Before you begin reading this guide, we recommend you run Elasticsearch Error Check-Up which analyzes 2 JSON files to detect many errors.
To easily locate the root cause and resolve this issue try AutoOps for Elasticsearch & OpenSearch. It diagnoses problems by analyzing hundreds of metrics collected by a lightweight agent and offers guidance for resolving them.
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. The process of updating distributed systems like Elasticsearch can be intricate, given the extensive data quantities, the involvement of numerous nodes, and the diverse configurations that may exist within your cluster.
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.
Bear in mind that any changes to your system could lead to data loss if the instructions are not adhered to accurately. Thoroughly test and strategize your upgrade, and ensure you create a backup of your data prior to executing any updates.
For guides on how to upgrade specific versions, see:
- How to Upgrade Elasticsearch from Version 5 to Version 6
- How to Upgrade Elasticsearch from Version 6 to Version 7
- How to Upgrade Elasticsearch from Version 7 to Version 8
What should I check before upgrading versions?
Elasticsearch nodes cannot be downgraded after upgrading. Before starting the upgrade process you should:
- Check the deprecation log and resolve any issues.
- Review the breaking changes to make sure you know of any functionalities which may changes or disappear. This would mainly affect node configuration, index mappings and templates, and cluster settings.
- Check the ES plugin compatibility to ensure they are compatible with the new version.
- Set up a test environment to test the upgrade process in a testing or staging environment first before upgrading your production cluster to avoid any issues.
- Take a backup and snapshots from your data, as the only way to “reverse” a failed upgrade is to create a new cluster with the old version and restore the data from snapshots.
How to perform offline upgrades – full cluster restart upgrades
A complete cluster restart upgrade involves simultaneously stopping all Elasticsearch nodes, updating them, and subsequently restarting each one. Inevitably, this upgrade approach will necessitate the downtime of your Elasticsearch cluster throughout the entire process.
Generally, offline upgrades are simpler than online ones because there’s no need to handle a cluster with varying node versions concurrently.
The steps are:
- Disable shard allocation
- Stop all Elasticsearch nodes and upgrade them
- Upgrade any plugins
- Start the Elasticsearch cluster
- Re-enable shard allocation
- Upgrade client libraries to new version
- Restart master eligible nodes
- Restart non-master eligible nodes
Keep in mind that during a full cluster restart, the master nodes need to be initiated prior to the non-master nodes. This is essential for allowing the master nodes to establish the cluster so that other nodes can join, which is in contrast to a rolling upstart where non-master nodes should be upgraded before the master nodes.
How to perform online upgrades – rolling restart upgrades
A rolling restart upgrade allows for updating a cluster without incurring any downtime. In this scenario, every node is sequentially upgraded and rebooted, without ever halting the entire Elasticsearch cluster.
Rolling restart upgrades cannot be performed when there is a change in MAJOR versions, except for specific exceptions:
- Upgrading Elasticsearch version 5.6.16 to version 6.x.x
- Upgrading Elasticsearch version 6.8.23 to version 7.x.x
- Upgrading Elasticsearch version 7.17.5 to version 8.x.x
For this reason, when performing a rolling restart upgrade between major versions, it is imperative to ALWAYS utilize the most recent minor version as an intermediary step for upgrading to the subsequent major version. For instance, if you are operating Elasticsearch 5.x.x, you can first update to 5.6.16 and then proceed to 6.8.23.
How to upgrade nodes in a rolling upgrade
The process for upgrading your nodes is as follows, upgrading all NON master-eligible nodes first.
- Make sure your cluster status is green and stable
Ensure that all replicas are available so that shutting down the node will not cause data loss.
- Disable unnecessary indexing
Wherever possible, you should stop all indexing processes to increase the cluster’s stability.
- Disable shard allocation
It is important to disable shard allocation so that when you stop a node for upgrade the cluster does not reallocate shards to another node. (See command below).
- Stop Elasticsearch
Stop Elasticsearch before moving on to the next step.
- Upgrade Elasticsearch
The method used to upgrade will depend upon your installation method.
- Upgrade plugins
Elasticsearch will not start if the plugin is not the same version as Elasticsearch.
- Start Elasticsearch
Start Elasticsearch before moving on to the next step.
- Re-enable shard allocation
Using the command given below.
- Check that the upgraded node has rejoined the cluster
Using the command below, you can check how many nodes are in the cluster.
- Wait for cluster status to turn green
The command provided below will also show you the progress of the shard recovery process on the upgraded node, until the cluster reaches a green state.
- Repeat
Repeat the full process above for each node.
To disable shard allocation, run:
PUT _cluster/settings
{
"persistent": {
"cluster.routing.allocation.enable": "primaries"
}
}To re-enable shard allocation, run:
PUT _cluster/settings
{
"persistent": {
"cluster.routing.allocation.enable": null
}
}Get cluster status and see how many nodes are in the cluster using:
GET _cluster/health
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.
Easily upgrade versions with OMC
To upgrade Elasticsearch versions without risking data loss, you can use the free Opster Management Console. The OMC enables version upgrades with no downtime and upgrading from significantly older versions to new ones at the click of a button, completely for free.
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;
