DOCS-7353: limited circumstances for writing to the config db

source/reference/config-database.txt

@@ -18,17 +18,12 @@ The ``config`` database supports :term:`sharded cluster`
 operation. See the :doc:`/sharding` section of this manual for full
 documentation of sharded clusters.
 
-.. important:: Consider the schema of the ``config`` database
+.. important:: The schema of the ``config`` database is
    *internal* and may change between releases of MongoDB. The
    ``config`` database is not a dependable API, and users should not
    write data to the ``config`` database in the course of normal
    operation or maintenance.
 
-.. warning:: Modification of the ``config`` database on a functioning
-   system may lead to instability or inconsistent data sets. If you
-   must modify the ``config`` database, use :program:`mongodump` to
-   create a full backup of the ``config`` database.
-
 To access the ``config`` database, connect to a :program:`mongos`
 instance in a sharded cluster, and use the following helper:
 
@@ -42,6 +37,28 @@ You can return a list of the collections, with the following helper:
 
    show collections
 
+The config database is for internal use only, and during normal
+operations you should never manually insert or store data in it.
+However, if you need to verify the write availability of a config
+server, you can insert a document into a test collection (after
+making sure that no collection of that name already exists):
+
+.. warning:: Modification of the ``config`` database on a functioning
+   system may lead to instability or inconsistent data sets. If you
+   must modify the ``config`` database, use :program:`mongodump` to
+   create a full backup of the ``config`` database.
+
+.. code-block:: javascript
+
+   db.testConfigServerWriteAvail.insert( { a : 1 } )
+
+If the operation succeeds, the config server is available to process
+writes.
+
+Future releases of the server may include different collections in
+the config database, so be careful when selecting a name for your
+test collection.
+
 Collections
 -----------