DOCSP-23350 write-blocking note (#52)

* DOCSP-23350 write-blokcing note

* staging updates

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Author: Dave Cuthbert

source/reference/mongosync.txt

@@ -61,23 +61,26 @@ Global Options
    For more information on connecting ``mongosync``, see
    :ref:`Connections <c2c-connecting>`.
 
-.. option:: --verbosity <level>
+.. option:: --config <filename>
 
-   *Default*: ``INFO``
+   Sets the path to the configuration file.
 
-   Sets the verbosity level to use in log messages.
-   {+c2c-product-name+} logs all messages at the specified level and
-   any messages at lower levels.
+   For more information, see :ref:`c2c-mongosync-config`.
 
-   The ``--verbosity`` option supports the following values:
+.. option:: --help, h
 
-   - ``TRACE``
-   - ``DEBUG``
-   - ``INFO``
-   - ``WARN``
-   - ``ERROR``
-   - ``FATAL``
-   - ``PANIC``
+   Prints usage information to stdout.
+
+.. option:: --id <ID>
+
+   Sets an identifier for the ``mongosync`` instance.
+
+   Use this option when running multiple instances of ``mongosync`` on
+   a sharded cluster, to synchronize the shards individually. 
+   
+   The ``--id`` option must correspond to the shard ID of the shard it
+   syncs.  To find the shard ID, use the :dbcommand:`listShards`
+   command.
 
 .. option:: --logPath <DIR>
 
@@ -91,31 +94,28 @@ Global Options
    Sets the port used by the HTTP server for the {+c2c-product-name+}
    HTTP API.
 
-.. option:: --id <ID>
-
-   Sets an identifier for the ``mongosync`` instance.
+.. option:: --verbosity <level>
 
-   Use this option when running multiple instances of ``mongosync`` on
-   a sharded cluster, to synchronize the shards individually. 
-   
-   The ``--id`` option must correspond to the shard ID of the shard it
-   syncs.  To find the shard ID, use the :dbcommand:`listShards`
-   command.
+   *Default*: ``INFO``
 
-.. option:: --config <filename>
+   Sets the verbosity level to use in log messages.
+   {+c2c-product-name+} logs all messages at the specified level and
+   any messages at lower levels.
 
-   Sets the path to the configuration file.
+   The ``--verbosity`` option supports the following values:
 
-   For more information, see :ref:`c2c-mongosync-config`.
+   - ``TRACE``
+   - ``DEBUG``
+   - ``INFO``
+   - ``WARN``
+   - ``ERROR``
+   - ``FATAL``
+   - ``PANIC``
 
 .. option:: --version, -v
 
    Prints ``mongosync`` version information to stdout.
 
-.. option:: --help, h
-
-   Prints usage information to stdout.
-
 Behavior
 --------
 
@@ -182,6 +182,29 @@ show an empty ``system.views`` collection in that database. The empty
 ``system.views`` collection will not change the accuracy of user
 data on the destination.
 
+Write Blocking
+~~~~~~~~~~~~~~
+
+By default, write-blocking is not enabled during synchronization. When
+write-blocking is enabled, ``mongosync`` blocks writes:
+
+- On the destination cluster during replication
+- On the source cluster while committing
+
+To enable write-blocking, use the :ref:`start API <c2c-api-start>`
+to set ``enableUserWriteBlocking`` to ``true``.
+
+:ref:`Reverse synchronization <c2c-api-reverse>` requires
+write-blocking. To enable reverse synchronization between the source
+and destination clusters, use the ``start API`` to initiate replication
+with ``reversible`` and ``enableUserWriteBlocking`` set to ``true``.
+The options must be set when replication begins, they cannot be updated
+later.
+
+To set ``enableUserWriteBlocking``, the ``mongosync`` user must have a
+role that includes the ``setUserWriteBlockMode`` and
+``bypassWriteBlockingMode`` ActionTypes.
+
 Examples
 --------