Add the rabbitmq module

rabbitmq/docs/01-the-on-ramp.md

@@ -0,0 +1,59 @@
+[Module README](../README.md) · [Exchanges →](02-exchanges.md)
+
+# 1. The on-ramp
+
+Two dependency decisions decide whether anything works, and neither produces a helpful error.
+
+## Use the starter, not `spring-rabbit`
+
+Boot 4 moved every auto-configuration out of `spring-boot-autoconfigure` into a per-technology
+module. RabbitMQ's lives in `spring-boot-amqp`, package
+`org.springframework.boot.amqp.autoconfigure`, and a bare `org.springframework.amqp:spring-rabbit`
+dependency does not bring it. You get no `RabbitTemplate`, no `RabbitAdmin`, no listener
+container factory — and a context that starts cleanly.
+
+```xml
+<dependency>
+  <groupId>org.springframework.boot</groupId>
+  <artifactId>spring-boot-starter-amqp</artifactId>
+</dependency>
+```
+
+## Boot does not give you a JSON converter
+
+Spring Kafka defaults to serializers you configure. Spring AMQP defaults to
+`SimpleMessageConverter`, which handles `String`, `byte[]` and `Serializable` and refuses
+everything else:
+
+```
+IllegalArgumentException: SimpleMessageConverter only supports String, byte[] and
+Serializable payloads, received: com.ankurm.rabbit.OrderMessage
+```
+
+One bean fixes both directions, because the auto-configured `RabbitTemplate` and the listener
+container factory both look for a `MessageConverter`:
+
+```java
+@Bean
+MessageConverter messageConverter() {
+    return new JacksonJsonMessageConverter();
+}
+```
+
+**Note the class name.** Spring AMQP 4.1 ships `Jackson2JsonMessageConverter` (Jackson 2) and
+`JacksonJsonMessageConverter` (Jackson 3) side by side — exactly as Spring Kafka ships
+`JsonSerializer` and `JacksonJsonSerializer`. Boot 4 is a Jackson 3 application. The rule across
+both stacks is the same: **if the class name contains a `2`, it belongs to the previous major
+version of Jackson.**
+
+## Versions
+
+Boot 4.1.1 manages Spring AMQP **4.1.1** and `com.rabbitmq:amqp-client` **5.30.0**. Maven Central
+has amqp-client 5.35.0; overriding the managed version to reach it is a change you should have a
+reason for.
+
+One API change to know about: `RabbitAdmin.QUEUE_MESSAGE_COUNT` now holds a `Long`. Every older
+example casts it to `Integer`, which is a `ClassCastException` at runtime and not at compile
+time.
+
+[Exchanges &rarr;](02-exchanges.md)