DOCSP-15941 default read concern is always local

Author: jmd-mongo

source/includes/fact-readConcern-option-description.rst

@@ -1,14 +1,12 @@
 Possible read concern levels are:
 
 - :readconcern:`"local"`. This is the default read concern level for
-  read operations against primary and read operations against
-  secondaries when associated with :ref:`causally consistent sessions
-  <causal-consistency>`.
+  read operations against the primary and secondaries.
 
-- :readconcern:`"available"`. This is the default for reads against
-  secondaries when when not associated with :ref:`causally consistent
-  sessions <causal-consistency>`. The query returns the instance's most
-  recent data.
+- :readconcern:`"available"`. Available for read operations against
+  the primary and secondaries. :readconcern:`"available"` behaves the 
+  same as :readconcern:`"local"` against the primary and non-sharded
+  secondaries. The query returns the instance's most recent data.
 
 - :readconcern:`"majority"`. Available for replica sets that use
   :ref:`WiredTiger storage engine <storage-wiredtiger>`.

source/reference/mongodb-defaults.txt

@@ -49,8 +49,7 @@ follows:
           - This read concern :red:`does not` guarantee :ref:`causal
             consistency <sessions>`.
 
-   * - | Reads against secondaries if the reads are 
-       | associated with :ref:`causally consistent sessions <sessions>`.
+   * - | Reads against secondaries.
 
      - :readconcern:`"local"`
 
@@ -63,22 +62,6 @@ follows:
             consistency <sessions>`.
 
 
-   * - | Reads against secondaries if the reads are :red:`not`
-       | associated with :ref:`causally consistent sessions <sessions>`.
-
-     - :readconcern:`"available"`
-
-       .. note::
-
-          - This read concern can return data that may be rolled
-            back.
-
-          - This read concern :red:`does not` guarantee :ref:`causal
-            consistency <sessions>`.
-
-          - For sharded collections, this read concern can also return
-            :red:`orphaned documents`.
-
 .. _mongodb-default-rc-outside-transactions:
 
 .. _mongodb-default-rc-txns:

source/reference/read-concern-local.txt

@@ -17,12 +17,8 @@ A query with read concern ``"local"`` returns data from the instance
 with no guarantee that the data has been written to a majority of the
 replica set members (i.e. may be rolled back).
 
-Read concern ``"local"`` is the default for:
-
-- read operations against primary
-
-- read operations against secondaries if the reads are associated with
-  :ref:`causally consistent sessions <sessions>`.
+Read concern ``"local"`` is the default for read operations against the
+primary and secondaries.
 
 .. include:: /includes/fact-readConcern-most-recent-data-in-node.rst
 

source/reference/read-concern.txt

@@ -54,19 +54,7 @@ The following read concern levels are available:
        the data has been written to a majority of the replica set
        members (i.e. may be rolled back).
 
-       Default for:
-          - reads against primary
-
-          - reads against secondaries if the reads are
-            associated with :ref:`causally consistent sessions <sessions>`. 
-
-            .. note::
-
-               Causally consistent client sessions only guarantee
-               causal consistency if the associated read operations use
-               :readconcern:`"majority"` read concern and the
-               associated write operations use
-               :writeconcern:`"majority"` write concern.
+       Default for reads against the primary and secondaries.
 
        **Availability:** Read concern :readconcern:`"local"` is available
        for use with or without causally consistent sessions and
@@ -81,9 +69,6 @@ The following read concern levels are available:
        that the data has been written to a majority of the replica set
        members (i.e. may be rolled back).
 
-       **Default for:** reads against secondaries if the reads are :red:`not`
-       associated with :ref:`causally consistent sessions <sessions>`.
-
        **Availability:** Read concern :readconcern:`"available"` is
        :red:`unavailable for use` with causally consistent sessions and
        transactions.

source/release-notes/5.0-compatibility.txt

@@ -166,6 +166,20 @@ Read Concern ``snapshot`` on Capped Collections
 
 .. include:: /includes/extracts/transactions-capped-collection-read-change.rst
 
+
+``local`` is the Default Read Concern
+-------------------------------------
+
+Starting in MongoDB 5.0, :readconcern:`"local"` is the default read 
+concern level for read operations against the primary and secondaries.
+
+This may introduce a significant latency increase for count queries that 
+use a filter and for :ref:`covered queries <covered-queries>`.
+
+You can opt out of this behavior by setting the cluster-wide 
+:ref:`read concern <read-concern>` with 
+:dbcommand:`setDefaultRWConcern`.
+
 Update Operator Changes
 -----------------------