DOCS-14330 v4.2 (#2539)

* DOCS-14330 Add deployment blurb back to shardedcluster page

* internal feedback

* XR feedback 1

* *

* **

Author: ianf-mongodb

source/core/sharded-cluster-components.txt

@@ -58,14 +58,19 @@ Sharding requires at least two shards to distribute sharded data. Single
 shard sharded clusters may be useful if you plan on enabling sharding in
 the  near future, but do not need to at the time of deployment.
 
+.. _sharded-cluster-components-distribution:
+
 Number of ``mongos`` and Distribution
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Deploying multiple :binary:`~bin.mongos` routers supports high
-availability and scalability. For shard-level high availability, a
-common pattern is to place :binary:`~bin.mongos` instances on the same
-hardware that :binary:`~bin.mongod` instances are already running on.
-Another option is to embed :binary:`~bin.mongos` routers with
+availability and scalability. If a proxy or load balancer is between the 
+application and the ``mongos`` routers, you must configure it for 
+:term:`client affinity`. Client affinity allows every connection from a 
+single client to reach the same ``mongos``. For shard-level high 
+availability, a common pattern is to place ``mongos`` instances on the 
+same hardware that ``mongos`` instances are already running on.
+Another option is to embed ``mongos`` routers with
 application tier infrastructure.
 
 There is no limit to the number of :binary:`~bin.mongos` routers you can

source/reference/glossary.txt

@@ -154,6 +154,9 @@ Glossary
 
       Client can also refer to a single thread or process.
 
+   client affinity
+      A consistent client connection to a specified data source.
+
    cluster
       See :term:`sharded cluster`.