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@@ -0,0 +1,93 @@
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+[[discovery-file]]
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+=== File-Based Discovery Plugin
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+
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+The file-based discovery plugin uses a list of hosts/ports in a `unicast_hosts.txt` file
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+in the `config/discovery-file` directory for unicast discovery.
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+
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+[[discovery-file-install]]
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+[float]
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+==== Installation
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+
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+This plugin can be installed using the plugin manager:
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+
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+[source,sh]
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+----------------------------------------------------------------
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+sudo bin/elasticsearch-plugin install discovery-file
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+----------------------------------------------------------------
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+
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+The plugin must be installed on every node in the cluster, and each node must
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+be restarted after installation. Note that installing the plugin will add a
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+`discovery-file` directory to the `config` folder, and a default `unicast_hosts.txt`
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+file that must be edited with the correct unicast hosts list before starting the node.
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+
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+[[discovery-file-remove]]
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+[float]
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+==== Removal
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+
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+The plugin can be removed with the following command:
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+
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+[source,sh]
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+----------------------------------------------------------------
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+sudo bin/elasticsearch-plugin remove discovery-file
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+----------------------------------------------------------------
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+
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+The node must be stopped before removing the plugin.
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+
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+[[discovery-file-usage]]
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+[float]
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+==== Using the file-based discovery plugin
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+
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+The file-based discovery plugin provides the ability to specify the
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+unicast hosts list through a simple `unicast_hosts.txt` file that can
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+be dynamically updated at any time. The discovery type for this plugin
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+is still the default `zen` plugin, so no changes are required to the
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+`elasticsearch.yml` config file. This plugin simply provides a facility
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+to supply the unicast hosts list for zen discovery through an external
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+file that can be updated at any time by a side process.
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+
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+For example, this gives a convenient mechanism for an Elasticsearch instance
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+that is run in docker containers to be dynamically supplied a list of IP
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+addresses to connect to for zen discovery when those IP addresses may not be
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+known at node startup.
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+
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+Note that the file-based discovery plugin is meant to augment the unicast
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+hosts list in `elasticsearch.yml` (if specified), not replace it. Therefore,
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+if there are valid unicast host entries in `discovery.zen.ping.unicast.hosts`,
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+they will be used in addition to those supplied in `unicast_hosts.txt`.
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+
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+Anytime a change is made to the `unicast_hosts.txt` file, even as Elasticsearch
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+continues to run, the new changes will be picked up by the plugin and the
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+new hosts list will be used for the next pinging round for master election.
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+
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+Upon installation of the plugin, a default `unicast_hosts.txt` file will
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+be found in the `$CONFIG_DIR/discovery-file` directory. This default file
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+will contain some comments about what the file should contain. All comments
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+for this file must appear on their lines starting with `#` (i.e. comments
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+cannot start in the middle of a line).
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+
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+[[discovery-file-format]]
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+[float]
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+==== unicast_hosts.txt file format
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+
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+The format of the file is to specify one unicast host entry per line.
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+Each unicast host entry consists of the host (host name or IP address) and
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+an optional transport port number. If the port number is specified, is must
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+come immediately after the host (on the same line) separated by a `:`.
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+If the port number is not specified, a default value of 9300 is used.
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+
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+For example, this is an example of `unicast_hosts.txt` for a cluster with
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+four nodes that participate in unicast discovery, some of which are not
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+running on the default port:
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+
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+[source,txt]
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+----------------------------------------------------------------
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+10.10.10.5
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+10.10.10.6:9305
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+10.10.10.5:10005
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+# an IPv6 address
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+[2001:0db8:85a3:0000:0000:8a2e:0370:7334]:9301
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+----------------------------------------------------------------
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+
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+Host names are allowed instead of IP addresses (similar to
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+`discovery.zen.ping.unicast.hosts`), and IPv6 addresses must be
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+specified in brackets with the port coming after the brackets.
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Ali Beyad