Initial (very brief) 4.0.0 release notes

Author: michaelklishin

release-notes/4.0.0.md

@@ -0,0 +1,76 @@
+## RabbitMQ 4.0.0
+
+RabbitMQ `4.0.0` is a new major release.
+
+Starting June 1st, 2024, community support for this series will only be provided to [regularly contributing users](https://github.com/rabbitmq/rabbitmq-server/blob/main/COMMUNITY_SUPPORT.md) and those who hold a valid [commercial support license](https://tanzu.vmware.com/rabbitmq/oss).
+
+## Highlights
+
+Some key improvements in this release are listed below.
+
+ * [Khepri](https://www.youtube.com/watch?v=whVqpgvep90), an [alternative schema data store](https://github.com/rabbitmq/rabbitmq-server/pull/7206) developed to replace Mnesia,
+   has matured
+ * AMQP 1.0 is now a core protocol that is always enabled. Its plugin is now a no-op that only exists to simplify upgrades.
+ * The AMQP 1.0 implementation is now significantly more efficient: its peak throughput is [more than double than that of 3.13.x](https://github.com/rabbitmq/rabbitmq-server/pull/9022)
+   on some workloads
+ * [AMQP 1.0 clients now can manage topologies](https://github.com/rabbitmq/rabbitmq-server/pull/10559) similarly to how AMQP 0-9-1 clients do it
+ * The AMQP 1.0 convention (address format) used for interacting with with AMQP 0-9-1 entities [is now easier to reason about](https://github.com/rabbitmq/rabbitmq-server/pull/11618)
+ * Mirroring (replication) of classic queues [was removed](https://github.com/rabbitmq/rabbitmq-server/pull/9815) after several years of depracation. For replicated messaging data types,
+   use quorum queues and/or streams. Non-replicated classic queues remain and their development continues
+ * Classic queue [storage efficiency improvements](https://github.com/rabbitmq/rabbitmq-server/pull/11112), in particular recovery time and storage of multi-MiB messages
+ * Node with multiple enabled plugins little on disk data to recover now [start up to 20-30% faster](https://github.com/rabbitmq/rabbitmq-server/pull/10989)
+ * CQv1, [the original classic queue storage layer, was removed](https://github.com/rabbitmq/rabbitmq-server/pull/10656) except for the part that's necessary for upgrades
+ * Several I/O-related metrics are dropped, they should be [monitored at the infrastructure and kernel layers](https://www.rabbitmq.com/docs/monitoring#system-metrics)
+
+See Compatibility Notes below to learn about **breaking or potentially breaking changes** in this release.
+
+## Release Artifacts
+
+RabbitMQ releases are distributed via [GitHub](https://github.com/rabbitmq/rabbitmq-server/releases).
+[Debian](https://rabbitmq.com/install-debian.html) and [RPM packages](https://rabbitmq.com/install-rpm.html) are available via Cloudsmith mirrors.
+
+[Community Docker image](https://hub.docker.com/_/rabbitmq/), [Chocolatey package](https://community.chocolatey.org/packages/rabbitmq), and the [Homebrew formula](https://www.rabbitmq.com/docs/install-homebrew)
+are other installation options. They are updated with a delay.
+
+
+## Erlang/OTP Compatibility Notes
+
+This release [requires Erlang 26.2](https://www.rabbitmq.com/docs/which-erlang).
+
+[Provisioning Latest Erlang Releases](https://www.rabbitmq.com/docs/which-erlang#erlang-repositories) explains
+what package repositories and tools can be used to provision latest patch versions of Erlang 26.x.
+
+
+## Upgrading to 4.0
+
+### Documentation guides on upgrades
+
+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 `3.13.x`.
+
+This release requires **all feature flags** in the 3.x series (specifically `3.13.x`) to be enabled before upgrading,
+there is no upgrade path from 3.12.14 (or a later patch release) straight to `4.0.0`.
+
+### Required Feature Flags
+
+This release does not [graduate](https://www.rabbitmq.com/docs/feature-flags#graduation) any feature flags.
+
+However, all users are highly encouraged to enable all feature flags before upgrading to this release from
+3.13.x.
+
+### Mixed version cluster compatibility
+
+RabbitMQ 4.0.0 nodes can run alongside `3.13.x` nodes. `4.0.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 4.0.0, these irregularities will go away.
+
+Mixed version clusters are a mechanism that allows rolling upgrade and are not meant to be run for extended
+periods of time (no more than a few hours).
+
+### Recommended Post-upgrade Procedures
+
+TBD