release-notes/4.2.0: Mention Khepri enabled by default

Author: dumbbell

release-notes/4.2.0.md

@@ -18,3 +18,33 @@ Two new optional built-in interceptors were added to RabbitMQ:
 2. Setting client ID of publishing MQTT client
 
 Detailed information can be found in the [Message Interceptor](https://www.rabbitmq.com/docs/next/message-interceptors) documentation.
+
+### Khepri enabled by default
+
+RabbitMQ supports two databases to [store
+metadata](https://www.rabbitmq.com/docs/metadata-store) such as virtual hosts,
+topology, runtime parameters, policies, internal users and so on: Mnesia and
+Khepri. That metadata store is also at the heart of clustering in RabbitMQ. As
+of RabbitMQ 4.2.0, **Khepri is the default metadata store for new
+deployments**.
+
+[Khepri](https://www.rabbitmq.com/docs/metadata-store#khepri) is based on the
+same Raft consensus algorithm used by quorum queues and streams. The goal is to
+have a consistent well defined behaviour around all queries and updates of
+metadata across an entire cluster, especially when the cluster suffers
+increased latency or network issues for instance. It also comes with increased
+performance in several use cases, even though this was not a goal.
+
+A new RabbitMQ 4.2.0+ node will use Khepri by default. If you upgrade an
+existing node or cluster, it will continue to use whatever metadata store it
+was using so far.
+
+If you did not enable Khepri yet, [it is recommended you enable
+it](https://www.rabbitmq.com/docs/metadata-store/how-to-enable-khepri):
+
+```
+rabbitmqctl enable_feature_flag khepri_db
+```
+
+Khepri will become mandatory in a future minor version. Mnesia support will be
+dropped in a future major version. These exact versions are to be decided.