherwise you can install the plugin as the user that owns all of the Elasticsearch files.
- In the case of DEB or RPM package installation, it is important to check the permissions of the plugins directory after you install it. You can 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 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.
Overview
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.
Common problems and important points
- The major problem with upgrades is version incompatibility. 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 cannot be done. You need to make sure to backup your data before an upgrade.
- Elasticsearch continuously removes or deprecates some of its 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.
How to easily upgrade versions
To upgrade Elasticsearch versions without risking data loss, you can use Opster’s Multi-Cluster Load Balancer. The MCLB enables version upgrades with no downtime and upgrading from significantly older versions to new ones (such as bringing your Elasticsearch from v1 to v7 for instance).
With Opster’s Load Balancer, you can mirror data to newer versions of Elasticsearch without disabling the older version. This allows you to improve and adjust data structure and application API calls until all of the data in the new version is present and operational, while staying available the entire time.
You can learn more about the Multi-Cluster Load Balancer here, and book a demo to try it out.
Log Context
Log “Un-assigning persistent tasks :” classname is TransportSetUpgradeModeAction.java.
We extracted the following from Elasticsearch source code for those seeking an in-depth context :
// 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> chainTaskExecutor =
new TypedChainTaskExecutor(client.threadPool().executor(executor());
r -> true;
