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

The zen discovery is the built in discovery module for elasticsearch and
the default. It provides both unicast and multicast 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 (these mechanisms 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 which existing nodes 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 `network.bind_host`

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

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

The unicast discovery allows for 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`. Note that IPv6 hosts must be bracketed. Defaults to
`127.0.0.1, [::1]`
|=======================================================================

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

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

As part of the 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 for the
tweaking of election time to handle cases of slow or congested networks
(higher values assure less chance of failure). Once a node joins, it
will send a join request to the master (`discovery.zen.join_timeout`)
with a timeout defaulting at 20 times the ping timeout.

When the master node stops or has encountered a problem, the cluster nodes
start pinging again and will elect a new master. This pinging round also
serves as a protection against (partial) network failures where node may unjustly
think that the master has failed. In this case the node will simply hear from
other nodes about the currently active master.

If `discovery.zen.master_election.filter_client` is `true`, pings from client nodes (nodes where `node.client` is
`true`, or both `node.data` and `node.master` are `false`) are ignored during master election; the default value is
`true`. If `discovery.zen.master_election.filter_data` is `true`, pings from non-master-eligible data nodes (nodes
where `node.data` is `true` and `node.master` is `false`) are ignored during master election; the default value is
`false`. Pings from master-eligible nodes are always observed during master election.

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` sets the minimum
number of master eligible nodes that need to join a newly elected master in order for an election to
complete and for the elected node to accept it's mastership. The same setting controls the minimum number of
active master eligible nodes that should be a part of any active cluster. If this requirement is not met the
active master node will step down and a new mastser election will be begin.

This setting must be set to a quorum of your master eligible nodes. It is recommended to avoid
having only two master eligible nodes, since a quorum of two is two. Therefore, a loss
of either master node will result in an inoperable 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>>

[float]
[[no-master-block]]
==== No master block

For the cluster to be fully operational, it must have an active master and the
number of running master eligible nodes must satisfy the
`discovery.zen.minimum_master_nodes` setting if set. The
`discovery.zen.no_master_block` settings controls what operations should be
rejected when there is no active master.

The `discovery.zen.no_master_block` setting has two valid options:

[horizontal]
`all`:: All operations on the node--i.e. both read & writes--will be rejected. This also applies for api cluster state
read or write operations, like the get index settings, put mapping and cluster state api.
`write`:: (default) Write operations will be rejected. Read operations will succeed, based on the last known cluster configuration.
This may result in partial reads of stale data as this node may be isolated from the rest of the cluster. 

The `discovery.zen.no_master_block` setting doesn't apply to nodes based apis (for example cluster stats, node info and
node stats apis) which will not be blocked and try to execute on any node possible.
