[[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 <>, 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. Secondly 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]] ==== Hosts providers By default the cluster formation module offers two hosts providers to configure the list of seed nodes: a _settings_-based and a _file_-based hosts provider. It can be extended to support cloud environments and other forms of hosts providers via {plugins}/discovery.html[discovery plugins]. Hosts providers are configured using the `discovery.zen.hosts_provider` setting, which defaults to the _settings_-based hosts provider. Multiple hosts providers can be specified as a list. [float] [[settings-based-hosts-provider]] ===== Settings-based hosts provider The settings-based hosts provider uses a node setting to configure a static list of hosts to use as seed nodes. These hosts can be specified as hostnames or IP addresses; hosts specified as hostnames are resolved to IP addresses during each round of discovery. Note that if you are in an environment where DNS resolutions vary with time, you might need to adjust your <>. The list of hosts is set using the <> static setting. For example: [source,yaml] -------------------------------------------------- discovery.zen.ping.unicast.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> A hostname that resolves to multiple IP addresses will try all resolved addresses. Additionally, the `discovery.zen.ping.unicast.hosts.resolve_timeout` configures the amount of time to wait for DNS lookups on each round of discovery. This is specified as a <> and defaults to 5s. Unicast discovery uses the <> module to perform the discovery. [float] [[file-based-hosts-provider]] ===== File-based hosts provider The file-based hosts provider configures a list of hosts via an external file. Elasticsearch 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 Elasticsearch 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.zen.hosts_provider: 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 Elasticsearch 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 unicast host entries in `discovery.zen.ping.unicast.hosts`, they are used in addition to those supplied in `unicast_hosts.txt`. The `discovery.zen.ping.unicast.hosts.resolve_timeout` setting also applies to DNS lookups for nodes specified by address via file-based discovery. This is specified as a <> and defaults to 5s. The format of the file is to specify 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, 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. 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). [float] [[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. [float] [[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. [float] [[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.