[[modules-discovery-zen]]
=== Zen Discovery

The zen discovery is the built in discovery module for elasticsearch and
the default. It provides both multicast and unicast discovery as well
being easily extended to support cloud environments.

The zen discovery is integrated with other modules, for example, all
communication between nodes is done using the
<<modules-transport,transport>> module.

It is separated into several sub modules, which are explained below:

[float]
[[ping]]
==== Ping

This is the process where a node uses the discovery mechanisms to find
other nodes. There is support for both multicast and unicast based
discovery (can be used in conjunction as well).

[float]
[[multicast]]
===== Multicast

Multicast ping discovery of other nodes is done by sending one or more
multicast requests where existing nodes that exists will receive and
respond to. It provides the following settings with the
`discovery.zen.ping.multicast` prefix:

[cols="<,<",options="header",]
|=======================================================================
|Setting |Description
|`group` |The group address to use. Defaults to `224.2.2.4`.

|`port` |The port to use. Defaults to `54328`.

|`ttl` |The ttl of the multicast message. Defaults to `3`.

|`address` |The address to bind to, defaults to `null` which means it
will bind to all available network interfaces.

|`enabled` |Whether multicast ping discovery is enabled. Defaults to `true`.
|=======================================================================

[float]
[[unicast]]
===== Unicast

The unicast discovery allows to perform the discovery when multicast is
not enabled. It basically requires a list of hosts to use that will act
as gossip routers. It provides the following settings with the
`discovery.zen.ping.unicast` prefix:

[cols="<,<",options="header",]
|=======================================================================
|Setting |Description
|`hosts` |Either an array setting or a comma delimited setting. Each
value is either in the form of `host:port`, or in the form of
`host[port1-port2]`.
|=======================================================================

The unicast discovery uses the
<<modules-transport,transport>> module to
perform the discovery.

[float]
[[master-election]]
==== Master Election

As part of the initial ping process a master of the cluster is either
elected or joined to. This is done automatically. The
`discovery.zen.ping_timeout` (which defaults to `3s`) allows to
configure the election to handle cases of slow or congested networks
(higher values assure less chance of failure). 

Nodes can be excluded from becoming a master by setting `node.master` to
`false`. Note, once a node is a client node (`node.client` set to
`true`), it will not be allowed to become a master (`node.master` is
automatically set to `false`).

The `discovery.zen.minimum_master_nodes` allows to control the minimum
number of master eligible nodes a node should "see" in order to operate
within the cluster. Its recommended to set it to a higher value than 1
when running more than 2 nodes in the cluster.

[float]
[[fault-detection]]
==== Fault Detection

There are two fault detection processes running. The first is by the
master, to ping all the other nodes in the cluster and verify that they
are alive. And on the other end, each node pings to master to verify if
its still alive or an election process needs to be initiated.

The following settings control the fault detection process using the
`discovery.zen.fd` prefix:

[cols="<,<",options="header",]
|=======================================================================
|Setting |Description
|`ping_interval` |How often a node gets pinged. Defaults to `1s`.

|`ping_timeout` |How long to wait for a ping response, defaults to
`30s`.

|`ping_retries` |How many ping failures / timeouts cause a node to be
considered failed. Defaults to `3`.
|=======================================================================

[float]
==== External Multicast

The multicast discovery also supports external multicast requests to
discover nodes. The external client can send a request to the multicast
IP/group and port, in the form of:

[source,js]
--------------------------------------------------
{
    "request" : {
        "cluster_name": "test_cluster"
    }
}
--------------------------------------------------

And the response will be similar to node info response (with node level
information only, including transport/http addresses, and node
attributes):

[source,js]
--------------------------------------------------
{
    "response" : {
        "cluster_name" : "test_cluster",
        "transport_address" : "...",
        "http_address" : "...",
        "attributes" : {
            "..."
        }
    }
}
--------------------------------------------------

Note, it can still be enabled, with disabled internal multicast
discovery, but still have external discovery working by keeping
`discovery.zen.ping.multicast.enabled` set to `true` (the default), but,
setting `discovery.zen.ping.multicast.ping.enabled` to `false`.

[float]
==== Cluster state updates

The master node is the only node in a cluster that can make changes to the
cluster state. The master node processes one cluster state update at a time,
applies the required changes and publishes the updated cluster state to all
the other nodes in the cluster. Each node receives the publish message,
updates its own cluster state and replies to the master node, which waits for
all nodes to respond, up to a timeout, before going ahead processing the next
updates in the queue. The `discovery.zen.publish_timeout` is set by default
to 30 seconds and can be changed dynamically through the
<<cluster-update-settings,cluster update settings api>> coming[1.1.0, The
setting existed before but wasn't dynamic].
