ODCS-12081: blurb about txns and disabling majority read concern

Author: kay-kim

source/core/transactions.txt

@@ -273,17 +273,20 @@ Read Concern
 
 Multi-document transactions support read concern
 :readconcern:`"snapshot"`, :readconcern:`"local"`, and
-:readconcern:`"majority"`:
+:readconcern:`"majority"` :
 
 - For :readconcern:`"local"` and :readconcern:`"majority"` read
   concern, MongoDB may sometimes substitute a stronger read concern.
+  Specifically, in MongoDB 4.0, all multi-documents transactions have
+  "snapshot" isolation. For details, see :ref:`readconcern-local-txn`
+  and :ref:`readconcern-majority-txn`.
 
 - For :readconcern:`"majority"` read concern, if the transaction
   commits with :ref:`write concern "majority"
   <transactions-write-concern>`, transaction operations are guaranteed
   to have read majority-committed data. Otherwise, the
   :readconcern:`"majority"` read concern provides no guarantees that
-  read operations read majority-committed data.
+  read operations read majority-committed data. [#psa]_
 
 - For :readconcern:`"snapshot"` read concern, if the transaction
   commits with :ref:`write concern "majority"
@@ -307,6 +310,16 @@ If unspecified at the transaction start, transactions use the
 session-level read concern or, if that is unset, the client-level read
 concern.
 
+.. [#psa]
+
+   For a three-member Primary-Secondary-Arbiter (PSA) replica set
+   architecture, you may have :ref:`disabled read concern "majority"
+   <4.0-disable-read-concern-majority>` to avoid cache pressure.
+   Disabling :readconcern:`"majority"` does not affect transactions;
+   i.e. you can specify read concern :readconcern:`"majority"` for
+   transactions even if read concern :readconcern:`"majority"` is
+   disabled.
+
 .. _transactions-write-concern:
 
 Write Concern

source/includes/extracts-transactions.yaml

@@ -141,13 +141,19 @@ content: |
 ---
 ref: transactions-read-concern-tip
 content: |
-   In MongoDB 4.0, all multi-documents transactions have ``snapshot`` isolation. In
-   future versions of MongoDB, the server will optimize around your specified read
-   concern (isolation level).  
-
-   For :readconcern:`"local"` and :readconcern:`"majority"` read concern, MongoDB
-   may provide stronger isolation guarantees than specified. If stronger guarantees
-   are needed, be explicit as future versions of the server may include
-   optimizations that remove stronger isolation guarantees than explicitly
-   specified.
+
+   For :readconcern:`"local"` and :readconcern:`"majority"` read
+   concern, MongoDB may provide stronger isolation guarantees than
+   specified. Specifically, in MongoDB 4.0, all multi-documents
+   transactions have :readconcern:`"snapshot"` isolation.
+
+   In future versions of MongoDB, the server will optimize around the
+   specified read concern (isolation level) instead of providing
+   stronger isolation guarantees than explicitly specified.
+   
+   As such, if stronger guarantees than :readconcern:`"local"` and
+   :readconcern:`"majority"` read concern are needed, explicitly
+   specify the appropriate read concern instead of relying on MongoDB
+   to promote the read concern level.
+
 ...

source/includes/options-mongod.yaml

@@ -2214,6 +2214,11 @@ description: |
       Disabling :readconcern:`"majority"` read concern disables support
       for :doc:`/changeStreams`.
 
+      Disabling :readconcern:`"majority"` does not affect
+      :doc:`multi-document transactions </core/transactions>`; i.e. you
+      can specify read concern "majority" for multi-document
+      transactions even if read concern "majority" is disabled.
+
    {{role}} has no effect for MongoDB versions: 4.0.0, 4.0.1, 4.0.2, 3.6.0.
 
 ---

source/reference/read-concern-local.txt

@@ -32,6 +32,8 @@ Causally Consistent Sessions
 Read concern :readconcern:`local` is available for use with causally
 consistent sessions.
 
+.. _readconcern-local-txn:
+
 Read Concern ``"local"`` and Transactions
 -----------------------------------------
 

source/reference/read-concern-majority.txt

@@ -31,7 +31,6 @@ data read in transactions.
 
 .. include:: /includes/fact-readConcern-most-recent-data-in-node.rst
 
-
 Example
 -------
 
@@ -96,6 +95,8 @@ Causally Consistent Sessions
 Read concern :readconcern:`majority` is available for use with causally
 consistent sessions.
 
+.. _readconcern-majority-txn:
+
 Read Concern ``"majority"`` and Transactions
 --------------------------------------------
 
@@ -156,6 +157,11 @@ read concern by setting either:
    Disabling :readconcern:`"majority"` read concern disables support
    for :doc:`/changeStreams`.
 
+   Disabling :readconcern:`"majority"` does not affect
+   :doc:`multi-document transactions </core/transactions>`; i.e. you
+   can specify read concern :readconcern:`"majority"` for transactions
+   even if read concern :readconcern:`"majority"` is disabled.
+
 .. [#]
 
    For MongoDB 3.6 discussion, see :v3.6:`Disable Read Concern Majority