DOCSP-5241: [Charts] As a SaaS user, I can change the read preference for an existing data source (#175)

* DOCSP-5241: [Charts] As a SaaS user, I can change the read preference for an existing data source

Author: steveren

source/data-sources.txt

@@ -141,6 +141,21 @@ You will see your newly added data source listed in the
 :guilabel:`Data Sources` view. To learn how to manage existing
 data sources, see :doc:`manage-data-sources`.
 
+Read Preference
+~~~~~~~~~~~~~~~
+
+Each data source has a read preference which determines the
+:manual:`replica set member </core/replica-set-members>` that the
+data source reads data from. By default, a newly added data source
+has a read preference of Secondary, meaning that it reads from a
+secondary node within the Atlas cluster. If another data source also
+reads from the same Atlas cluster as the newly created data source,
+the newly created data source has the same read preference as the
+existing data source.
+
+You can :ref:`change the read preference <mod-data-source-permissions>`
+of a data source in the :guilabel:`Data Sources` view.
+
 .. class:: hidden
 
    .. toctree::

source/manage-data-sources.txt

@@ -106,6 +106,65 @@ modify user permissions:
 
 #. Click :guilabel:`Apply Changes`.
 
+.. _modify-data-source-read-preference:
+
+Modify Data Source Read Preference
+----------------------------------
+
+A data source ``Owner`` can modify the read preference of a data
+source. A data source's read preference determines which
+:manual:`replica set member </core/replica-set-members>` the data
+source reads data from.
+
+Read preference options are:
+
+- :manual:`Secondary </core/replica-set-members/#secondaries>`
+- :atlas:`Analytics Node </faq/#analytics-nodes-overview>`
+- :manual:`Nearest </reference/read-preference/#nearest>`
+- :manual:`Primary </core/replica-set-members/#replica-set-primary-member>`
+
+If your Atlas cluster includes one or more :atlas:`analytics nodes
+</faq/#analytics-nodes-overview>`, they are the best choice for your
+data source read preference. Analytics nodes provide :manual:`workload
+isolation </core/workload-isolation>` and reduce strain on your
+cluster's data-bearing nodes.
+
+.. important::
+
+   Not all Atlas clusters include analytics nodes, and the
+   :guilabel:`Analytics Node` option appears in the read preference
+   selection UI whether analytics nodes are available or not. Consult
+   your :atlas:`cluster configuration </scale-cluster/>` and ensure
+   that your cluster has analytics nodes available before choosing
+   :guilabel:`Analytics Node` for your data source's read preference.
+
+   If you select :guilabel:`Analytics Node` for your data source's
+   read preference when no analytics nodes are available, any charts
+   which use the data source will produce an error the next time they
+   read from the data source.
+
+If your Atlas cluster does not have analytics nodes available, the next
+best choice is :guilabel:`Secondary`. Reading from a secondary node
+reduces strain on your primary node.
+
+To modify the read preference for a data source:
+
+1. From the :guilabel:`Data Sources` view, click the ellipsis
+   (:guilabel:`...`) button on the right-hand side of the data source
+   you want to modify.
+
+#. Click :guilabel:`Edit Read Preference`.
+
+#. Select your desired setting from the dropdown menu.
+
+#. Click the :guilabel:`Save` button.
+
+.. note::
+
+   After changing a data source's read preference, it takes a few minutes
+   for the change to take effect. All data sources which read from the
+   same Atlas cluster will use the selected read preference setting.
+
 .. _charts-remove-data-source:
 
 Remove a Data Source