[DOCS] Add docs for runtime fields

docs/reference/mapping/types.asciidoc

@@ -108,11 +108,12 @@ as-you-type completion.
 === Arrays
 In {es}, arrays do not require a dedicated field data type. Any field can contain
 zero or more values by default, however, all values in the array must be of the
-same field type. See <<array>>.
+same field type.
+
+See <<array>> to learn more.
 
 [discrete]
 === Multi-fields
-
 It is often useful to index the same field in different ways for different
 purposes. For instance, a `string` field could be mapped as
 a `text` field for full-text search, and as a `keyword` field for
@@ -124,6 +125,23 @@ the <<analysis-standard-analyzer,`standard` analyzer>>, the
 This is the purpose of _multi-fields_. Most field types support multi-fields
 via the <<multi-fields>> parameter.
 
+[discrete]
+=== Runtime
+Oftentimes, you just want to make your data available to {es} without indexing
+or preprocessing. _Runtime fields_ extend the flexibility of the
+<<search-search,search API>> to quickly make data searchable, even with minimal
+structure.
+
+// tag::runtime-fields-description[]
+Runtime fields are not indexed and do have <<doc-values,doc_values>>, meaning
+Lucene is unaware of their existence. However, runtime fields are accessible
+from the search API like any other field that has `doc_values` and is
+searchable. You can retrieve and query these fields, as well as aggregate and
+sort on them.
+// end::runtime-fields-description[]
+
+See <<runtime>> to learn more.
+
 include::types/alias.asciidoc[]
 
 include::types/array.asciidoc[]
@@ -168,6 +186,8 @@ include::types/rank-feature.asciidoc[]
 
 include::types/rank-features.asciidoc[]
 
+include::types/runtime.asciidoc[]
+
 include::types/search-as-you-type.asciidoc[]
 
 include::types/text.asciidoc[]

docs/reference/mapping/types/runtime.asciidoc

@@ -0,0 +1,94 @@
+[[runtime]]
+=== Runtime
+
+////
+[source,console]
+----
+PUT /my-index
+----
+// TESTSETUP
+////
+
+Typically, you must index fields to {es} before they can be retrieved,
+aggregated, or searched. With runtime fields, you can explicitly define a field
+in the mapping and access it at runtime without indexing your data. This
+flexibility allows you to more quickly ingest raw data into the Elastic Stack
+and immediately access it. By dynamically evaluating runtime fields at query
+time, you can choose which fields to index and optimize disk space.
+
+include::{es-ref-dir}/mapping/types.asciidoc[tag=runtime-fields-description]
+
+// Each runtime field is of the `runtime` data type, and has its own
+// <<mapping-types,field type>>, such as `boolean`, `long`, or `keyword`. The
+// field type identifies the data type In
+// the following example, the data type is `runtime` and the runtime field type is
+// `keyword`.
+
+==== Specifying runtime fields
+Runtime fields do not have access to the
+<<mapping-source-field,`_source` field>>. You specify runtime fields in the
+mapping by <<modules-scripting-using,defining a script>>, which always has
+access to the `source` field. At search time, the script runs and generates
+values for each scripted field.
+
+The script in the following request extracts the day of the week from the
+`timestamp` field, which is defined as a `date` data type.
+
+[source,console]
+----
+PUT /my-index/_mappings
+{
+  "properties" : {
+    "day_of_week" : {
+      "type" : "runtime", <1>
+      "runtime_type" : "keyword", <2>
+      "script" : {
+        "source" : "emit(doc['timestamp'].value.dayOfWeekEnum.getDisplayName(TextStyle.FULL, Locale.ROOT))"
+      }
+    }
+  }
+}
+----
+
+<1> Runtime fields are of the `runtime` data type.
+<2> Each runtime has its own field type, defined by `runtime_type`.
+
+[[runtime-params]]
+==== Parameters for `runtime` fields
+Runtime fields accept the following parameters:
+
+`type`::
+The type of runtime computation to perform at query time. Currently, runtime
+fields only support the `runtime` data type.
+
+`runtime_type`::
+The <<mapping-types,field type>> for each scripted field. {es}
+supports `boolean`, `date`, `double`, `ip`, `keyword`, and `long`.
+
+==== Retrieving runtime fields
+Because runtime fields are not part of the `_source` field, they are not
+returned in search hits by default. You can request runtime fields by using the
+<<search-fields,`fields`>> parameter, or by using the
+<<docvalue-fields,`docvalue_fields`>> parameter to return `doc_values` for one
+or more fields in the search response.
+
+Use the <<search-field-caps,field capabilities API>> to return runtime fields
+like any other field. For example, a runtime field with a `runtime_type` of
+`keyword` returns as any other field that belongs to the `keyword` family.
+
+The following request uses the search API to retrieve the `day_of_week` field
+that the previous request defined in the mapping.
+
+[source,console]
+----
+GET /my-index/_search
+{
+  "aggs": {
+    "days_of_week": {
+      "terms": {
+        "field": "day_of_week"
+      }
+    }
+  }
+}
+----