File-Based Discovery Plugin

The file-based discovery plugin uses a list of hosts/ports in a unicast_hosts.txt file in the config/discovery-file directory for unicast discovery.

This plugin can be installed using the plugin manager:

sudo bin/elasticsearch-plugin install discovery-file

The plugin must be installed on every node in the cluster, and each node must be restarted after installation.

This plugin can be downloaded for offline install from https://artifacts.elastic.co/downloads/elasticsearch-plugins/discovery-file/discovery-file-5.6.16.zip.

The plugin can be removed with the following command:

sudo bin/elasticsearch-plugin remove discovery-file

The node must be stopped before removing the plugin.

The file-based discovery plugin provides the ability to specify the unicast hosts list through a simple unicast_hosts.txt file that can be dynamically updated at any time. To enable, add the following in elasticsearch.yml:

discovery.zen.hosts_provider: file

This plugin simply provides a facility to supply the unicast hosts list for zen discovery through an external file that can be updated at any time by a side process.

For example, this gives a convenient mechanism for an Elasticsearch instance that is run in docker containers to be dynamically supplied a list of IP addresses to connect to for zen discovery when those IP addresses may not be known at node startup.

Note that the file-based discovery plugin is meant to augment the unicast hosts list in elasticsearch.yml (if specified), not replace it. Therefore, if there are valid unicast host entries in discovery.zen.ping.unicast.hosts, they will be used in addition to those supplied in unicast_hosts.txt.

Anytime a change is made to the unicast_hosts.txt file, even as Elasticsearch continues to run, the new changes will be picked up by the plugin and the new hosts list will be used for the next pinging round for master election.

Upon installation of the plugin, a default unicast_hosts.txt file will be found in the $CONFIG_DIR/discovery-file directory. This default file will contain some comments about what the file should contain. All comments for this file must appear on their lines starting with # (i.e. comments cannot start in the middle of a line).

The format of the file is to specify one unicast host entry per line. Each unicast host entry consists of the host (host name or IP address) and an optional transport port number. If the port number is specified, is must come immediately after the host (on the same line) separated by a :. If the port number is not specified, a default value of 9300 is used.

For example, this is an example of unicast_hosts.txt for a cluster with four nodes that participate in unicast discovery, some of which are not running on the default port:

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

Host names are allowed instead of IP addresses (similar to discovery.zen.ping.unicast.hosts), and IPv6 addresses must be specified in brackets with the port coming after the brackets.