Merge pull request #6150 from rabbitmq/mergify/bp/v3.9.x/pr-6149

3.9.24 release notes (backport #6147) (backport #6148) (backport #6149)

release-notes/3.9.24.md

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+RabbitMQ `3.9.24` is a maintenance release in the `3.9.x` release series.
+
+Please refer to the **Upgrading to 3.9** section from [v3.9.0 release notes](https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.9.0) if upgrading from a version prior to 3.9.0.
+
+This release requires at least Erlang 24.2, and supports Erlang 25. [RabbitMQ and Erlang/OTP Compatibility Matrix](https://www.rabbitmq.com/which-erlang.html) has more details on Erlang version requirements for RabbitMQ.
+
+
+### Minimum Supported Erlang Version
+
+Erlang 23 support has reached its [end of support](https://www.rabbitmq.com/which-erlang.html).
+
+Starting with this release, RabbitMQ requires Erlang 24.2 or later versions. Nodes **will fail to start** on older Erlang releases.
+
+Erlang 25 as our new baseline means much improved performance on ARM64 architectures, [profiling with flame graphs](https://blog.rabbitmq.com/posts/2022/05/flame-graphs/)
+across all architectures, and the most recent TLS 1.3 implementation available to all RabbitMQ 3.11 users.
+
+### Open File Handle Limit is Capped by Default on Linux, *BSD, macOS
+
+Nodes now default to 65536 concurrent client connections instead of using the effective kernel open file handle limit.
+Users who want to override this default, that is, have nodes that should support more concurrent connections and open files,
+now have to perform an additional configuration step:
+
+ * Pick a new limit value they would like to use, for instance, 100K
+ * Set the maximum open file handle limit (for example, via `systemd` or similar tooling) for the OS user used by RabbitMQ to 100K
+ * Set the [`ERL_MAX_PORTS` environment variable](https://www.rabbitmq.com/configure.html#customise-environment) to 100K
+
+This change was introduced because of a change in several Linux distributions: they now use a default open file handle limit so high,
+they cause a significant (say, 1.5 GiB) memory preallocated the Erlang runtime.
+
+For example, [RHEL 9 and CentOS Stream 9](https://access.redhat.com/solutions/1479623) are examples of such distributions.
+The new default [comes from modern systemd](https://github.com/systemd/systemd/commit/a8b627aaed409a15260c25988970c795bf963812).
+
+See [rabbitmq/rabbitmq-server#5684](https://github.com/rabbitmq/rabbitmq-server/pull/5684) and [docker-library/rabbitmq#545](https://github.com/docker-library/rabbitmq/issues/545#issuecomment-1224977154)
+for details.
+
+
+## Changes Worth Mentioning
+
+Release notes can be found on GitHub at [rabbitmq-server/release-notes](https://github.com/rabbitmq/rabbitmq-server/tree/v3.9.x/release-notes).
+
+
+## Changes Worth Mentioning
+
+Release notes can be found on GitHub at [rabbitmq-server/release-notes](https://github.com/rabbitmq/rabbitmq-server/tree/v3.10.x/release-notes).
+
+### Core Server
+
+#### Bug Fixes
+
+  * Intra-cluster communication link metric collector could run into an exception when
+    peer connection has just been re-established, e.g. after a peer node restart.
+
+    GitHub issue: [#6127](https://github.com/rabbitmq/rabbitmq-server/pull/6127)
+
+### MQTT Plugin
+
+#### Bug Fixes
+
+  * When a node was [put into maintenance mode](https://www.rabbitmq.com/upgrade.html#maintenance-mode), it closed all MQTT client connections
+    cluster-wide instead of just local client connections.
+
+    GitHub issue: [#6108](https://github.com/rabbitmq/rabbitmq-server/pull/6108)
+
+
+## Dependency Upgrades
+
+None in this release.
+
+
+## Source Code Archives
+
+To obtain source code of the entire distribution, please download the archive named `rabbitmq-server-3.9.24.tar.xz`
+instead of the source tarball produced by GitHub.