DOCSP-50758 -- add $function page (#13619)

* DCOSP-50758 -- add  page

* DCOSP-50758 -- add  example

* DCOSP-50758 -- add  fields and behavior

* DCOSP-50758 -- add  fields and behavior

* DOCSP-50758 -- external review revisions

* DOCSP-50758 -- external review revisions

* DOCSP-50758 -- remove schema validation note

Author: jvincent-mongodb

content/atlas/source/atlas-stream-processing/sp-agg-function.txt

@@ -0,0 +1,117 @@
+.. _streams-agg-pipeline-function:
+.. _atlas-sp-agg-function:
+
+==============================================
+``$function`` Stage (Stream Processing)
+==============================================
+
+.. default-domain:: mongodb
+
+.. meta::
+   :keywords: atlas stream processing, $function aggregation pipeline stage
+   :description: Learn how to use the $function stage to aggregate data across
+                 overlapping or staggered timeframes.
+
+.. facet::
+   :name: genre
+   :values: reference
+
+.. contents:: On this page
+   :local:
+   :backlinks: none
+   :depth: 2
+   :class: singlecol
+
+.. _atlas-sp-agg-function-def:
+
+.. note::
+
+  **Currently, this functionality is only available in Atlas Stream Processing 
+  Instances deployed to AWS cloud.**
+
+
+Definition
+~~~~~~~~~~
+
+The :expression:`$function` stage specifies a pipeline stage in which you can run 
+custom JavaScript code locally within the pipeline. Currently, this functionality 
+is only available in Atlas clusters deployed to AWS infrastructure.
+
+.. _atlas-sp-agg-function-syntax:
+
+Syntax
+------
+
+The :expression:`$function` operator has the following syntax:
+
+.. code-block:: javascript
+
+   { 
+     $function: {
+       body: <code>,
+       args: <array expression>,
+       lang: "js"
+     }
+   }
+
+.. list-table::
+   :header-rows: 1
+   :widths: 20 20 80
+
+   * - Field
+     - Type
+     - Description
+     
+
+   * - :ref:`body <function-body>`
+
+     - String or Code
+
+     - .. _function-body:
+
+       The function definition. You can specify the function
+       definition as either BSON type Code or String. See also
+       :ref:`lang <function-lang>`.
+
+       ``function(arg1, arg2, ...) { ... }``
+
+       or
+
+       ``"function(arg1, arg2, ...) { ... }"``
+
+
+   * - :ref:`args <function-args>`
+
+     - Array
+
+     - .. _function-args:
+
+       Arguments passed to the function :ref:`body <function-body>`.
+       If the :ref:`body <function-body>` function does not take an
+       argument, you can specify an empty array ``[ ]``.
+
+       The array elements can be any BSON type, including Code. See
+       :ref:`function-example-where-alternative`.
+
+   * - :ref:`lang <function-lang>`
+
+     - String
+
+     - .. _function-lang:
+
+       The language used in the :ref:`body <function-body>`. You
+       must specify ``lang: "js"``.
+
+
+.. _atlas-sp-agg-function-behavior:
+
+Behavior
+--------
+
+The :expression:`$function` stage runs the specified function
+on each document in the stream. The function can take arguments
+from the document or from the :ref:`args <function-args>` array.
+
+The function can return any BSON type, including an array or a
+Code type. The returned value is used as the output of the
+:expression:`$function` stage.