DOCS-16771-read-pref-update (#8433) (#8462)

* DOCS-16771-read-pref-update

* DOCS-16771-read-pref-update

* DOCS-16771-read-pref-update

* DOCS-16771-read-pref-update

* DOCS-16771-read-pref-update

* DOCS-16771-read-pref-update

* DOCS-16771-read-pref-update

* DOCS-16771-read-pref-update

* DOCS-16771-read-pref-update

* DOCS-16771-read-pref-update

* DOCS-16771-read-pref-update

---------

Co-authored-by: jason-price-mongodb <[email protected]>

Author: jason-price-mongodb

source/core/read-preference.txt

@@ -228,10 +228,7 @@ Read Preference and Transactions
 Additional Considerations
 -------------------------
 
-For :doc:`aggregation pipeline </core/aggregation-pipeline>`
-operations that include the :pipeline:`$out` or :pipeline:`$merge`
-stages, the pipeline runs on the :term:`primary` member regardless of
-read preference setting.
+.. include:: /includes/merge-and-read-preference.rst
 
 For :dbcommand:`mapReduce` operations, only "inline"
 :dbcommand:`mapReduce` operations that do not write data support read

source/includes/merge-and-read-preference.rst

@@ -0,0 +1,20 @@
+Consider the following points when using :pipeline:`$merge` or
+:pipeline:`$out` stages in an :ref:`aggregation pipeline
+<aggregation-pipeline>`:
+
+- Starting in MongoDB 5.0, pipelines with a ``$merge`` stage can run on
+  replica set :term:`secondary` nodes if all the nodes in the cluster
+  have the :ref:`featureCompatibilityVersion <view-fcv>` set to ``5.0``
+  or higher and the :ref:`read preference <read-preference>`
+  allows secondary reads.
+
+  - ``$merge`` and ``$out`` stages run on secondary nodes, but write
+    operations are sent to the :term:`primary` node.
+
+  - Not all driver versions support ``$merge`` operations sent to
+    the secondary nodes. For details, see the :driver:`driver </>`
+    documentation.
+
+- In earlier MongoDB versions, pipelines with ``$out`` or ``$merge``
+  stages always run on the primary node and read preference isn't
+  considered.

source/reference/operator/aggregation/merge.txt

@@ -40,20 +40,7 @@ Definition
    - Can output to the same collection that is being aggregated. For more
      information, see :ref:`merge-behavior-same-collection`.
 
-   - Pipelines with the :pipeline:`$merge` stage can run on 
-     replica set secondary nodes if all the nodes in cluster have 
-     :ref:`featureCompatibilityVersion <view-fcv>` set 
-     to ``5.0`` or higher and the :doc:`/core/read-preference`
-     allows secondary reads. 
-     
-     - Read operations of the :pipeline:`$merge` statement are sent to 
-       secondary nodes, while the write operations occur only on the 
-       primary node.
-       
-     - Not all driver versions support targeting of :pipeline:`$merge` 
-       operations to replica set secondary nodes. Check your 
-       :driver:`driver </>` documentation to see when your driver added support 
-       for :pipeline:`$merge` read operations running on secondary nodes.
+   - .. include:: /includes/merge-and-read-preference.rst
 
    - Creates a new collection if the output collection does not already
      exist.