DOCSP-46686: Causal consistency (#188)

Author: mcmorisi

source/write/transactions.txt

@@ -28,8 +28,7 @@ all data changes before they ever become visible.
 
 In MongoDB, transactions run within logical **sessions**. A
 session is a grouping of related read or write operations that you intend to run 
-sequentially. Sessions enable **causal consistency** for a
-group of operations and allow you to run operations in an
+sequentially. Sessions allow you to run operations in an
 **ACID-compliant transaction**, which is a transaction that meets an expectation
 of atomicity, consistency, isolation, and durability. MongoDB guarantees that the 
 data involved in your transaction operations remains consistent, even if the 
@@ -47,6 +46,64 @@ creating a new client each time.
    ``ClientSession`` with a different ``MongoClient`` results in operation
    errors.
 
+Causal Consistency
+~~~~~~~~~~~~~~~~~~
+
+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()`` to insert a document, then call
+       ``update_one()`` 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()`` to retrieve a document, then call
+       ``delete_one()`` 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`` read concern
+- Write operations that have a ``majority`` 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/>`
+
 Sample Data
 ~~~~~~~~~~~