DOCS-13389 Doc for the oplogFetcherUsesExhaust parameter

Author: kanchana-mongodb

source/core/replica-set-sync.txt

@@ -202,7 +202,7 @@ Replication
 Secondary members replicate data continuously after the initial sync.
 Secondary members copy the :doc:`oplog </core/replica-set-oplog>` from
 their *sync from* source and apply these operations in an asynchronous
-process. [#slow-oplogs]_
+process. [#slow-oplogs]_ 
 
 Secondaries may automatically change their *sync from* source as needed
 based on changes in the ping time and state of other members'
@@ -213,6 +213,33 @@ for more information on sync source selection criteria.
 
    .. include:: /includes/extracts/4.2-changes-slow-oplog-log-message-footnote.rst
 
+.. _replica-set-streaming-replication:
+
+Streaming Replication 
+~~~~~~~~~~~~~~~~~~~~~
+
+Starting in MongoDB 4.4, *sync from* sources send a continuous stream 
+of :doc:`oplog </core/replica-set-oplog>` entries to their syncing 
+secondaries. Streaming replication mitigates replication lag in 
+high-load and high-latency networks. It also: 
+
+- Reduces staleness for reads from secondaries.
+- Reduces risk of losing write operations with :ref:`w: 1 <wc-w>` due to 
+  primary failover.
+- Reduces latency on write operations with :writeconcern:`w: "majority" 
+  <"majority">` and :ref:`w: >1 <wc-w>` (that is, any write concern that 
+  requires waiting for replication).
+
+Prior to MongoDB 4.4, secondaries fetched batches of :doc:`oplog 
+</core/replica-set-oplog>` entries by issuing a request to their *sync 
+from* source and waiting for a response. This required a network roundtrip 
+for each batch of :doc:`oplog </core/replica-set-oplog>` entries. MongoDB 
+4.4 adds the :parameter:`oplogFetcherUsesExhaust` startup parameter for 
+disabling streaming replication and using the older replication behavior. 
+Set the :parameter:`oplogFetcherUsesExhaust` parameter to ``false`` only if 
+there are any resource constraints on the *sync from* source or if you wish 
+to limit MongoDB's usage of network bandwidth for replication.
+
 .. _replica-set-internals-multi-threaded-replication:
 
 Multithreaded Replication

source/reference/parameters.txt

@@ -2106,6 +2106,30 @@ Replication Parameters
    configuration file setting or the 
    :option:`--setParameter <mongod --setParameter>` command line option.
 
+.. parameter:: oplogFetcherUsesExhaust
+
+   .. versionadded:: 4.4
+
+   |mongod-only|
+
+   *Type*: boolean
+
+   *Default*: true 
+
+   Enables or disables :ref:`streaming replication 
+   <replica-set-streaming-replication>`. Set the value to ``true`` to enable 
+   streaming replication. 
+   
+   Set the value to ``false`` to disable streaming replication. If 
+   disabled, secondaries fetch batches of :doc:`oplog </core/replica-set-oplog>` 
+   entries by issuing a request to their *sync from* source and waiting for a 
+   response. This requires a network roundtrip for each batch of :doc:`oplog 
+   </core/replica-set-oplog>` entries.
+
+   You can only set this parameter on startup, using either the 
+   :setting:`setParameter` configuration file setting or the 
+   :option:`--setParameter <mongod --setParameter>` command line option.
+
 .. parameter:: oplogInitialFindMaxSeconds
 
    .. versionadded:: 3.6

source/release-notes/4.4.txt

@@ -204,6 +204,22 @@ synchronization process from the beginning.
 Prior to MongoDB 4.4, the secondary would restart the entire initial
 sync if it encountered an error during the process.
 
+Streaming Replication 
+~~~~~~~~~~~~~~~~~~~~~
+
+Starting in MongoDB 4.4, *sync from* sources send a continuous 
+stream of :doc:`oplog </core/replica-set-oplog>` entries to their 
+syncing secondaries. 
+
+Prior to MongoDB 4.4, secondaries fetched batches of :doc:`oplog 
+</core/replica-set-oplog>` entries by issuing a request to their *sync 
+from* source and waiting for a response. This required a network roundtrip 
+for each batch of :doc:`oplog </core/replica-set-oplog>` entries. MongoDB 
+4.4 adds the :parameter:`oplogFetcherUsesExhaust` startup parameter for 
+disabling streaming replication and using the older replication behavior.
+
+For details, see :ref:`replica-set-streaming-replication`.
+
 Rollback Directory
 ~~~~~~~~~~~~~~~~~~