[DOCS] Adding node.max_local_storage_nodes setting

This setting was missing from the docs, so I added it. However, I also
completely rewrote the nodes documentation page because it was mostly
talking about client nodes with some issues, without ever discussing
master nodes, or even tribe nodes. All nodes should be listed on a
"nodes" documentation page.

Fixes #15903
Fixed #14429

docs/reference/modules/node.asciidoc

@@ -1,32 +1,271 @@
 [[modules-node]]
 == Node
 
-*elasticsearch* allows to configure a node to either be allowed to store
-data locally or not. Storing data locally basically means that shards of
-different indices are allowed to be allocated on that node. By default,
-each node is considered to be a data node, and it can be turned off by
-setting `node.data` to `false`.
-
-This is a powerful setting allowing to simply create smart load
-balancers that take part in some of different API processing. Lets take
-an example:
-
-We can start a whole cluster of data nodes which do not even start an
-HTTP transport by setting `http.enabled` to `false`. Such nodes will
-communicate with one another using the
-<<modules-transport,transport>> module. In front
-of the cluster we can start one or more "non data" nodes which will
-start with HTTP enabled. All HTTP communication will be performed
-through these "non data" nodes.
-
-The benefit of using that is first the ability to create smart load
-balancers. These "non data" nodes are still part of the cluster, and
-they redirect operations exactly to the node that holds the relevant
-data. The other benefit is the fact that for scatter / gather based
-operations (such as search), these nodes will take part of the
-processing since they will start the scatter process, and perform the
-actual gather processing.
-
-This relieves the data nodes to do the heavy duty of indexing and
-searching, without needing to process HTTP requests (parsing), overload
-the network, or perform the gather processing.
+Any time that you start an instance of Elasticsearch, you are starting a
+_node_. A collection of connected nodes is  called a
+<<modules-cluster,cluster>>. If you are running a single node of Elasticsearch,
+then you have a cluster of one node.
+
+Every node in the cluster can handle <<modules-http,HTTP>> and
+<<modules-transport,Transport>> traffic by default. The transport layer
+is used exclusively for communication between nodes and between nodes and the
+{javaclient}/transport-client.html[Java `TransportClient`]; the HTTP layer is
+used only by external REST clients.
+
+All nodes know about all the other nodes in the cluster and can forward client
+requests to the appropriate node. Besides that, each node serves one or more
+purpose:
+
+<<master-node,Master-eligible node>>::
+
+A node that has `node.master` set to `true` (default), which makes it eligible
+to be <<modules-discovery-zen,elected as the _master_ node>>, which controls
+the cluster.
+
+<<data-node,Data node>>::
+
+A node that has `node.data` set to `true` (default). Data nodes hold data and
+perform data related operations such as CRUD, search, and aggregations.
+
+<<client-node,Client node>>::
+
+A client node has both `node.master` and `node.data` set to `false`. It can
+neither hold data nor become the master node.  It behaves as a ``smart
+router'' and is used to forward cluster-level requests to the master node and
+data-related requests (such as search) to the appropriate data nodes.
+
+<<modules-tribe,Tribe node>>::
+
+A tribe node, configured via the `tribe.*` settings, is a special type  of
+client node that can connect to multiple clusters and perform search and other
+operations across all connected clusters.
+
+By default a node is both a master-eligible node and a data node. This is very
+convenient for small clusters but, as the cluster grows, it becomes important
+to consider separating dedicated master-eligible nodes from dedicated data
+nodes.
+
+[NOTE]
+[[coordinating-node]]
+.Coordinating node
+===============================================
+
+Requests like search requests or bulk-indexing requests may involve data held
+on different data nodes. A search request, for example, is executed in two
+phases which are coordinated by the node which receives the client request --
+the _coordinating node_.
+
+In the _scatter_ phase, the coordinating node forwards the request to the data
+nodes which hold the data.  Each data node executes the request locally and
+returns its results to the coordinating node. In the _gather_  phase, the
+coordinating node reduces each data node's results into a single global
+resultset.
+
+This means that a _client_ node needs to have enough memory and CPU in order to
+deal with the gather phase.
+
+===============================================
+
+[float]
+[[master-node]]
+=== Master Eligible Node
+
+The master node is responsible for lightweight cluster-wide actions such as
+creating or deleting an index, tracking which nodes are part of the cluster,
+and deciding which shards to allocate to which nodes. It is important for
+cluster health to have a stable master node.
+
+Any master-eligible node (all nodes by default) may be elected to become the
+master node by the <<modules-discovery-zen,master election process>>.
+
+Indexing and searching your data is CPU-, memory-, and I/O-intensive work
+which can put pressure on a node's resources. To ensure that your master
+node is stable and not under pressure, it is a good idea in a bigger
+cluster to split the roles between dedicated master-eligible nodes and
+dedicated data nodes.
+
+While master nodes can also behave as <<coordinating-node,coordinating nodes>>
+and route search and indexing requests from clients to data nodes, it is
+better _not_ to use dedicated master nodes for this purpose. It is important
+for the stability of the cluster that master-eligible nodes do as little work
+as possible.
+
+To create a standalone master-eligible node, set:
+
+[source,yaml]
+-------------------
+node.master: true <1>
+node.data: false <2>
+-------------------
+<1> The `node.master` role is enabled by default.
+<2> Disable the `node.data` role (enabled by default).
+
+[float]
+[[split-brain]]
+==== Avoiding split brain with `minimum_master_nodes`
+
+To prevent data loss, it is vital to configure the
+`discovery.zen.minimum_master_nodes` setting (which defaults to `1`) so that
+each master-eligible node knows the _minimum number of master-eligible nodes_
+that must be visible in order to form a cluster.
+
+To explain, imagine that you have a cluster consisting of two master-eligible
+nodes. A network failure breaks communication between these two nodes.  Each
+node sees one master-eligible node... itself. With `minimum_master_nodes` set
+to the default of `1`,  this is sufficient to form a cluster. Each node elects
+itself as the new master (thinking that the other master-eligible node has
+died) and the result is two clusters, or a _split brain_.  These two nodes
+will never rejoin until one node is restarted.  Any data that has been written
+to the restarted node will be lost.
+
+Now imagine that you have a cluster with three master-eligible nodes, and
+`minimum_master_nodes` set to `2`.  If a network split separates one node from
+the other two nodes, the side with one node cannot see enough master-eligible
+nodes and will realise that it cannot elect itself as master.  The side with
+two nodes will elect a new master (if needed) and continue functioning
+correctly.  As soon as the network split is resolved, the single node will
+rejoin the cluster and start serving requests again.
+
+This setting should be set to a _quorum_ of master-eligible nodes:
+
+  (master_eligible_nodes / 2) + 1
+
+In other words, if there are three master-eligible nodes, then minimum master
+nodes should be set to `(3 / 2) + 1` or `2`:
+
+[source,yaml]
+----------------------------
+discovery.zen.minimum_master_nodes: 2 <1>
+----------------------------
+<1> Defaults to `1`.
+
+This setting can also be changed dynamically on a live cluster with the
+<<cluster-update-settings,cluster update settings API>>:
+
+[source,js]
+----------------------------
+PUT _cluster/settings
+{
+  "transient": {
+    "discovery.zen.minimum_master_nodes": 2
+  }
+}
+----------------------------
+// AUTOSENSE
+
+TIP: An advantage of splitting the master and data roles between dedicated
+nodes is that you can have just three master-eligible nodes and set
+`minimum_master_nodes` to `2`. You never have to change this setting, no
+matter how many dedicated data nodes you add to the cluster.
+
+
+[float]
+[[data-node]]
+=== Data Node
+
+Data nodes hold the shards that contain the documents you have indexed. Data
+nodes handle data related operations like CRUD, search, and aggregations.
+These operations are I/O-, memory-, and CPU-intensive. It is important to
+monitor these resources and to add more data nodes if they are overloaded.
+
+The main benefit of having dedicated data nodes is the separation of the
+master and data roles.
+
+To create a dedicated data node, set:
+
+[source,yaml]
+-------------------
+node.master: false <1>
+node.data: true <2>
+-------------------
+<1> Disable the `node.master` role (enabled by default).
+<2> The `node.data` role is enabled by default.
+
+[float]
+[[client-node]]
+=== Client Node
+
+If you take away the ability to be able to handle master duties and take away
+the ability to hold data, then you are left with a _client_ node that can only
+route requests, handle the search reduce phase, and distribute bulk indexing.
+Essentially, client nodes behave as smart load balancers.
+
+Standalone client nodes can benefit large clusters by offloading the
+coordinating node role from data and master-eligible nodes.  Client nodes join
+the cluster and receive the full <<cluster-state,cluster state>>, like every
+other node, and they use the cluster state to route requests directly to the
+appropriate place(s).
+
+WARNING: Adding too many client nodes to a cluster can increase the burden on
+the entire cluster because the elected master node must await acknowledgement
+of cluster state updates from every node! The benefit of client nodes should
+not be overstated -- data nodes can happily serve the same purpose as client
+nodes.
+
+To create a deciated client node, set:
+
+[source,yaml]
+-------------------
+node.master: false <1>
+node.data: false <2>
+-------------------
+<1> Disable the `node.master` role (enabled by default).
+<2> Disable the `node.data` role (enabled by default).
+
+[float]
+== Node data path settings
+
+[float]
+[[data-path]]
+=== `path.data`
+
+Every data and master-eligible node requires access to a data directory where
+shards and index and cluster metadata will be stored. The `path.data` defaults
+to `$ES_HOME/data` but can be configured in the `elasticsearch.yml` config
+file an absolute path or a path relative to `$ES_HOME` as follows:
+
+[source,yaml]
+-----------------------
+path.data:  /var/elasticsearch/data
+-----------------------
+
+Like all node settings, it can also be specified on the command line as:
+
+[source,sh]
+-----------------------
+./bin/elasticsearch --path.data /var/elasticsearch/data
+-----------------------
+
+TIP: When using the `.zip` or `.tar.gz` distributions, the `path.data` setting
+should be configured to locate the data directory outside the Elasticsearch
+home directory, so that the home directory can be deleted without deleting
+your data! The RPM and Debian distributions do this for you already.
+
+
+[float]
+[[max-local-storage-nodes]]
+=== `node.max_local_storage_nodes`
+
+The <<data-path,data path>> can be shared by multiple nodes, even by nodes
+from different clusters.  This is very useful for testing failover and
+different configurations on your development machine.  In production, however,
+it is recommended to run only one node of Elasticsearch per server.
+
+To prevent more than one node from sharing the same data path, add this
+setting to the `elasticsearch.yml` config file:
+
+[source,yaml]
+------------------------------
+node.max_local_storage_nodes: 1
+------------------------------
+
+WARNING: Never run different node types (i.e. master, data, client) from the
+same data directory. This can lead to unexpected data loss.
+
+[float]
+== Other node settings
+
+More node settings can be found in <<modules,Modules>>.  Of particular note are
+the <<cluster-name,`cluster.name`>>, the <<node-name,`node.name`>> and the
+<<modules-network,network settings>>.
+