Some key improvements in this release are listed below.
### Initial Support for AMQP 1.0 Filter Expressions
Support for the `properties` and `appliation-properties` filters of [AMQP Filter Expressions Version 1.0 Working Draft 09](https://groups.oasis-open.org/higherlogic/ws/public/document?document_id=66227).
### Feature Flags Quality of Life Improvements
Graduated (mandatory) [feature flags](https://www.rabbitmq.com/docs/feature-flags) several minors ago has proven that they could use some user experience improvements.
For example, certain required feature flags will now be enabled on node boot when all nodes in the cluster support them.
See core server changes below as well as the [GitHub project dedicated to feature flags improvements](https://github.com/orgs/rabbitmq/projects/4/views/1)
for the complete list of related changes.
## Breaking Changes and Compatibility Notes
### MQTT
* The default MQTT [Maximum Packet Size](https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901086) changed from 256 MiB to 16 MiB.
This default can be overridden by [configuring](https://www.rabbitmq.com/docs/configure#config-file) `mqtt.max_packet_size_authenticated`.
Note that this value must not be greater than `max_message_size` (which also defaults to 16 MiB).
See the [Upgrading guide](https://www.rabbitmq.com/docs/upgrade) for documentation on upgrades and [GitHub releases](https://github.com/rabbitmq/rabbitmq-server/releases)
for release notes of individual releases.
This release series only supports upgrades from `4.0.x`.
[Blue/Green Deployment](https://www.rabbitmq.com/docs/blue-green-upgrade)-style upgrades are avaialble for migrations from 3.12.x and 3.13.x series
to `4.1.x`.
### Required Feature Flags
None/TBD.
### Mixed version cluster compatibility
RabbitMQ 4.1.0 nodes can run alongside `4.0.x` nodes. `4.1.x`-specific features can only be made available when all nodes in the cluster
upgrade to 4.0.0 or a later patch release in the new series.
While operating in mixed version mode, some aspects of the system may not behave as expected. The list of known behavior changes will be covered in future updates.
Once all nodes are upgraded to 4.1.0, these irregularities will go away.
Mixed version clusters are a mechanism that allows rolling upgrade and are not meant to be run for extended
*`properties` and `appliation-properties` filters of [AMQP Filter Expressions Version 1.0 Working Draft 09](https://groups.oasis-open.org/higherlogic/ws/public/document?document_id=66227)
when consuming from a stream via AMQP 1.0. String prefix and suffix matching is also supported.
This feature adds the ability to RabbitMQ to have multiple concurrent clients each consuming only a subset of messages while maintaining message order.
It also reduces network traffic between RabbitMQ and clients by only dispatching those messages that the clients are actually interested in.
* AMQP 1.0 connections that use OAuth 2.0 now can renew their JWT tokens
This allows clients to set a new token proactively before the current one [expires](/docs/oauth2#token-expiration), ensuring uninterrupted connectivity.
If a client does not set a new token before the existing one expires, RabbitMQ will automatically close the AMQP 1.0 connection.
* Support for Multiple Routing Keys in AMQP 1.0 via `x-cc` Message Annotation.
AMQP 1.0 publishers now can set multiple routing keys by using the `x-cc` message annotation.
This annotation allows publishers to specify a [list](https://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-types-v1.0-os.html#type-list)
of routing keys ([strings](https://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-types-v1.0-os.html#type-string)) for more flexible message distribution,
similar to the [CC](https://www.rabbitmq.com/docs/sender-selected) header in AMQP 0.9.1.
This feature allows operators to gain insights into the message sizes being published to RabbitMQ,
such as average message size, number of messages per pre-defined bucket (which can both be computed accurately), and percentiles (which will be approximated).
Each metric is labelled by protocol (AMQP 1.0, AMQP 0.9.1, MQTT 5.0, MQTT 3.1.1, and MQTT 3.1).
* The `rabbitmq_event_exchange` plugin now can be configured to internally publish AMQP 1.0 instead of AMQP 0.9.1 messages to the `amq.rabbitmq.event` topic exchange.
This allows AMQP 1.0 consumers to receive event properties containing complex types such as [lists](https://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-types-v1.0-os.html#type-list)
or [maps](https://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-types-v1.0-os.html#type-map), for example queue arguments for the `queue.created`
event or client provided properties for the `connection.created` event.
