[DOCS] Adds overview and API ref for cluster voting configurations (#36954)

Author: lcawl

docs/reference/cluster.asciidoc

@@ -104,3 +104,5 @@ include::cluster/tasks.asciidoc[]
 include::cluster/nodes-hot-threads.asciidoc[]
 
 include::cluster/allocation-explain.asciidoc[]
+
+include::cluster/voting-exclusions.asciidoc[]

docs/reference/cluster/voting-exclusions.asciidoc

@@ -0,0 +1,76 @@
+[[voting-config-exclusions]]
+== Voting configuration exclusions API
+++++
+<titleabbrev>Voting Configuration Exclusions</titleabbrev>
+++++
+
+Adds or removes master-eligible nodes from the
+<<modules-discovery-voting,voting configuration exclusion list>>.
+
+[float]
+=== Request
+
+`POST _cluster/voting_config_exclusions/<node_name>` +
+
+`DELETE _cluster/voting_config_exclusions`
+
+[float]
+=== Path parameters
+
+`node_name`::
+  A <<cluster-nodes,node filter>> that identifies {es} nodes.
+
+[float]
+=== Description
+  
+By default, if there are more than three master-eligible nodes in the cluster
+and you remove fewer than half of the master-eligible nodes in the cluster at
+once, the <<modules-discovery-voting,voting configuration>> automatically
+shrinks.
+
+If you want to shrink the voting configuration to contain fewer than three nodes
+or to remove half or more of the master-eligible nodes in the cluster at once,
+you must use this API to remove departed nodes from the voting configuration
+manually. It adds an entry for that node in the voting configuration exclusions
+list. The cluster then tries to reconfigure the voting configuration to remove
+that node and to prevent it from returning.
+
+If the API fails, you can safely retry it.  Only a successful response
+guarantees that the node has been removed from the voting configuration and will
+not be reinstated.
+
+NOTE: Voting exclusions are required only when you remove at least half of the
+master-eligible nodes from a cluster in a short time period. They are not
+required when removing master-ineligible nodes or fewer than half of the
+master-eligible nodes.
+
+The <<modules-discovery-settings,`cluster.max_voting_config_exclusions`
+setting>> limits the size of the voting configuration exclusion list. The
+default value is `10`. Since voting configuration exclusions are persistent and
+limited in number, you must clear the voting config exclusions list once the
+exclusions are no longer required.
+
+There is also a
+<<modules-discovery-settings,`cluster.auto_shrink_voting_configuration` setting>>,
+which is set to true by default. If it is set to false, you must use this API to
+maintain the voting configuration.
+
+For more information, see <<modules-discovery-removing-nodes>>.
+
+[float]
+=== Examples
+
+Add `nodeId1` to the voting configuration exclusions list:
+[source,js]
+-------------------------------------------------- 
+POST /_cluster/voting_config_exclusions/nodeId1
+--------------------------------------------------
+// CONSOLE
+// TEST[catch:bad_request]
+
+Remove all exclusions from the list:
+[source,js]
+--------------------------------------------------
+DELETE /_cluster/voting_config_exclusions
+--------------------------------------------------
+// CONSOLE

docs/reference/modules/discovery.asciidoc

@@ -13,6 +13,16 @@ module. This module is divided into the following sections:
     unknown, such as when a node has just started up or when the previous
     master has failed.
 
+<<modules-discovery-quorums>>::
+
+    This section describes how {es} uses a quorum-based voting mechanism to
+    make decisions even if some nodes are unavailable.
+
+<<modules-discovery-voting>>::
+
+    This section describes the concept of voting configurations, which {es}
+    automatically updates as nodes leave and join the cluster.
+
 <<modules-discovery-bootstrap-cluster>>::
 
     Bootstrapping a cluster is required when an Elasticsearch cluster starts up
@@ -40,26 +50,27 @@ module. This module is divided into the following sections:
     Cluster state publishing is the process by which the elected master node
     updates the cluster state on all the other nodes in the cluster.
 
-<<modules-discovery-quorums>>::
+<<cluster-fault-detection>>::
+
+    {es} performs health checks to detect and remove faulty nodes.
 
-    This section describes the detailed design behind the master election and
-    auto-reconfiguration logic.
-    
 <<modules-discovery-settings,Settings>>::
 
     There are settings that enable users to influence the discovery, cluster
     formation, master election and fault detection processes.    
 
 include::discovery/discovery.asciidoc[]
 
+include::discovery/quorums.asciidoc[]
+
+include::discovery/voting.asciidoc[]
+
 include::discovery/bootstrapping.asciidoc[]
 
 include::discovery/adding-removing-nodes.asciidoc[]
 
 include::discovery/publishing.asciidoc[]
 
-include::discovery/quorums.asciidoc[]
-
 include::discovery/fault-detection.asciidoc[]
 
-include::discovery/discovery-settings.asciidoc[]
+include::discovery/discovery-settings.asciidoc[]

docs/reference/modules/discovery/adding-removing-nodes.asciidoc

@@ -12,6 +12,7 @@ cluster, and to scale the cluster up and down by adding and removing
 master-ineligible nodes only. However there are situations in which it may be
 desirable to add or remove some master-eligible nodes to or from a cluster.
 
+[[modules-discovery-adding-nodes]]
 ==== Adding master-eligible nodes
 
 If you wish to add some nodes to your cluster, simply configure the new nodes
@@ -24,6 +25,7 @@ cluster. You can use the `cluster.join.timeout` setting to configure how long a
 node waits after sending a request to join a cluster. Its default value is `30s`.
 See <<modules-discovery-settings>>.
 
+[[modules-discovery-removing-nodes]]
 ==== Removing master-eligible nodes
 
 When removing master-eligible nodes, it is important not to remove too many all
@@ -50,7 +52,7 @@ will never automatically move a node on the voting exclusions list back into the
 voting configuration. Once an excluded node has been successfully
 auto-reconfigured out of the voting configuration, it is safe to shut it down
 without affecting the cluster's master-level availability. A node can be added
-to the voting configuration exclusion list using the following API:
+to the voting configuration exclusion list using the <<voting-config-exclusions>> API. For example:
 
 [source,js]
 --------------------------------------------------

docs/reference/modules/discovery/discovery-settings.asciidoc

@@ -3,6 +3,15 @@
 
 Discovery and cluster formation are affected by the following settings:
 
+`cluster.auto_shrink_voting_configuration`::
+
+    Controls whether the <<modules-discovery-voting,voting configuration>>
+    sheds departed nodes automatically, as long as it still contains at least 3
+    nodes. The default value is `true`. If set to `false`, the voting
+    configuration never shrinks automatically and you must remove departed
+    nodes manually with the <<voting-config-exclusions,voting configuration
+    exclusions API>>.
+
 [[master-election-settings]]`cluster.election.back_off_time`::
 
     Sets the amount to increase the upper bound on the wait before an election
@@ -152,9 +161,11 @@ APIs are not be blocked and can run on any available node.
 
     Provides a list of master-eligible nodes in the cluster. The list contains
     either an array of hosts or a comma-delimited string. Each value has the
-    format `host:port` or `host`, where `port` defaults to the setting `transport.profiles.default.port`. Note that IPv6 hosts must be bracketed.
+    format `host:port` or `host`, where `port` defaults to the setting
+    `transport.profiles.default.port`. Note that IPv6 hosts must be bracketed.
     The default value is `127.0.0.1, [::1]`. See <<unicast.hosts>>.
 
 `discovery.zen.ping.unicast.hosts.resolve_timeout`::
 
-    Sets the amount of time to wait for DNS lookups on each round of discovery. This is specified as a <<time-units, time unit>> and defaults to `5s`.
+    Sets the amount of time to wait for DNS lookups on each round of discovery.
+    This is specified as a <<time-units, time unit>> and defaults to `5s`.

docs/reference/modules/discovery/fault-detection.asciidoc

@@ -2,8 +2,9 @@
 === Cluster fault detection
 
 The elected master periodically checks each of the nodes in the cluster to
-ensure that they are still connected and healthy. Each node in the cluster also periodically checks the health of the elected master. These checks
-are known respectively as _follower checks_ and _leader checks_.
+ensure that they are still connected and healthy. Each node in the cluster also
+periodically checks the health of the elected master. These checks are known
+respectively as _follower checks_ and _leader checks_.
 
 Elasticsearch allows these checks to occasionally fail or timeout without
 taking any action. It considers a node to be faulty only after a number of
@@ -16,4 +17,4 @@ and retry setting values and attempts to remove the node from the cluster.
 Similarly, if a node detects that the elected master has disconnected, this
 situation is treated as an immediate failure. The node bypasses the timeout and
 retry settings and restarts its discovery phase to try and find or elect a new
-master.
+master.

docs/reference/modules/discovery/quorums.asciidoc

@@ -18,13 +18,13 @@ cluster. In many cases you can do this simply by starting or stopping the nodes
 as required. See <<modules-discovery-adding-removing-nodes>>.
 
 As nodes are added or removed Elasticsearch maintains an optimal level of fault
-tolerance by updating the cluster's _voting configuration_, which is the set of
-master-eligible nodes whose responses are counted when making decisions such as
-electing a new master or committing a new cluster state. A decision is made only
-after more than half of the nodes in the voting configuration have responded.
-Usually the voting configuration is the same as the set of all the
-master-eligible nodes that are currently in the cluster. However, there are some
-situations in which they may be different.
+tolerance by updating the cluster's <<modules-discovery-voting,voting
+configuration>>, which is the set of master-eligible nodes whose responses are
+counted when making decisions such as electing a new master or committing a new
+cluster state. A decision is made only after more than half of the nodes in the
+voting configuration have responded.  Usually the voting configuration is the
+same as the set of all the master-eligible nodes that are currently in the
+cluster. However, there are some situations in which they may be different.
 
 To be sure that the cluster remains available you **must not stop half or more
 of the nodes in the voting configuration at the same time**. As long as more
@@ -38,46 +38,6 @@ cluster-state update that adjusts the voting configuration to match, and this
 can take a short time to complete. It is important to wait for this adjustment
 to complete before removing more nodes from the cluster.
 
-[float]
-==== Setting the initial quorum
-
-When a brand-new cluster starts up for the first time, it must elect its first
-master node. To do this election, it needs to know the set of master-eligible
-nodes whose votes should count. This initial voting configuration is known as
-the _bootstrap configuration_ and is set in the
-<<modules-discovery-bootstrap-cluster,cluster bootstrapping process>>.
-
-It is important that the bootstrap configuration identifies exactly which nodes
-should vote in the first election. It is not sufficient to configure each node
-with an expectation of how many nodes there should be in the cluster. It is also
-important to note that the bootstrap configuration must come from outside the
-cluster: there is no safe way for the cluster to determine the bootstrap
-configuration correctly on its own.
-
-If the bootstrap configuration is not set correctly, when you start a brand-new
-cluster there is a risk that you will accidentally form two separate clusters
-instead of one. This situation can lead to data loss: you might start using both
-clusters before you notice that anything has gone wrong and it is impossible to
-merge them together later.
-
-NOTE: To illustrate the problem with configuring each node to expect a certain
-cluster size, imagine starting up a three-node cluster in which each node knows
-that it is going to be part of a three-node cluster. A majority of three nodes
-is two, so normally the first two nodes to discover each other form a cluster
-and the third node joins them a short time later. However, imagine that four
-nodes were erroneously started instead of three. In this case, there are enough
-nodes to form two separate clusters. Of course if each node is started manually
-then it's unlikely that too many nodes are started. If you're using an automated
-orchestrator, however, it's certainly possible to get into this situation--
-particularly if the orchestrator is not resilient to failures such as network
-partitions.
-
-The initial quorum is only required the very first time a whole cluster starts
-up. New nodes joining an established cluster can safely obtain all the
-information they need from the elected master. Nodes that have previously been
-part of a cluster will have stored to disk all the information that is required
-when they restart.
-
 [float]
 ==== Master elections
 
@@ -104,92 +64,3 @@ and then started again then it will automatically recover, such as during a
 action with the APIs described here in these cases, because the set of master
 nodes is not changing permanently.
 
-[float]
-==== Automatic changes to the voting configuration
-
-Nodes may join or leave the cluster, and Elasticsearch reacts by automatically
-making corresponding changes to the voting configuration in order to ensure that
-the cluster is as resilient as possible. 
-
-The default auto-reconfiguration
-behaviour is expected to give the best results in most situations. The current
-voting configuration is stored in the cluster state so you can inspect its
-current contents as follows:
-
-[source,js]
---------------------------------------------------
-GET /_cluster/state?filter_path=metadata.cluster_coordination.last_committed_config
---------------------------------------------------
-// CONSOLE
-
-NOTE: The current voting configuration is not necessarily the same as the set of
-all available master-eligible nodes in the cluster. Altering the voting
-configuration involves taking a vote, so it takes some time to adjust the
-configuration as nodes join or leave the cluster. Also, there are situations
-where the most resilient configuration includes unavailable nodes, or does not
-include some available nodes, and in these situations the voting configuration
-differs from the set of available master-eligible nodes in the cluster.
-
-Larger voting configurations are usually more resilient, so Elasticsearch
-normally prefers to add master-eligible nodes to the voting configuration after
-they join the cluster. Similarly, if a node in the voting configuration
-leaves the cluster and there is another master-eligible node in the cluster that
-is not in the voting configuration then it is preferable to swap these two nodes
-over. The size of the voting configuration is thus unchanged but its
-resilience increases.
-
-It is not so straightforward to automatically remove nodes from the voting
-configuration after they have left the cluster. Different strategies have
-different benefits and drawbacks, so the right choice depends on how the cluster
-will be used. You can control whether the voting configuration automatically shrinks by using the following setting:
-
-`cluster.auto_shrink_voting_configuration`::
-
-    Defaults to `true`, meaning that the voting configuration will automatically
-    shrink, shedding departed nodes, as long as it still contains at least 3
-    nodes.  If set to `false`, the voting configuration never automatically
-    shrinks; departed nodes must be removed manually using the
-    <<modules-discovery-adding-removing-nodes,voting configuration exclusions API>>.
-
-NOTE: If `cluster.auto_shrink_voting_configuration` is set to `true`, the
-recommended and default setting, and there are at least three master-eligible
-nodes in the cluster, then Elasticsearch remains capable of processing
-cluster-state updates as long as all but one of its master-eligible nodes are
-healthy.
-
-There are situations in which Elasticsearch might tolerate the loss of multiple
-nodes, but this is not guaranteed under all sequences of failures. If this
-setting is set to `false` then departed nodes must be removed from the voting
-configuration manually, using the
-<<modules-discovery-adding-removing-nodes,voting exclusions API>>, to achieve
-the desired level of resilience.
-
-No matter how it is configured, Elasticsearch will not suffer from a "split-brain" inconsistency.
-The `cluster.auto_shrink_voting_configuration` setting affects only its availability in the
-event of the failure of some of its nodes, and the administrative tasks that
-must be performed as nodes join and leave the cluster.
-
-[float]
-==== Even numbers of master-eligible nodes
-
-There should normally be an odd number of master-eligible nodes in a cluster.
-If there is an even number, Elasticsearch leaves one of them out of the voting
-configuration to ensure that it has an odd size. This omission does not decrease
-the failure-tolerance of the cluster. In fact, improves it slightly: if the
-cluster suffers from a network partition that divides it into two equally-sized
-halves then one of the halves will contain a majority of the voting
-configuration and will be able to keep operating. If all of the master-eligible
-nodes' votes were counted, neither side would contain a strict majority of the
-nodes and so the cluster would not be able to make any progress.
-
-For instance if there are four master-eligible nodes in the cluster and the
-voting configuration contained all of them, any quorum-based decision would
-require votes from at least three of them. This situation means that the cluster
-can tolerate the loss of only a single master-eligible node. If this cluster
-were split into two equal halves, neither half would contain three
-master-eligible nodes and the cluster would not be able to make any progress.
-If the voting configuration contains only three of the four master-eligible
-nodes, however, the cluster is still only fully tolerant to the loss of one
-node, but quorum-based decisions require votes from two of the three voting
-nodes. In the event of an even split, one half will contain two of the three
-voting nodes so that half will remain available.