[[modules-discovery-hosts-providers]]
=== Discovery

Discovery is the process by which the cluster formation module finds other
nodes with which to form a cluster. This process runs when you start an
Elasticsearch node or when a node believes the master node failed and continues
until the master node is found or a new master node is elected.

This process starts with a list of _seed_ addresses from one or more
<<built-in-hosts-providers,seed hosts providers>>, together with the addresses
of any master-eligible nodes that were in the last-known cluster. The process
operates in two phases: First, each node probes the seed addresses by
connecting to each address and attempting to identify the node to which it is
connected and to verify that it is master-eligible. Secondly, if successful, it
shares with the remote node a list of all of its known master-eligible peers
and the remote node responds with _its_ peers in turn. The node then probes all
the new nodes that it just discovered, requests their peers, and so on.

If the node is not master-eligible then it continues this discovery process
until it has discovered an elected master node. If no elected master is
discovered then the node will retry after `discovery.find_peers_interval` which
defaults to `1s`.

If the node is master-eligible then it continues this discovery process until
it has either discovered an elected master node or else it has discovered
enough masterless master-eligible nodes to complete an election. If neither of
these occur quickly enough then the node will retry after
`discovery.find_peers_interval` which defaults to `1s`.

[[built-in-hosts-providers]]
==== Seed hosts providers

By default the cluster formation module offers two seed hosts providers to
configure the list of seed nodes: a _settings_-based and a _file_-based seed
hosts provider.  It can be extended to support cloud environments and other
forms of seed hosts providers via {plugins}/discovery.html[discovery plugins].
Seed hosts providers are configured using the `discovery.seed_providers`
setting, which defaults to the _settings_-based hosts provider. This setting
accepts a list of different providers, allowing you to make use of multiple
ways to find the seed hosts for your cluster.

Each seed hosts provider yields the IP addresses or hostnames of the seed
nodes. If it returns any hostnames then these are resolved to IP addresses
using a DNS lookup. If a hostname resolves to multiple IP addresses then {es}
tries to find a seed node at all of these addresses. If the hosts provider does
not explicitly give the TCP port of the node by then, it will implicitly use the
first port in the port range given by `transport.profiles.default.port`, or by
`transport.port` if `transport.profiles.default.port` is not set. The number of
concurrent lookups is controlled by
`discovery.seed_resolver.max_concurrent_resolvers` which defaults to `10`, and
the timeout for each lookup is controlled by `discovery.seed_resolver.timeout`
which defaults to `5s`. Note that DNS lookups are subject to
<<networkaddress-cache-ttl,JVM DNS caching>>.

[discrete]
[[settings-based-hosts-provider]]
===== Settings-based seed hosts provider

The settings-based seed hosts provider uses a node setting to configure a
static list of the addresses of the seed nodes. These addresses can be given as
hostnames or IP addresses; hosts specified as hostnames are resolved to IP
addresses during each round of discovery.

The list of hosts is set using the <<unicast.hosts,`discovery.seed_hosts`>>
static setting. For example:

[source,yaml]
--------------------------------------------------
discovery.seed_hosts:
   - 192.168.1.10:9300
   - 192.168.1.11 <1>
   - seeds.mydomain.com <2>
--------------------------------------------------
<1> The port will default to `transport.profiles.default.port` and fallback to
    `transport.port` if not specified.
<2> If a hostname resolves to multiple IP addresses, {es} will attempt to
    connect to every resolved address.

[discrete]
[[file-based-hosts-provider]]
===== File-based seed hosts provider

The file-based seed hosts provider configures a list of hosts via an external
file.  {es} reloads this file when it changes, so that the list of seed nodes
can change dynamically without needing to restart each node. For example, this
gives a convenient mechanism for an {es} instance that is run in a Docker
container to be dynamically supplied with a list of IP addresses to connect to
when those IP addresses may not be known at node startup.

To enable file-based discovery, configure the `file` hosts provider as follows
in the `elasticsearch.yml` file:

[source,yml]
----------------------------------------------------------------
discovery.seed_providers: file
----------------------------------------------------------------

Then create a file at `$ES_PATH_CONF/unicast_hosts.txt` in the format described
below. Any time a change is made to the `unicast_hosts.txt` file the new
changes will be picked up by {es} and the new hosts list will be used.

Note that the file-based discovery plugin augments the unicast hosts list in
`elasticsearch.yml`: if there are valid seed addresses in
`discovery.seed_hosts` then {es} uses those addresses in addition to those
supplied in `unicast_hosts.txt`.

The `unicast_hosts.txt` file contains one node entry per line. Each node 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, {es} will implicitly use the first port in the port range given by
`transport.profiles.default.port`, or by `transport.port` if
`transport.profiles.default.port` is not set.

For example, this is an example of `unicast_hosts.txt` for a cluster with four
nodes that participate in 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 and are resolved by DNS as
described above. IPv6 addresses must be given in brackets with the port, if
needed, coming after the brackets.

You can also add comments to this file. All comments must appear on their lines
starting with `#` (i.e. comments cannot start in the middle of a line).

[discrete]
[[ec2-hosts-provider]]
===== EC2 hosts provider

The {plugins}/discovery-ec2.html[EC2 discovery plugin] adds a hosts provider
that uses the https://github.com/aws/aws-sdk-java[AWS API] to find a list of
seed nodes.

[discrete]
[[azure-classic-hosts-provider]]
===== Azure Classic hosts provider

The {plugins}/discovery-azure-classic.html[Azure Classic discovery plugin] adds
a hosts provider that uses the Azure Classic API find a list of seed nodes.

[discrete]
[[gce-hosts-provider]]
===== Google Compute Engine hosts provider

The {plugins}/discovery-gce.html[GCE discovery plugin] adds a hosts provider
that uses the GCE API find a list of seed nodes.
