nixos/apache-kafka: Add manual chapter

nixos/modules/services/misc/apache-kafka.nix

@@ -214,6 +214,10 @@ in {
         SuccessExitStatus = "0 143";
       };
     };
-
   };
+
+  meta.doc = ./kafka.md;
+  meta.maintainers = with lib.maintainers; [
+    srhb
+  ];
 }

nixos/modules/services/misc/kafka.md

@@ -0,0 +1,63 @@
+# Apache Kafka {#module-services-apache-kafka}
+
+[Apache Kafka](https://kafka.apache.org/) is an open-source distributed event
+streaming platform
+
+## Basic Usage {#module-services-apache-kafka-basic-usage}
+
+The Apache Kafka service is configured almost exclusively through its
+[settings](#opt-services.apache-kafka.settings) option, with each attribute
+corresponding to the [upstream configuration
+manual](https://kafka.apache.org/documentation/#configuration) broker settings.
+
+## KRaft {#module-services-apache-kafka-kraft}
+
+Unlike in Zookeeper mode, Kafka in
+[KRaft](https://kafka.apache.org/documentation/#kraft) mode requires each log
+dir to be "formatted" (which means a cluster-specific a metadata file must
+exist in each log dir)
+
+The upstream intention is for users to execute the [storage
+tool](https://kafka.apache.org/documentation/#kraft_storage) to achieve this,
+but this module contains a few extra options to automate this:
+
+- [](#opt-services.apache-kafka.clusterId)
+- [](#opt-services.apache-kafka.formatLogDirs)
+- [](#opt-services.apache-kafka.formatLogDirsIgnoreFormatted)
+
+## Migrating to settings {#module-services-apache-kafka-migrating-to-settings}
+
+Migrating a cluster to the new `settings`-based changes requires adapting removed options to the corresponding upstream settings.
+
+This means that the upstream [Broker Configs documentation](https://kafka.apache.org/documentation/#brokerconfigs) should be followed closely.
+
+Note that dotted options in the upstream docs do _not_ correspond to nested Nix attrsets, but instead as quoted top level `settings` attributes, as in `services.apache-kafka.settings."broker.id"`, *NOT* `services.apache-kafka.settings.broker.id`.
+
+Care should be taken, especially when migrating clusters from the old module, to ensure that the same intended configuration is reproduced faithfully via `settings`.
+
+To assist in the comparison, the final config can be inspected by building the config file itself, ie. with: `nix-build <nixpkgs/nixos> -A config.services.apache-kafka.configFiles.serverProperties`.
+
+Notable changes to be aware of include:
+
+- Removal of `services.apache-kafka.extraProperties` and `services.apache-kafka.serverProperties`
+  - Translate using arbitrary properties using [](#opt-services.apache-kafka.settings)
+  - [Upstream docs](https://kafka.apache.org/documentation.html#brokerconfigs)
+  - The intention is for all broker properties to be fully representable via [](#opt-services.apache-kafka.settings).
+  - If this is not the case, please do consider raising an issue.
+  - Until it can be remedied, you *can* bail out by using [](#opt-services.apache-kafka.configFiles.serverProperties) to the path of a fully rendered properties file.
+
+- Removal of `services.apache-kafka.hostname` and `services.apache-kafka.port`
+  - Translate using: `services.apache-kafka.settings.listeners`
+  - [Upstream docs](https://kafka.apache.org/documentation.html#brokerconfigs_listeners)
+
+- Removal of `services.apache-kafka.logDirs`
+  - Translate using: `services.apache-kafka.settings."log.dirs"`
+  - [Upstream docs](https://kafka.apache.org/documentation.html#brokerconfigs_log.dirs)
+
+- Removal of `services.apache-kafka.brokerId`
+  - Translate using: `services.apache-kafka.settings."broker.id"`
+  - [Upstream docs](https://kafka.apache.org/documentation.html#brokerconfigs_broker.id)
+
+- Removal of `services.apache-kafka.zookeeper`
+  - Translate using: `services.apache-kafka.settings."zookeeper.connect"`
+  - [Upstream docs](https://kafka.apache.org/documentation.html#brokerconfigs_zookeeper.connect)