mongodb · kay-kim · Oct 9, 2018 · Sep 4, 2018 · Sep 28, 2018
diff --git a/source/changeStreams.txt b/source/changeStreams.txt
@@ -101,6 +101,11 @@ You can open change streams against:
 
 .. _open-change-stream:
 
+.. important::
+
+   Change stream is only available if :readconcern:`"majority"` read
+   concern support is enabled (default).
+
 Open A Change Stream
 --------------------
 

diff --git a/source/core/transactions.txt b/source/core/transactions.txt
@@ -280,17 +280,20 @@ 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"
@@ -314,6 +317,16 @@ 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

diff --git a/source/includes/extracts-transactions.yaml b/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.
+
 ...
diff --git a/source/includes/fact-enable-majority-readConcern.rst b/source/includes/fact-enable-majority-readConcern.rst
@@ -1,2 +1,6 @@
 To use :term:`read concern` level of :readconcern:`"majority"`, replica
 sets must use :ref:`WiredTiger storage engine <storage-wiredtiger>`.
+
+For MongoDB 4.0.3+ (and 3.6.1+), you can disable read concern
+:readconcern:`"majority"`. For more information, see
+:ref:`4.0-disable-read-concern-majority`.
diff --git a/source/includes/options-conf.yaml b/source/includes/options-conf.yaml
@@ -1900,13 +1900,10 @@ replacement:
 program: conf
 name: replication.enableMajorityReadConcern
 directive: setting
-description: |
-
-   .. deprecated:: 3.6
-
-      Starting in MongoDB 3.6, :readconcern:`"majority"` read concern
-      is always enabled, and this setting has no effect.
-
+inherit:
+  name: enableMajorityReadConcern
+  program: mongod
+  file: options-mongod.yaml
 ---
 program: conf
 name: sharding.configsvrMode

diff --git a/source/includes/options-mongod.yaml b/source/includes/options-mongod.yaml
@@ -2203,13 +2203,38 @@ program: mongod
 name: enableMajorityReadConcern
 directive: option
 optional: true
-args: null
+default: true
 description: |
 
-   .. deprecated:: 3.6
+   Starting in MongoDB 3.6, MongoDB enables support for
+   :readconcern:`"majority"` read concern by default.
+
+   For MongoDB 4.0.3+ and 3.6.1+, you can disable read concern
+   :readconcern:`"majority"` to prevent the storage cache pressure from
+   immobilizing a deployment with a three-member
+   primary-secondary-arbiter (PSA) architecture. For more information
+   about disabling read concern :readconcern:`"majority"`, see
+   :ref:`4.0-disable-read-concern-majority`.
+
+   To disable, set {{role}} to false.
+
+   .. important::
+
+      In general, avoid disabling :readconcern:`"majority"` read concern
+      unless necessary. However, if you have a three-member replica set
+      with a primary-secondary-arbiter (PSA) architecture or a sharded
+      cluster with a three-member PSA shards, disable to prevent the
+      storage cache pressure from immobilizing the deployment.
+
+      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.
 
-      Starting in MongoDB 3.6, :readconcern:`"majority"` read concern
-      is always enabled, and this option has no effect.
+   {{role}} has no effect for MongoDB versions: 4.0.0, 4.0.1, 4.0.2, 3.6.0.
 
 ---
 program: mongod

diff --git a/source/includes/steps-change-replica-set-wiredtiger.yaml b/source/includes/steps-change-replica-set-wiredtiger.yaml
@@ -5,6 +5,7 @@ pre: |
 action:
   language: sh
   code: |
+    use admin
     db.shutdownServer()
 ---
 title: "Prepare a data directory for the new ``mongod`` running with WiredTiger."
@@ -21,19 +22,41 @@ pre: |
 ---
 title: "Start ``mongod`` with WiredTiger."
 ref: change-wt-repl-sync-start-mongod-w-wiredtiger
-pre: |
+content: |
    Start :binary:`~bin.mongod`, specifying ``wiredTiger`` as the
    :option:`--storageEngine <mongod --storageEngine>` and the prepared data directory for
    WiredTiger as the :option:`--dbpath <mongod --dbpath>`. Specify additional options,
    such as :option:`--bind_ip <mongod --bind_ip>`, as appropriate for this replica set
    member.
 
    .. include:: /includes/warning-bind-ip-security-considerations.rst
+
+   .. tabs::
+
+      tabs:
+         - id: general
+           name: General Use (For Most Architectures)
+           content: |
+             .. code-block:: sh
+
+                mongod --storageEngine wiredTiger --dbpath <newWiredTigerDBPath> --replSet <replSetName> --bind_ip localhost,<hostname(s)|ip address(es)>
+
+
+         - id: psa36
+           name: PSA Architecture
+           content: |
+
+             .. important::
+
+                If you are running a three-member PSA architecture and
+                MongoDB version 4.0.3+, include
+                :option:`--enableMajorityReadConcern false <mongod
+                --enableMajorityReadConcern>` to disable read concern ``majority``. |seemore|
+
+             .. code-block:: sh
+
+                mongod --storageEngine wiredTiger --dbpath <newWiredTigerDBPath> --replSet <replSetName> --bind_ip localhost,<hostname(s)|ip address(es)> --enableMajorityReadConcern false
 
-action:
-  language: sh
-  code: |
-    mongod --storageEngine wiredTiger --dbpath <newWiredTigerDBPath> --replSet <replSetName> --bind_ip localhost,<hostname(s)|ip address(es)>
 post: |
   Since no data exists in the ``--dbpath``, the ``mongod`` will perform an
   :doc:`initial sync </tutorial/resync-replica-set-member>`. The length of the

diff --git a/source/reference/configuration-options.txt b/source/reference/configuration-options.txt
@@ -724,7 +724,7 @@ LDAP Parameters
       oplogSizeMB: <int>
       replSetName: <string>
       secondaryIndexPrefetch: <string>
-      enableMajorityReadConcern: <boolean>  ## Deprecated in 3.6
+      enableMajorityReadConcern: <boolean>  
 
 .. include:: /includes/option/setting-conf-replication.oplogSizeMB.rst
 

diff --git a/source/reference/method/Mongo.watch.txt b/source/reference/method/Mongo.watch.txt
@@ -73,6 +73,12 @@ Full Document Lookup of Update Operations
 
 .. include:: /includes/extracts/changestream-full-document-lookup.rst
 
+Availability
+~~~~~~~~~~~~
+
+Change stream is only available if :readconcern:`"majority"` read
+concern support is enabled (default).
+
 Access Control
 --------------
 

diff --git a/source/reference/method/db.collection.watch.txt b/source/reference/method/db.collection.watch.txt
@@ -52,6 +52,11 @@ Behavior
   - For a sharded cluster, you must issue :method:`db.watch()` on a
     :binary:`~bin.mongos` instance.
 
+Availability
+~~~~~~~~~~~~
+
+Change stream is only available if :readconcern:`"majority"` read
+concern support is enabled (default).
 
 Resumability
 ~~~~~~~~~~~~

diff --git a/source/reference/method/db.watch.txt b/source/reference/method/db.watch.txt
@@ -79,6 +79,12 @@ Full Document Lookup of Update Operations
 
 .. include:: /includes/extracts/changestream-full-document-lookup.rst
 
+Availability
+~~~~~~~~~~~~
+
+Change stream is only available if :readconcern:`"majority"` read
+concern support is enabled (default).
+
 Access Control
 --------------
 

diff --git a/source/reference/read-concern-local.txt b/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
 -----------------------------------------
 

diff --git a/source/reference/read-concern-majority.txt b/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
 --------------------------------------------
 
@@ -122,4 +123,46 @@ Read Your Own Writes
 
 .. include:: /includes/fact-read-own-writes.rst
 
+.. _4.0-disable-read-concern-majority:
+
+Disable Read Concern Majority
+-----------------------------
+
+*For 3-Member Primary-Secondary-Arbiter Architecture*
+
+For MongoDB 4.0.3+ (and 3.6.1+ [#]_), you can disable read concern
+:readconcern:`"majority"` if you have a three-member replica set with a
+primary-secondary-arbiter (PSA) architecture or a sharded cluster with
+a three-member PSA shards.
+
+With a three-member PSA architecture, the cache pressure will increase
+if any data bearing node is down. To prevent the storage cache pressure
+from immobilizing a deployment with a PSA architecture, you can disable
+read concern by setting either:
+
+- :option:`--enableMajorityReadConcern <mongod
+  --enableMajorityReadConcern>` command line option to ``false``.
+
+- :setting:`replication.enableMajorityReadConcern` configuration file
+  setting to ``false``.
+
+.. important::
+
+   In general, avoid disabling :readconcern:`"majority"` read concern
+   unless necessary. However, if you have a three-member replica set
+   with a primary-secondary-arbiter (PSA) architecture or a sharded
+   cluster with a three-member PSA shards, disable to prevent the
+   storage cache pressure from immobilizing the deployment.
+
+   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
+   </reference/read-concern-majority/#disable-read-concern-majority>`.