Update docs for cluster API

docs/reference/cluster.asciidoc

@@ -6,23 +6,69 @@
 ["float",id="cluster-nodes"]
 == Node specification
 
-Most cluster level APIs allow to specify which nodes to execute on (for
-example, getting the node stats for a node). Nodes can be identified in
-the APIs either using their internal node id, the node name, address,
-custom attributes, or just the `_local` node receiving the request. For
-example, here are some sample executions of nodes info:
+Some cluster-level APIs may operate on a subset of the nodes which can be
+specified with _node filters_. For example, the <<tasks,Task Management>>,
+<<cluster-nodes-stats,Nodes Stats>>, and <<cluster-nodes-info,Nodes Info>> APIs
+can all report results from a filtered set of nodes rather than from all nodes.
+
+_Node filters_ are written as a comma-separated list of individual filters,
+each of which adds or removes nodes from the chosen subset. Each filter can be
+one of the following:
+
+* `_all`, to add all nodes to the subset.
+* `_local`, to add the local node to the subset.
+* `_master`, to add the currently-elected master node to the subset.
+* a node id, to add this node to the set.
+* a pattern, using `*` wildcards, which adds all nodes to the subset
+  whose name, address or hostname matches the pattern.
+* `master:true`, `data:true`, `ingest:true` or `coordinating_only:true`, which
+  respectively add to the subset all master-eligible nodes, all data nodes,
+  all ingest nodes, and all coordinating-only nodes.
+* `master:false`, `data:false`, `ingest:false` or `coordinating_only:false`,
+  which respectively remove from the subset all master-eligible nodes, all data
+  nodes, all ingest nodes, and all coordinating-only nodes.
+* a pair of patterns, using `*` wildcards, of the form `attrname:attrvalue`,
+  which adds to the subset all nodes with a custom node attribute whose name
+  and value match the respective patterns. Custom node attributes are
+  configured by setting properties in the configuration file of the form
+  `node.attr.attrname: attrvalue`.
+
+NOTE: node filters run in the order in which they are given, which is important
+if using filters that remove nodes from the set. For example
+`_all,master:false` means all the nodes except the master-eligible ones, but
+`master:false,_all` means the same as `_all` because the `_all` filter runs
+after the `master:false` filter.
+
+NOTE: if no filters are given, the default is to select all nodes. However, if
+any filters are given then they run starting with an empty chosen subset. This
+means that filters such as `master:false` which remove nodes from the chosen
+subset are only useful if they come after some other filters. When used on its
+own, `master:false` selects no nodes.
+
+Here are some examples of the use of node filters with the
+<<cluster-nodes-info,Nodes Info>> APIs.
 
 [source,js]
 --------------------------------------------------
-# Local
+# If no filters are given, the default is to select all nodes
+GET /_nodes
+# Explicitly select all nodes
+GET /_nodes/_all
+# Select just the local node
 GET /_nodes/_local
-# Address
-GET /_nodes/10.0.0.3,10.0.0.4
-GET /_nodes/10.0.0.*
-# Names
+# Select the elected master node
+GET /_nodes/_master
+# Select nodes by name, which can include wildcards
 GET /_nodes/node_name_goes_here
 GET /_nodes/node_name_goes_*
-# Attributes (set something like node.attr.rack: 2 in the config)
+# Select nodes by address, which can include wildcards
+GET /_nodes/10.0.0.3,10.0.0.4
+GET /_nodes/10.0.0.*
+# Select nodes by role
+GET /_nodes/_all,master:false
+GET /_nodes/data:true,ingest:true
+GET /_nodes/coordinating_only:true
+# Select nodes by custom attribute (e.g. with something like `node.attr.rack: 2` in the configuration file)
 GET /_nodes/rack:2
 GET /_nodes/ra*:2
 GET /_nodes/ra*:2*

docs/reference/cluster/nodes-hot-threads.asciidoc

@@ -1,12 +1,23 @@
 [[cluster-nodes-hot-threads]]
 == Nodes hot_threads
 
-An API allowing to get the current hot threads on each node in the
-cluster. Endpoints are `/_nodes/hot_threads`, and
-`/_nodes/{nodesIds}/hot_threads`.
+This API yields a breakdown of the hot threads on each selected node in the
+cluster. Its endpoints are `/_nodes/hot_threads` and
+`/_nodes/{nodes}/hot_threads`:
 
-The output is plain text with a breakdown of each node's top hot
-threads. Parameters allowed are:
+[source,js]
+--------------------------------------------------
+GET /_nodes/hot_threads
+GET /_nodes/nodeId1,nodeId2/hot_threads
+--------------------------------------------------
+// CONSOLE
+
+The first command gets the hot threads of all the nodes in the cluster. The
+second command gets the hot threads of only `nodeId1` and `nodeId2`. Nodes can
+be selected using <<cluster-nodes,node filters>>.
+
+The output is plain text with a breakdown of each node's top hot threads.  The
+allowed parameters are:
 
 [horizontal]
 `threads`:: 	number of hot threads to provide, defaults to 3.

docs/reference/cluster/stats.asciidoc

@@ -211,4 +211,14 @@ Will return, for example:
 ////
 The TESTRESPONSE above replace all the fields values by the expected ones in the test,
 because we don't really care about the field values but we want to check the fields names.
-////
+////
+
+This API can be restricted to a subset of the nodes using the `?nodeId`
+parameter, which accepts <<cluster-nodes,node filters>>:
+
+[source,js]
+--------------------------------------------------
+GET /_cluster/stats?nodeId=node1,node*,master:false
+--------------------------------------------------
+// CONSOLE
+