Before you begin reading this guide, we recommend you try running the Elasticsearch Error Check-Up which can resolve issues that cause many errors.
This guide will help you check for common problems that cause the log ” Cannot start plugin due to incorrect Lucene version plugin . node . ” to appear. It’s important to understand the issues related to the log, so to get started, read the general overview on common issues and tips related to the Elasticsearch concepts: lucene, node, plugin and plugins.
Advanced users might want to skip right to the common problems section in each concept or try running the Check-Up which analyses ES to pinpoint the cause of many errors and provides suitable actionable recommendations how to resolve them (free tool that requires no installation).
Lucene
What it is:
Lucene or Apache Lucene is an open-source Java library used as a search engine. Elasticsearch is built on top of Lucene.
Elasticsearch converts Lucene into a distributed system/search engine for scaling horizontally. Elasticsearch also provides other features like thread-pool, queues, node/cluster monitoring API, data monitoring API, Cluster management, etc. In short, Elasticsearch extends Lucene and provides additional features beyond it.
Elasticsearch hosts data on data nodes. Each data node hosts one or more indices, and each index is divided into shards with each shard holding part of the index’s data. Each shard created in Elasticsearch is a separate Lucene instance or process.
Notes and good things to know:
When an index is created in ElasticSearch, it is divided into one or more primary shards for scaling the data and splitting it into multiple nodes/instances.
- As each shard is a separate instance of Lucene, creating too many shards will consume unnecessary resources and damage performance.
It takes proper planning to decide the number of primary shards for your index, taking into account the index size, max growth, and the number of data nodes.
Previous versions of Elasticsearch defaulted to creating five shards per index. Starting with 7.0.0, the default is now one shard per index.
Nodes in Elasticsearch
What it is:
Simply put 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 stores the data, and participates 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.
Coordinator or client node:
Coordinator Nodes are nodes that do not hold any configured role. They don’t hold data, are not part of the master eligible group nor execute ingest pipelines. Coordinator nodes serve incoming search requests and act as the query coordinator running query and fetch phases, send requests to every node that holds a shard being queried. The client node also distributes bulk indexing operations and route queries to shards based on the node’s responsiveness.
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
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:
- 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 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.
Log context
Log “cannot start plugin due to incorrect Lucene version: plugin [{}]; node [{}].” classname is PluginsService.java
We extracted the following from Elasticsearch source code for those seeking an in-depth context :
}
} catch (Throwable t) {
// We don't have the expected version... Let's fail after.
logger.debug("exception raised while checking plugin Lucene version."; t);
}
logger.error("cannot start plugin due to incorrect Lucene version: plugin [{}]; node [{}].";
luceneVersion; Constants.LUCENE_MAIN_VERSION);
return false;
}
}
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