Merge pull request #97 from norareidy/DOCSP-48660-causal-consistency-edits

DOCSP-48660: Causal consistency edits

Author: norareidy

dbx/causal-consistency.rst

@@ -1,11 +1,29 @@
-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
+MongoDB enables **causal consistency** in certain client
+sessions. The causal consistency model guarantees that in a
+distributed system, 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.
 
+To guarantee causal consistency, client sessions must fulfill the
+following requirements:
+
+- When starting a session, the driver must enable the causal consistency
+  option. This option is enabled by default.
+
+- Operations must run in a single session on a single thread. Otherwise,
+  the sessions or threads must communicate the operation time and cluster
+  time values to each other. To view an example of two sessions that communicate
+  these values, see the :manual:`Causal Consistency examples </core/read-isolation-consistency-recency/#examples>`
+  in the {+mdb-server+} manual.
+
+- You must use a |majority-rc| read concern.
+
+- You must use a |majority-wc| write concern. This is the default write concern
+  value.
+  
 The following table describes the guarantees that causally
 consistent sessions provide:
 
@@ -24,27 +42,21 @@ consistent sessions provide:
        a preceding read operation.
 
    * - Monotonic writes
-     - If a write operation must precede other write operations, the driver
+     - If a write operation must precede other write operations, the server
        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 
+       |update-one-method| to modify the inserted document, the server runs the 
        insert operation first.
 
    * - Writes follow reads
-     - If a write operation must follow other read operations, the driver runs
+     - If a write operation must follow other read operations, the server 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
+       |delete-one-method| to delete the retrieved document, the server 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