Merge pull request #92 from norareidy/causal-consistency-shared

Add duplicate causal consistency info

Author: norareidy

dbx/causal-consistency.rst

@@ -0,0 +1,54 @@
+MongoDB enables **causal consistency** in client sessions.
+The causal consistency model guarantees that operations within a session
+run in a causal order. Clients observe results that are consistent
+with the causal relationships, or the dependencies between
+operations. For example, if you perform a series of operations where
+one operation logically depends on the result of another, any subsequent
+reads reflect the dependent relationship.
+
+The following table describes the guarantees that causally
+consistent sessions provide:
+
+.. list-table::
+   :widths: 40 60
+   :header-rows: 1
+
+   * - Guarantee
+     - Description
+
+   * - Read your writes
+     - Read operations reflect the results of preceding write operations.
+
+   * - Monotonic reads
+     - Read operations do not return results that reflect an earlier data state than
+       a preceding read operation.
+
+   * - Monotonic writes
+     - If a write operation must precede other write operations, the driver
+       runs this write operation first.
+
+       For example, if you call |insert-one-method| to insert a document, then call
+       |update-one-method| to modify the inserted document, the driver runs the 
+       insert operation first.
+
+   * - Writes follow reads
+     - If a write operation must follow other read operations, the driver runs
+       the read operations first.
+
+       For example, if you call |find-one-method| to retrieve a document, then call
+       |delete-one-method| to delete the retrieved document, the driver runs the find
+       operation first.
+
+In a causally consistent session, MongoDB ensures a
+causal relationship between the following operations:
+
+- Read operations that have a |majority-rc| read concern
+- Write operations that have a |majority-wc| write concern
+
+.. tip::
+
+   To learn more about the concepts mentioned in this section, see the 
+   following {+mdb-server+} manual entries:
+
+   - :manual:`Causal Consistency </core/read-isolation-consistency-recency/#causal-consistency>`
+   - :manual:`Causal Consistency and Read and Write Concerns </core/causal-consistency-read-write-concerns/>`