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DOCS-11128: rephrase afterClusterTime reference in read concern docs #3180

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Dec 22, 2017
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DOCS-11128: rephrase afterClusterTime reference in read concern docs #3180

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18 changes: 10 additions & 8 deletions source/reference/read-concern.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -134,18 +134,20 @@ The following read concern levels are available:
.. versionadded:: 3.6

MongoDB introduces the ``afterClusterTime`` option that can be set by
the drivers. Read operations with a specified ``afterClusterTime``
return data that meets the level requirement and the specified after
cluster time requirement.
the drivers.

Read operations with a specified ``afterClusterTime``
return data that meet both the :ref:`read concern level <read-concern-levels>`
requirement and the specified ``afterClusterTime`` requirement.

.. important::

Do not manually set the ``afterClusterTime``. MongoDB drivers set
this value automatically for operations associated with
:ref:`causally consistent sessions <causal-consistency>`.
causally consistent sessions.

:ref:`Causally consistent sessions<causal-consistency>` use
``afterClusterTime`` to provide :ref:`causal consistency <causal-consistency>`.
Causally consistent sessions use the ``afterClusterTime`` value to
provide causal consistency.

``afterClusterTime`` is available for :readconcern:`"local"` (default)
and :readconcern:`"majority"` read concern levels:
Expand All @@ -155,8 +157,8 @@ and :readconcern:`"majority"` read concern levels:
readConcern: { level: <"majority"|"local"> , afterClusterTime: <Timestamp> }

For read operations not associated with causally consistent sessions,
i.e. the ``afterClusterTime`` is unset, reads against secondary members
have the default read concern level :readconcern:`"available"`.
i.e. where the ``afterClusterTime`` is unset, reads against secondary members
have the default read concern level: :readconcern:`"available"`.

.. _read-concern-storage-engine-drivers:

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