DOCSP-14267- isLagged Update (#407)

* DOCSP-14267 transfers text with expansion from old ticket

* DOCSP-14267 transfers text with expansion from old ticket

* DOCS-14267 Updates text per internal feedback

* DOCS-14267 refactors explanation, per feedback

* DOCSP-14267 Updates per feedback

* DOCSP-14267 Updates to example explanation

* DOCSP-14267 vale cleanup

* DOCSP-14267 Fixes from Jason feedback

* DOCSP-14267 Fixes from Jason feedback

* DOCSP-14267 updates from Jason feedback

* DOCSP-14267 Fixes from Jason feedback

* DOCSP-14267 Fixes from Jason feedback

* DOCSP-14267 Fixes from Jason feedback

* DOCSP-14267 Fixes from Jason feedback

* DOCSP-14267 refactor to include troubleshooting content

* DOCSP-14267 Fixes from Jason feedback

* DOCSP-14267 Fixes from Jason feedback

* DOCSP-14267 Fixes from Jason feedback

* DOCSP-14267 Fixes from Jason feedback

* DOCSP-14267 Fixes from Jason feedback

* DOCSP-14267 Fixes per Gregory Noma feedback

Author: kennethdyer

source/reference/command/serverStatus.txt

@@ -874,7 +874,12 @@ flowControl
    committed lag is greater than some percentage of the configured
    :parameter:`flowControlTargetLagSeconds`.
 
-   When run on a secondary, the returned boolean is a placeholder.
+   Replication lag can occur without engaging flow control.
+   An unresponsive :term:`secondary` might lag without the replica set
+   receiving sufficient load to engage flow control, leaving 
+   the :serverstatus:`flowControl.isLagged` value at ``false``.
+
+   For additional information, see :ref:`Flow Control <flow-control>`.
 
    .. versionadded:: 4.2
 

source/tutorial/troubleshoot-replica-sets.txt

@@ -143,6 +143,56 @@ Flow Control
 
 .. include:: /includes/extracts/4.2-changes-flow-control-specific-desc.rst
 
+Replication lag can occur without the replica set receiving sufficient load
+to engage flow control, such as in the case of an unresponsive 
+:term:`secondary`.
+
+To view the status of flow control, run the following commands on the 
+:term:`primary`:
+
+#. Run the :method:`rs.printSecondaryReplicationInfo` method to determine if
+   any nodes are lagging:
+
+   .. code-block:: javascript
+   
+      rs.printSecondaryReplicationInfo()    
+
+   Example output:
+
+   .. code-block:: javascript 
+      :copyable: false
+   
+      source: 192.0.2.2:27017
+      {
+        syncedTo: 'Mon Jan 31 2022 18:58:50 GMT+0000 (Coordinated Universal Time)',
+        replLag: '0 secs (0 hrs) behind the primary '
+      }
+      ---
+      source: 192.0.2.3:27017
+      {
+        syncedTo: 'Mon Jan 31 2022 18:58:05 GMT+0000 (Coordinated Universal Time)',
+        replLag: '45 secs (0 hrs) behind the primary '
+      }
+
+#. Run the :dbcommand:`serverStatus` command and use the 
+   :serverstatus:`flowControl.isLagged` value to determine whether the
+   replica set has engaged flow control:
+
+   .. code-block:: javascript
+
+      db.runCommand( { serverStatus: 1 } ).flowControl.isLagged     
+
+   Example output:
+
+   .. code-block:: javascript
+      :copyable: false
+
+      false
+   
+   If flow control has not engaged, investigate the :term:`secondary`
+   to determine the cause of the replication lag, such as limitations in 
+   the hardware, network, or application.
+
 For information on flow control statistics, see:
 
 - :serverstatus:`flowControl <flowControl>`