[[modules-discovery-ec2]]
=== EC2 Discovery

EC2 discovery allows to use the EC2 APIs to perform automatic discovery
(similar to multicast in non hostile multicast environments). Here is a
simple sample configuration:

[source,js]
--------------------------------------------------
cloud:
    aws:
        access_key: AKVAIQBF2RECL7FJWGJQ
        secret_key: vExyMThREXeRMm/b/LRzEB8jWwvzQeXgjqMX+6br

discovery:
    type: ec2
--------------------------------------------------

You'll need to install the `cloud-aws` plugin. Please check the
https://github.com/elasticsearch/elasticsearch-cloud-aws[plugin website]
to find the most up-to-date version to install before (re)starting
elasticsearch.

The following are a list of settings (prefixed with `discovery.ec2`)
that can further control the discovery:

[cols="<,<",options="header",]
|=======================================================================
|Setting |Description
|`groups` |Either a comma separated list or array based list of
(security) groups. Only instances with the provided security groups will
be used in the cluster discovery.

|`host_type` |The type of host type to use to communicate with other
instances. Can be one of `private_ip`, `public_ip`, `private_dns`,
`public_dns`. Defaults to `private_ip`.

|`availability_zones` |Either a comma separated list or array based list
of availability zones. Only instances within the provided availability
zones will be used in the cluster discovery.

|`any_group` |If set to `false`, will require all security groups to be
present for the instance to be used for the discovery. Defaults to
`true`.

|`ping_timeout` |How long to wait for existing EC2 nodes to reply during
discovery. Defaults to 3s.
|=======================================================================

[float]
==== Filtering by Tags

EC2 discovery can also filter machines to include in the cluster based
on tags (and not just groups). The settings to use include the
`discovery.ec2.tag.` prefix. For example, setting
`discovery.ec2.tag.stage` to `dev` will only filter instances with a tag
key set to `stage`, and a value of `dev`. Several tags set will require
all of those tags to be set for the instance to be included.

One practical use for tag filtering is when an EC2 cluster contains many
nodes that are not running elasticsearch. In this case (particularly
with high `ping_timeout` values) there is a risk that a new node's
discovery phase will end before it has found the cluster (which will
result in it declaring itself master of a new cluster with the same name
- highly undesirable). Tagging elasticsearch EC2 nodes and then
filtering by that tag will resolve this issue.

[float]
==== Region

The `cloud.aws.region` can be set to a region and will automatically use
the relevant settings for both `ec2` and `s3`. The available values are:
`us-east-1`, `us-west-1`, `ap-southeast-1`, `eu-west-1`.

[float]
==== Automatic Node Attributes

Though not dependent on actually using `ec2` as discovery (but still
requires the cloud aws plugin installed), the plugin can automatically
add node attributes relating to EC2 (for example, availability zone,
that can be used with the awareness allocation feature). In order to
enable it, set `cloud.node.auto_attributes` to `true` in the settings.