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

The zen discovery is the built in discovery module for elasticsearch and
the default. It provides unicast discovery, but can be extended to
support cloud environments and other forms of discovery.

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.

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

Unicast discovery requires a list of hosts to use that will act as gossip routers. These hosts can be specified as
hostnames or IP addresses; hosts specified as hostnames are resolved to IP addresses during each round of pinging. Note
that with the Java security manager in place, the JVM defaults to caching positive hostname resolutions indefinitely.
This can be modified by adding
http://docs.oracle.com/javase/8/docs/technotes/guides/net/properties.html[`networkaddress.cache.ttl=<timeout>`] to your
http://docs.oracle.com/javase/8/docs/technotes/guides/security/PolicyFiles.html[Java security policy]. Any hosts that
fail to resolve will be logged. Note also that with the Java security manager in place, the JVM defaults to caching
negative hostname resolutions for ten seconds. This can be modified by adding
http://docs.oracle.com/javase/8/docs/technotes/guides/net/properties.html[`networkaddress.cache.negative.ttl=<timeout>`]
to your http://docs.oracle.com/javase/8/docs/technotes/guides/security/PolicyFiles.html[Java security policy].

Unicast discovery 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 should be in the form of `host:port` or `host` (where `port` defaults to `9300`). Note that IPv6 hosts must be
bracketed. Defaults to `127.0.0.1, [::1]`
|`hosts.resolve_timeout` |The amount of time to wait for DNS lookups on each round of pinging. Specified as
<<time-units, time units>>. Defaults to 5s.
|=======================================================================

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 a 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.ignore_non_master_pings` is `true`, pings from nodes that are not master
eligible (nodes where `node.master` is `false`) are ignored during master election; the default value is
`false`.

Nodes can be excluded from becoming a master by setting `node.master` 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 its 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 master 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 eligible 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]
==== 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, acknowledges
it, but does *not* yet apply it. If the master does not receive acknowledgement from
at least `discovery.zen.minimum_master_nodes` nodes within a certain time (controlled by
the `discovery.zen.commit_timeout` setting and defaults to 30 seconds) the cluster state
change is rejected.

Once enough nodes have responded, the cluster state is committed and a message will
be sent to all the nodes. The nodes then proceed to apply the new cluster state to their
internal state. The master node 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 is measured from the moment the publishing started. Both
timeout settings 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).  Requests to these apis will not be blocked and can run on any available node.
