Initial documentation for index templates V2 (elastic#55755)

This commit adds the initial document for version two index templates. Since these are intended to
be used in favor of V1 index templates, this re-uses the anchors for index templates to point to the
V2 APIs, renaming the V1 template docs' anchors (this was a suggestion from the docs team).

There is more documentation that can be written, but this is a start.

Relates to elastic#53101

docs/build.gradle

@@ -81,6 +81,7 @@ testClusters.integTest {
 
   // TODO: remove this once cname is prepended to transport.publish_address by default in 8.0
   systemProperty 'es.transport.cname_in_publish_address', 'true'
+  systemProperty 'es.itv2_feature_enabled', 'true'
 }
 
 // build the cluster with all plugins

docs/reference/indices.asciidoc

@@ -55,6 +55,11 @@ index settings, aliases, mappings, and index templates.
 * <<indices-get-template>>
 * <<indices-template-exists>>
 
+[float]
+[[component-template-apis]]
+=== Component templates:
+* <<indices-component-template>>
+
 [float]
 [[monitoring]]
 === Monitoring:
@@ -125,6 +130,10 @@ include::indices/template-exists.asciidoc[]
 
 include::indices/open-close.asciidoc[]
 
+include::indices/index-templates.asciidoc[]
+
+include::indices/component-templates.asciidoc[]
+
 include::indices/templates.asciidoc[]
 
 include::indices/put-mapping.asciidoc[]

docs/reference/indices/component-templates.asciidoc

@@ -0,0 +1,272 @@
+[[indices-component-template]]
+=== Component template API
+++++
+<titleabbrev>Component template</titleabbrev>
+++++
+
+Component templates are building blocks that specify mappings, settings, or alias configuration, but
+don't apply to a set of indices themselves. To be used a component template must be specified in the
+`composed_of` of an <<indices-templates,index template>>.
+
+[source,console]
+--------------------------------------------------
+PUT _component_template/template_1
+{
+  "template": {
+    "settings": {
+      "number_of_shards": 1
+    },
+    "mappings": {
+      "_source": {
+        "enabled": false
+      },
+      "properties": {
+        "host_name": {
+          "type": "keyword"
+        },
+        "created_at": {
+          "type": "date",
+          "format": "EEE MMM dd HH:mm:ss Z yyyy"
+        }
+      }
+    }
+  }
+}
+--------------------------------------------------
+// TESTSETUP
+
+//////////////////////////
+
+[source,console]
+--------------------------------------------------
+DELETE _component_template/template_*
+--------------------------------------------------
+// TEARDOWN
+
+//////////////////////////
+
+[[put-component-template-api-request]]
+==== {api-request-title}
+
+`PUT /_component_template/<component-template>`
+
+
+[[put-component-template-api-desc]]
+==== {api-description-title}
+
+Use the PUT component template API to create or update a component template.
+
+// tag::component-template-def[]
+Component templates define <<index-modules-settings,settings>>, <<mapping,mappings>>, and
+<<indices-aliases,aliases>> that you can automatically apply when creating new indices. On their own
+component templates are not matched or applied to an index, instead they are used in the
+`composed_of` definition of an <<indices-templates,index template>> to make up the final index
+configuration.
+// end::component-template-def[]
+
+Component templates are only used during index creation. Changes to component templates do not
+affect existing indices. Settings and mappings specified in <<indices-create-index, create index>>
+API requests and <<indices-templates,index templates>> override any settings or mappings specified
+in a component template.
+
+===== Comments in component templates
+You can use C-style /* */ block comments in component templates.
+You can include comments anywhere in the request body,
+except before the opening curly bracket.
+
+[[getting-component-templates]]
+===== Getting component templates
+
+=== Get component template API
+++++
+<titleabbrev>Get component template</titleabbrev>
+++++
+
+Returns information about one or more component templates.
+
+[source,console]
+--------------------------------------------------
+GET /_component_template/template_1
+--------------------------------------------------
+
+[[get-component-template-api-request]]
+==== {api-request-title}
+
+`GET /_component-template/<component-template>`
+
+
+[[get-component-template-api-path-params]]
+==== {api-path-parms-title}
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=index-template]
++
+To return all component templates, omit this parameter or use a value of `*`.
+
+
+[[get-component-template-api-query-params]]
+==== {api-query-parms-title}
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=flat-settings]
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=local]
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=master-timeout]
+
+
+[[get-component-template-api-example]]
+==== {api-examples-title}
+
+
+[[get-component-template-api-wildcard-ex]]
+===== Get component templates using a wildcard expression
+
+[source,console]
+--------------------------------------------------
+GET /_component_template/temp*
+--------------------------------------------------
+
+
+[[get-component-template-api-all-ex]]
+===== Get all component templates
+
+[source,console]
+--------------------------------------------------
+GET /_component_template
+--------------------------------------------------
+
+[[put-component-template-api-path-params]]
+==== {api-path-parms-title}
+
+`<component-template>`::
+(Required, string)
+Name of the component template to create.
+
+
+[[put-component-template-api-query-params]]
+==== {api-query-parms-title}
+
+`create`::
+(Optional, boolean)
+If `true`, this request cannot replace or update existing component templates.
+Defaults to `false`.
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=master-timeout]
+
+[[put-component-template-api-request-body]]
+==== {api-request-body-title}
+
+`template`::
+(Required, object)
+This is the template to be applied, may optionally include a `mappings`,
+`settings`, or `aliases` configuration.
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=aliases]
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=mappings]
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=settings]
+
+`version`::
+(Optional, integer)
+Version number used to manage component templates externally.
+This number is not automatically generated or incremented by {es}.
+
+`_meta`::
+(Optional, object)
+Optional user metadata about the component template. May have any contents.
+This map is not automatically generated by {es}.
+
+[[put-component-template-api-example]]
+==== {api-examples-title}
+
+===== Component template with index aliases
+
+You can include <<indices-aliases,index aliases>> in a component template.
+
+[source,console]
+--------------------------------------------------
+PUT _component_template/template_1
+{
+  "template": {
+    "settings" : {
+        "number_of_shards" : 1
+    },
+    "aliases" : {
+        "alias1" : {},
+        "alias2" : {
+            "filter" : {
+                "term" : {"user" : "kimchy" }
+            },
+            "routing" : "kimchy"
+        },
+        "{index}-alias" : {} <1>
+    }
+  }
+}
+--------------------------------------------------
+<1> the `{index}` placeholder in the alias name will be replaced with the
+actual index name that the template gets applied to, during index creation.
+
+[[applying-component-templates]]
+===== Applying component templates
+
+Component templates on their own don't apply or interact with indices at all. In order for them to
+be effective, they must be used as part of index template's `composed_of` field to take effect. See
+the <<indices-templates,index templates>> documentation for more information.
+
+[[component-templates-version]]
+===== Component template versioning
+
+You can use the `version` parameter to add an optional version number to a component template. External
+systems can use these version numbers to simplify template management.
+
+The `version` parameter is completely optional and not automatically generated by {es}.
+
+To unset a `version`, replace the template without specifying one.
+
+[source,console]
+--------------------------------------------------
+PUT /_component_template/template_1
+{
+  "template": {
+    "settings" : {
+        "number_of_shards" : 1
+    }
+  },
+  "version": 123
+}
+--------------------------------------------------
+
+To check the `version`, you can use the <<getting-component-templates,get component template>> API.
+
+[[component-templates-metadata]]
+===== Component template metadata
+
+You can use the `_meta` parameter to add optional metadata to a component template. This is a
+user-defined map that can contain any data. This data will be stored in the cluster state however,
+so keeping it short is preferrable.
+
+The `_meta` parameter is completely optional and not automatically generated by {es}.
+
+To unset `_meta`, replace the template without specifying one.
+
+[source,console]
+--------------------------------------------------
+PUT /_component_template/template_1
+{
+  "template": {
+    "settings" : {
+        "number_of_shards" : 1
+    }
+  },
+  "_meta": {
+    "description": "set number of shards to one",
+    "serialization": {
+      "class": "MyComponentTemplate",
+      "id": 10
+    }
+  }
+}
+--------------------------------------------------
+
+To check the `_meta`, you can use the <<getting-component-templates,get component template>> API.

docs/reference/indices/delete-index-template.asciidoc

@@ -49,4 +49,4 @@ include::{docdir}/rest-api/common-parms.asciidoc[tag=index-template]
 [[delete-template-api-query-params]]
 ==== {api-query-parms-title}
 
-include::{docdir}/rest-api/common-parms.asciidoc[tag=timeoutparms]
+include::{docdir}/rest-api/common-parms.asciidoc[tag=timeoutparms]

docs/reference/indices/get-index-template.asciidoc

@@ -1,4 +1,4 @@
-[[indices-get-template]]
+[[indices-get-template-v1]]
 === Get index template API
 ++++
 <titleabbrev>Get index template</titleabbrev>
@@ -33,13 +33,13 @@ GET /_template/template_1
 --------------------------------------------------
 
 
-[[get-template-api-request]]
+[[get-template-v1-api-request]]
 ==== {api-request-title}
 
 `GET /_template/<index-template>`
 
 
-[[get-template-api-path-params]]
+[[get-template-v1-api-path-params]]
 ==== {api-path-parms-title}
 
 include::{docdir}/rest-api/common-parms.asciidoc[tag=index-template]
@@ -48,7 +48,7 @@ To return all index templates, omit this parameter
 or use a value of `_all` or `*`.
 
 
-[[get-template-api-query-params]]
+[[get-template-v1-api-query-params]]
 ==== {api-query-parms-title}
 
 include::{docdir}/rest-api/common-parms.asciidoc[tag=flat-settings]
@@ -60,11 +60,11 @@ include::{docdir}/rest-api/common-parms.asciidoc[tag=local]
 include::{docdir}/rest-api/common-parms.asciidoc[tag=master-timeout]
 
 
-[[get-template-api-example]]
+[[get-template-v1-api-example]]
 ==== {api-examples-title}
 
 
-[[get-template-api-multiple-ex]]
+[[get-template-v1-api-multiple-ex]]
 ===== Get multiple index templates
 
 [source,console]
@@ -73,7 +73,7 @@ GET /_template/template_1,template_2
 --------------------------------------------------
 
 
-[[get-template-api-wildcard-ex]]
+[[get-template-v1-api-wildcard-ex]]
 ===== Get index templates using a wildcard expression
 
 [source,console]
@@ -82,7 +82,7 @@ GET /_template/temp*
 --------------------------------------------------
 
 
-[[get-template-api-all-ex]]
+[[get-template-v1-api-all-ex]]
 ===== Get all index templates
 
 [source,console]