Add elasticsearch-node tool docs (#37812)

This commit, mostly authored by @DaveCTurner, 
adds documentation for elasticsearch-node tool #37696.

docs/reference/commands/index.asciidoc

@@ -10,6 +10,7 @@ tasks from the command line:
 * <<certgen>>
 * <<certutil>>
 * <<migrate-tool>>
+* <<node-tool>>
 * <<saml-metadata>>
 * <<setup-passwords>>
 * <<shard-tool>>
@@ -21,6 +22,7 @@ tasks from the command line:
 include::certgen.asciidoc[]
 include::certutil.asciidoc[]
 include::migrate-tool.asciidoc[]
+include::node-tool.asciidoc[]
 include::saml-metadata.asciidoc[]
 include::setup-passwords.asciidoc[]
 include::shard-tool.asciidoc[]

docs/reference/commands/node-tool.asciidoc

@@ -0,0 +1,334 @@
+[[node-tool]]
+== elasticsearch-node
+
+The `elasticsearch-node` command enables you to perform unsafe operations that
+risk data loss but which may help to recover some data in a disaster.
+
+[float]
+=== Synopsis
+
+[source,shell]
+--------------------------------------------------
+bin/elasticsearch-node unsafe-bootstrap|detach-cluster
+  [--ordinal <Integer>] [-E <KeyValuePair>]
+  [-h, --help] ([-s, --silent] | [-v, --verbose])
+--------------------------------------------------
+
+[float]
+=== Description
+
+Sometimes {es} nodes are temporarily stopped, perhaps because of the need to
+perform some maintenance activity or perhaps because of a hardware failure.
+After you resolve the temporary condition and restart the node,
+it will rejoin the cluster and continue normally. Depending on your
+configuration, your cluster may be able to remain completely available even
+while one or more of its nodes are stopped.
+
+Sometimes it might not be possible to restart a node after it has stopped. For
+example, the node's host may suffer from a hardware problem that cannot be
+repaired. If the cluster is still available then you can start up a fresh node
+on another host and {es} will bring this node into the cluster in place of the
+failed node.
+
+Each node stores its data in the data directories defined by the
+<<path-settings,`path.data` setting>>. This means that in a disaster you can
+also restart a node by moving its data directories to another host, presuming
+that those data directories can be recovered from the faulty host.
+
+{es} <<modules-discovery-quorums,requires a response from a majority of the
+master-eligible nodes>> in order to elect a master and to update the cluster
+state. This means that if you have three master-eligible nodes then the cluster
+will remain available even if one of them has failed. However if two of the
+three master-eligible nodes fail then the cluster will be unavailable until at
+least one of them is restarted.
+
+In very rare circumstances it may not be possible to restart enough nodes to
+restore the cluster's availability. If such a disaster occurs, you should
+build a new cluster from a recent snapshot and re-import any data that was
+ingested since that snapshot was taken.
+
+However, if the disaster is serious enough then it may not be possible to
+recover from a recent snapshot either. Unfortunately in this case there is no
+way forward that does not risk data loss, but it may be possible to use the
+`elasticsearch-node` tool to construct a new cluster that contains some of the
+data from the failed cluster.
+
+This tool has two modes:
+
+* `elastisearch-node unsafe-bootstap` can be used if there is at least one
+  remaining master-eligible node. It forces one of the remaining nodes to form
+  a brand-new cluster on its own, using its local copy of the cluster metadata.
+  This is known as _unsafe cluster bootstrapping_.
+
+* `elastisearch-node detach-cluster` enables you to move nodes from one cluster
+  to another. This can be used to move nodes into the new cluster created with
+  the `elastisearch-node unsafe-bootstap` command. If unsafe cluster bootstrapping was not
+  possible, it also enables you to
+  move nodes into a brand-new cluster.
+
+[[node-tool-unsafe-bootstrap]]
+[float]
+==== Unsafe cluster bootstrapping
+
+If there is at least one remaining master-eligible node, but it is not possible
+to restart a majority of them, then the `elasticsearch-node unsafe-bootstrap`
+command will unsafely override the cluster's <<modules-discovery-voting,voting
+configuration>> as if performing another
+<<modules-discovery-bootstrap-cluster,cluster bootstrapping process>>. 
+The target node can then form a new cluster on its own by using
+the cluster metadata held locally on the target node. 
+
+[WARNING]
+These steps can lead to arbitrary data loss since the target node may not hold the latest cluster
+metadata, and this out-of-date metadata may make it impossible to use some or
+all of the indices in the cluster.
+
+Since unsafe bootstrapping forms a new cluster containing a single node, once
+you have run it you must use the <<node-tool-detach-cluster,`elasticsearch-node
+detach-cluster` tool>> to migrate any other surviving nodes from the failed
+cluster into this new cluster.
+
+When you run the `elasticsearch-node unsafe-bootstrap` tool it will analyse the
+state of the node and ask for confirmation before taking any action. Before
+asking for confirmation it reports the term and version of the cluster state on
+the node on which it runs as follows:
+
+[source,txt]
+----
+Current node cluster state (term, version) pair is (4, 12)
+----
+
+If you have a choice of nodes on which to run this tool then you should choose
+one with a term that is as large as possible. If there is more than one
+node with the same term, pick the one with the largest version.
+This information identifies the node with the freshest cluster state, which minimizes the
+quantity of data that might be lost. For example, if the first node reports
+`(4, 12)` and a second node reports `(5, 3)`, then the second node is preferred
+since its term is larger.  However if the second node reports `(3, 17)` then
+the first node is preferred since its term is larger. If the second node
+reports `(4, 10)` then it has the same term as the first node, but has a
+smaller version, so the first node is preferred.
+
+[WARNING]
+Running this command can lead to arbitrary data loss. Only run this tool if you
+understand and accept the possible consequences and have exhausted all other
+possibilities for recovery of your cluster.
+
+The sequence of operations for using this tool are as follows:
+
+1. Make sure you have really lost access to at least half of the
+master-eligible nodes in the cluster, and they cannot be repaired or recovered
+by moving their data paths to healthy hardware.
+2. Stop **all** remaining nodes.
+3. Choose one of the remaining master-eligible nodes to become the new elected
+master as described above.
+4. On this node, run the `elasticsearch-node unsafe-bootstrap` command as shown
+below. Verify that the tool reported `Master node was successfully
+bootstrapped`.
+5. Start this node and verify that it is elected as the master node.
+6. Run the <<node-tool-detach-cluster,`elasticsearch-node detach-cluster`
+tool>>, described below, on every other node in the cluster.
+7. Start all other nodes and verify that each one joins the cluster.
+8. Investigate the data in the cluster to discover if any was lost during this
+process.
+
+When you run the tool it will make sure that the node that is being used to
+bootstrap the cluster is not running. It is important that all other
+master-eligible nodes are also stopped while this tool is running, but the tool
+does not check this.
+
+The message `Master node was successfully bootstrapped` does not mean that
+there has been no data loss, it just means that tool was able to complete its
+job.
+
+[[node-tool-detach-cluster]]
+[float]
+==== Detaching nodes from their cluster
+
+It is unsafe for nodes to move between clusters, because different clusters
+have completely different cluster metadata. There is no way to safely merge the
+metadata from two clusters together.
+
+To protect against inadvertently joining the wrong cluster, each cluster
+creates a unique identifier, known as the _cluster UUID_, when it first starts
+up. Every node records the UUID of its cluster and refuses to join a
+cluster with a different UUID.
+
+However, if a node's cluster has permanently failed then it may be desirable to
+try and move it into a new cluster. The `elasticsearch-node detach-cluster`
+command lets you detach a node from its cluster by resetting its cluster UUID.
+It can then join another cluster with a different UUID.
+
+For example, after unsafe cluster bootstrapping you will need to detach all the
+other surviving nodes from their old cluster so they can join the new,
+unsafely-bootstrapped cluster.
+
+Unsafe cluster bootstrapping is only possible if there is at least one
+surviving master-eligible node. If there are no remaining master-eligible nodes
+then the cluster metadata is completely lost. However, the individual data
+nodes also contain a copy of the index metadata corresponding with their
+shards. This sometimes allows a new cluster to import these shards as
+<<modules-gateway-dangling-indices,dangling indices>>. You can sometimes
+recover some indices after the loss of all master-eligible nodes in a cluster
+by creating a new cluster and then using the `elasticsearch-node
+detach-cluster` command to move any surviving nodes into this new cluster.
+
+There is a risk of data loss when importing a dangling index because data nodes
+may not have the most recent copy of the index metadata and do not have any
+information about <<docs-replication,which shard copies are in-sync>>. This
+means that a stale shard copy may be selected to be the primary, and some of
+the shards may be incompatible with the imported mapping.
+
+[WARNING]
+Execution of this command can lead to arbitrary data loss. Only run this tool
+if you understand and accept the possible consequences and have exhausted all
+other possibilities for recovery of your cluster.
+
+The sequence of operations for using this tool are as follows:
+
+1. Make sure you have really lost access to every one of the master-eligible
+nodes in the cluster, and they cannot be repaired or recovered by moving their
+data paths to healthy hardware.
+2. Start a new cluster and verify that it is healthy. This cluster may comprise
+one or more brand-new master-eligible nodes, or may be an unsafely-bootstrapped
+cluster formed as described above.
+3. Stop **all** remaining data nodes.
+4. On each data node, run the `elasticsearch-node detach-cluster` tool as shown
+below. Verify that the tool reported `Node was successfully detached from the
+cluster`.
+5. If necessary, configure each data node to
+<<modules-discovery-hosts-providers,discover the new cluster>>.
+6. Start each data node and verify that it has joined the new cluster.
+7. Wait for all recoveries to have completed, and investigate the data in the
+cluster to discover if any was lost during this process.
+
+The message `Node was successfully detached from the cluster` does not mean
+that there has been no data loss, it just means that tool was able to complete
+its job.
+
+[float]
+=== Parameters
+
+`unsafe-bootstrap`:: Specifies to unsafely bootstrap this node as a new
+one-node cluster.
+
+`detach-cluster`:: Specifies to unsafely detach this node from its cluster so
+it can join a different cluster.
+
+`--ordinal <Integer>`:: If there is <<max-local-storage-nodes,more than one
+node sharing a data path>> then this specifies which node to target. Defaults
+to `0`, meaning to use the first node in the data path.
+
+`-E <KeyValuePair>`:: Configures a setting.
+
+`-h, --help`:: Returns all of the command parameters.
+
+`-s, --silent`:: Shows minimal output.
+
+`-v, --verbose`:: Shows verbose output.
+
+[float]
+=== Examples
+
+[float]
+==== Unsafe cluster bootstrapping
+
+Suppose your cluster had five master-eligible nodes and you have permanently
+lost three of them, leaving two nodes remaining.
+
+* Run the tool on the first remaining node, but answer `n` at the confirmation
+  step.
+
+[source,txt]
+----
+node_1$ ./bin/elasticsearch-node unsafe-bootstrap
+
+    WARNING: Elasticsearch MUST be stopped before running this tool.
+
+Current node cluster state (term, version) pair is (4, 12)
+
+You should only run this tool if you have permanently lost half or more
+of the master-eligible nodes in this cluster, and you cannot restore the
+cluster from a snapshot. This tool can cause arbitrary data loss and its
+use should be your last resort. If you have multiple surviving master
+eligible nodes, you should run this tool on the node with the highest
+cluster state (term, version) pair.
+
+Do you want to proceed?
+
+Confirm [y/N] n
+----
+
+* Run the tool on the second remaining node, and again answer `n` at the
+  confirmation step.
+
+[source,txt]
+----
+node_2$ ./bin/elasticsearch-node unsafe-bootstrap
+
+    WARNING: Elasticsearch MUST be stopped before running this tool.
+
+Current node cluster state (term, version) pair is (5, 3)
+
+You should only run this tool if you have permanently lost half or more
+of the master-eligible nodes in this cluster, and you cannot restore the
+cluster from a snapshot. This tool can cause arbitrary data loss and its
+use should be your last resort. If you have multiple surviving master
+eligible nodes, you should run this tool on the node with the highest
+cluster state (term, version) pair.
+
+Do you want to proceed?
+
+Confirm [y/N] n
+----
+
+* Since the second node has a greater term it has a fresher cluster state, so
+  it is better to unsafely bootstrap the cluster using this node:
+
+[source,txt]
+----
+node_2$ ./bin/elasticsearch-node unsafe-bootstrap
+
+    WARNING: Elasticsearch MUST be stopped before running this tool.
+
+Current node cluster state (term, version) pair is (5, 3)
+
+You should only run this tool if you have permanently lost half or more
+of the master-eligible nodes in this cluster, and you cannot restore the
+cluster from a snapshot. This tool can cause arbitrary data loss and its
+use should be your last resort. If you have multiple surviving master
+eligible nodes, you should run this tool on the node with the highest
+cluster state (term, version) pair.
+
+Do you want to proceed?
+
+Confirm [y/N] y
+Master node was successfully bootstrapped
+----
+
+[float]
+==== Detaching nodes from their cluster
+
+After unsafely bootstrapping a new cluster, run the `elasticsearch-node
+detach-cluster` command to detach all remaining nodes from the failed cluster
+so they can join the new cluster:
+
+[source, txt]
+----
+node_3$ ./bin/elasticsearch-node detach-cluster
+
+    WARNING: Elasticsearch MUST be stopped before running this tool.
+
+You should only run this tool if you have permanently lost all of the
+master-eligible nodes in this cluster and you cannot restore the cluster
+from a snapshot, or you have already unsafely bootstrapped a new cluster
+by running `elasticsearch-node unsafe-bootstrap` on a master-eligible
+node that belonged to the same cluster as this node. This tool can cause
+arbitrary data loss and its use should be your last resort.
+
+Do you want to proceed?
+
+Confirm [y/N] y
+Node was successfully detached from the cluster
+----
+

docs/reference/modules/gateway.asciidoc

@@ -49,6 +49,7 @@ as long as the following conditions are met:
 
 NOTE: These settings only take effect on a full cluster restart.
 
+[[modules-gateway-dangling-indices]]
 === Dangling indices
 
 When a node joins the cluster, any shards stored in its local data