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Move file-based discovery to core (#33241)
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Today we support a static list of seed hosts in core Elasticsearch, and allow a
dynamic list of seed hosts to be provided via a file using the `discovery-file`
plugin. In fact the ability to provide a dynamic list of seed hosts is
increasingly useful, so this change moves this functionality to core
Elasticsearch to avoid the need for a plugin.

Furthermore, in order to start up nodes in integration tests we currently
assign a known port to each node before startup, which unfortunately sometimes
fails if another process grabs the selected port in the meantime. By moving the
`discovery-file` functionality into the core product we can use it to avoid
this race.

This change also moves the expected path to the file from
`$ES_PATH_CONF/discovery-file/unicast_hosts.txt` to
`$ES_PATH_CONF/unicast_hosts.txt`. An example of this file is not included in
distributions.

For BWC purposes the plugin still exists, but does nothing more than create the
example file in the old location, and issue a warning when it is used. We also
continue to support the old location for the file, but warn about its
deprecation.

Relates #29244
Closes #33030
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@DaveCTurner
DaveCTurner authored Aug 30, 2018
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73 changes: 8 additions & 65 deletions docs/plugins/discovery-file.asciidoc
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[[discovery-file]]
=== File-Based Discovery Plugin

The file-based discovery plugin uses a list of hosts/ports in a `unicast_hosts.txt` file
in the `config/discovery-file` directory for unicast discovery.
The functionality provided by the `discovery-file` plugin is now available in
Elasticsearch without requiring a plugin. This plugin still exists to ensure
backwards compatibility, but it will be removed in a future version.

On installation, this plugin creates a file at
`$ES_PATH_CONF/discovery-file/unicast_hosts.txt` that comprises comments that
describe how to use it. It is preferable not to install this plugin and instead
to create this file, and its containing directory, using standard tools.

:plugin_name: discovery-file
include::install_remove.asciidoc[]

[[discovery-file-usage]]
[float]
==== Using the file-based discovery plugin

The file-based discovery plugin provides the ability to specify the
unicast hosts list through a simple `unicast_hosts.txt` file that can
be dynamically updated at any time. To enable, add the following in `elasticsearch.yml`:

[source,yaml]
----
discovery.zen.hosts_provider: file
----

This plugin simply provides a facility to supply the unicast hosts list for
zen discovery through an external file that can be updated at any time by a side process.

For example, this gives a convenient mechanism for an Elasticsearch instance
that is run in docker containers to be dynamically supplied a list of IP
addresses to connect to for zen discovery when those IP addresses may not be
known at node startup.

Note that the file-based discovery plugin is meant to augment the unicast
hosts list in `elasticsearch.yml` (if specified), not replace it. Therefore,
if there are valid unicast host entries in `discovery.zen.ping.unicast.hosts`,
they will be used in addition to those supplied in `unicast_hosts.txt`.

Anytime a change is made to the `unicast_hosts.txt` file, even as Elasticsearch
continues to run, the new changes will be picked up by the plugin and the
new hosts list will be used for the next pinging round for master election.

Upon installation of the plugin, a default `unicast_hosts.txt` file will
be found in the `$CONFIG_DIR/discovery-file` directory. This default file
will contain some comments about what the file should contain. All comments
for this file must appear on their lines starting with `#` (i.e. comments
cannot start in the middle of a line).

[[discovery-file-format]]
[float]
==== unicast_hosts.txt file format

The format of the file is to specify one unicast host entry per line.
Each unicast host 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.
242 changes: 160 additions & 82 deletions docs/reference/modules/discovery/zen.asciidoc
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[[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.
Zen discovery is the built-in, default, discovery module for Elasticsearch. It
provides unicast and file-based discovery, and can be extended to support cloud
environments and other forms of discovery via plugins.

The zen discovery is integrated with other modules, for example, all
communication between nodes is done using the
<<modules-transport,transport>> module.
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.
This is the process where a node uses the discovery mechanisms to find other
nodes.

[float]
[[discovery-seed-nodes]]
==== Seed nodes

Zen discovery uses a list of _seed_ nodes in order to start off the discovery
process. At startup, or when electing a new master, Elasticsearch tries to
connect to each seed node in its list, and holds a gossip-like conversation with
them to find other nodes and to build a complete picture of the cluster. By
default there are two methods for configuring the list of seed nodes: _unicast_
and _file-based_. It is recommended that the list of seed nodes comprises the
list of master-eligible nodes in the cluster.

[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 if you are in an environment where DNS resolutions vary with
time, you might need to adjust your <<networkaddress-cache-ttl,JVM security
settings>>.
Unicast discovery configures a static list of hosts for 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 pinging. Note that
if you are in an environment where DNS resolutions vary with time, you might
need to adjust your <<networkaddress-cache-ttl,JVM security settings>>.

It is recommended that the unicast hosts list be maintained as the list of
master-eligible nodes in the cluster.
The list of hosts is set using the `discovery.zen.ping.unicast.hosts` static
setting. This is either an array of hosts or a comma-delimited string. Each
value should be in the form of `host:port` or `host` (where `port` defaults to
the setting `transport.profiles.default.port` falling back to
`transport.tcp.port` if not set). Note that IPv6 hosts must be bracketed. The
default for this setting is `127.0.0.1, [::1]`

Unicast discovery provides the following settings with the `discovery.zen.ping.unicast` prefix:
Additionally, the `discovery.zen.ping.unicast.resolve_timeout` configures the
amount of time to wait for DNS lookups on each round of pinging. This is
specified as a <<time-units, time unit>> and defaults to 5s.

[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 the setting `transport.profiles.default.port`
falling back to `transport.tcp.port` if not set). 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.
|=======================================================================
Unicast discovery uses the <<modules-transport,transport>> module to perform the
discovery.

The unicast discovery uses the <<modules-transport,transport>> module to perform the discovery.
[float]
[[file-based-hosts-provider]]
===== File-based

In addition to hosts provided by the static `discovery.zen.ping.unicast.hosts`
setting, it is possible to provide 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 for Zen discovery when those IP addresses may not be known at node
startup.

To enable file-based discovery, configure the `file` hosts provider as follows:

```
discovery.zen.hosts_provider: file
```

Then create a file at `$ES_PATH_CONF/unicast_hosts.txt` in
<<discovery-file-format,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` then they will be used in addition to those
supplied in `unicast_hosts.txt`.

The `discovery.zen.ping.unicast.resolve_timeout` setting also applies to DNS
lookups for nodes specified by address via file-based discovery. This is
specified as a <<time-units, time unit>> and defaults to 5s.

[[discovery-file-format]]
[float]
====== unicast_hosts.txt file format

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.

It is also possible to 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]
[[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`) determines how long the node
will wait before deciding on starting an election or joining an existing cluster.
Three pings will be sent over this timeout interval. In case where no decision can be
reached after the timeout, the pinging process restarts.
In slow or congested networks, three seconds might not be enough for a node to become
aware of the other nodes in its environment before making an election decision.
Increasing the timeout should be done with care in that case, as it will slow down the
election process.
Once a node decides to join an existing formed cluster, 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
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`) determines how long the node will wait before deciding on starting an
election or joining an existing cluster. Three pings will be sent over this
timeout interval. In case where no decision can be reached after the timeout,
the pinging process restarts. In slow or congested networks, three seconds
might not be enough for a node to become aware of the other nodes in its
environment before making an election decision. Increasing the timeout should
be done with care in that case, as it will slow down the election process. Once
a node decides to join an existing formed cluster, 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`.

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 begin.
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 begin.

This setting must be set to a <<minimum_master_nodes,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.
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.
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:
Expand All @@ -116,19 +188,21 @@ considered failed. Defaults to `3`.

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>>
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]]
Expand All @@ -143,10 +217,14 @@ 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.
`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.
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