DOCS-363 reference items for localthreshold settings

Author: Sam Kleinman

source/reference/configuration-options.txt

@@ -464,8 +464,8 @@ Settings
 
    Use the :option:`mongod --repair` option to access this
    functionality.
-   
-   .. note:: 
+
+   .. note::
 
       Because :program:`mongod` rewrites all of the database files
       during the repair routine, if you do not run :setting:`repair`
@@ -724,3 +724,36 @@ Sharding Cluster Options
    later, the new value will have no effect. See the
    ":ref:`sharding-balancing-modify-chunk-size`" procedure if you
    need to change the chunk size on an existing shard cluster.
+
+.. setting:: localThreshold
+
+   .. versionadded:: 2.2
+
+   :setting:`localThreshold` affects the logic that program:`mongos`
+   uses when selecting :term:`replica set` members to pass reads
+   operations to from clients. Specify a value to
+   :setting:`localThreshold` in milliseconds. The default value is
+   ``15``, which corresponds to the default value in all of the client
+   :doc:`drivers </applications/drivers>`.
+
+   This setting only affects :program:`mongos` processes.
+
+   When :program:`mongos` receives a request that permits reads to
+   :term:`secondary` members, the :program:`mongos` will:
+
+    - find the nearest suitable member of the set, in terms of ping time.
+
+    - construct a list of replica set members that is within a ping
+      time of 15 milliseconds of the nearest suitable member of the
+      set.
+
+      If you specify a value for :setting:`localThreshold`,
+      :program:`mongos` will construct the list of replica members
+      that are within the latency allowed by this value.
+
+    - The :program:`mongos` will select a member to read from at
+      random from this list.
+
+   See the :ref:`replica-set-read-preference-behavior-member-selection`
+   section of the :ref:`read preference <replica-set-read-preference>`
+   documentation for more information.

source/reference/mongos.txt

@@ -227,3 +227,34 @@ Options
    .. versionadded:: 2.1.2
 
    Disables the HTTP interface.
+
+.. option:: --localThreshold
+
+   .. versionadded:: 2.2
+
+   :option:`--localThreshold` affects the logic that program:`mongos`
+   uses when selecting :term:`replica set` members to pass reads
+   operations to from clients. Specify a value to
+   :option:`--localThreshold` in milliseconds. The default value is
+   ``15``, which corresponds to the default value in all of the client
+   :doc:`drivers </applications/drivers>`.
+
+   When :program:`mongos` receives a request that permits reads to
+   :term:`secondary` members, the :program:`mongos` will:
+
+    - find the nearest suitable member of the set, in terms of ping time.
+
+    - construct a list of replica set members that is within a ping
+      time of 15 milliseconds of the nearest suitable member of the
+      set.
+
+      If you specify a value for :option:`--localThreshold`,
+      :program:`mongos` will construct the list of replica members
+      that are within the latency allowed by this value.
+
+    - The :program:`mongos` will select a member to read from at
+      random from this list.
+
+   See the :ref:`replica-set-read-preference-behavior-member-selection`
+   section of the :ref:`read preference <replica-set-read-preference>`
+   documentation for more information.