[[discovery-file]]
=== 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.

:plugin_name: discovery-file
include::install_remove.asciidoc[]

[[discovery-file-usage]]
[float]
==== Using the file-based discovery 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`:

[source,yaml]
----
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).

[[discovery-file-format]]
[float]
==== unicast_hosts.txt file format

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:

[source,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
----------------------------------------------------------------

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.