Multicast socket closed while running; restarting. – How to solve related issues

Overview

The process known as discovery occurs when an Elasticsearch node starts, restarts or loses contact with the master node for any reason. In those cases, the node needs to contact other nodes in the cluster to find any existing master node or initiate the election of a new master node. 

How it works

Upon startup, each node looks for other nodes, firstly by contacting the IP addresses of eligible master nodes held in the previous cluster state.  If they are not available, it will look for nodes based upon the seed host provider mechanisms available.

Seed host providers may be defined in 3 ways: list based, file based or plugin based. All of these methods provide a list of IP addresses or hostnames which the node should contact in order to obtain a list of master eligible nodes. The node will contact all of these addresses in turn, until either an active master is found, or failing that, until sufficient nodes can be found to elect a new master node.

Examples

The simplest form is to define a list of seed host providers in elasticsearch.yml:

discovery.seed_hosts:
   - 192.168.1.10:9300
   - 192.168.1.11 
   - seeds.mydomain.com

An alternative way is to refer to a file using the following setting:

discovery.seed_providers: file

The file MUST be placed in the following filepath: $ES_PATH_CONF/unicast_hosts.txt

10.10.10.5
10.10.10.6:9305
10.10.10.5:10005
# an IPv6 address
[2001:0db8:85a3:0000:0000:8a2e:0370:7334]:9301

Note that the use of a port is optional. If not used, then the default port range of 9300-9400 will be used.

If you use AWS or GCS then you can install and use a plugin to obtain a list of seed hosts from an API.  A plugin also exists for Azure but is deprecated since version 5.

AWS plugin

A typical configuration could be as follows:

discovery.seed_providers: ec2
discovery.ec2.tag.role: master
discovery.ec2.tag.environment: dev
discovery.ec2.endpoint: ec2.us-east-1.amazonaws.com
cloud.node.auto_attributes: true
cluster.routing.allocation.awareness.attributes: aws_availability_zone

The above configuration would look for all nodes with a tag called “environment” set to “dev” and a tag called “role” set to “master”, in the AWS zone us-east-1. The last two lines set up cluster routing allocation awareness based upon aws availability zones. (Not necessary, but nice to have).

GCE plugin

A typical configuration could be as follows:

discovery.seed_providers: gce
cloud.gce.project_id: <your-google-project-id>
cloud.gce.zone: <your-zone>
discovery.gce.tags: <my-tag-name>

The above configuration would look for all virtual machines inside your project, zone and with a tag set to the tag name you provide.

Notes and good things to know

Cluster formation depends on correct setup of the network.host settings in elasticsearch.yml.  Make sure that the nodes can reach each other across the network using their IP addresses / hostname, and are not getting blocked due to firewall settings on the ports required.

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.