Add the rabbitmq module

rabbitmq/docs/02-exchanges.md

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+[← The on-ramp](01-the-on-ramp.md) · [Module README](../README.md) · [The silent drop →](03-the-silent-drop.md)
+
+# 2. Four exchange types
+
+A producer never publishes to a queue. It publishes to an **exchange** with a **routing key**,
+and bindings decide where that lands. The whole topology is in
+[`Topology.java`](../src/main/java/com/ankurm/rabbit/Topology.java) as beans; `RabbitAdmin`
+declares them when the connection opens.
+
+## Direct — exact match
+
+Binding key `new` receives routing key `new`. Nothing else. This is the workhorse: one queue per
+command type.
+
+## Fanout — routing key ignored entirely
+
+Every bound queue gets a copy. `audit.all` and `analytics.all` both receive it, and the routing
+key you passed is not consulted at all. Use it for broadcast; use it knowing that adding a queue
+adds a full copy of the traffic.
+
+## Topic — wildcards over dot-separated words
+
+`*` is **exactly one word**. `#` is **zero or more**. The distinction is the one people get
+wrong, so here it is against a real broker
+([`docs/output/topic-wildcards.txt`](output/topic-wildcards.txt)):
+
+```
+routing key              orders.eu    orders.high  note
+order.eu.high            true         true         matches both
+order.eu.low             true         false        matches order.eu.* only
+order.us.high            false        true         matches order.#.high only
+order.eu.west.high       false        true         matches order.#.high only - * is one word
+order.high               false        true         matches order.#.high - # can be zero words
+```
+
+Bindings are `order.eu.*` and `order.#.high`. Two rows are worth pausing on:
+
+- `order.eu.west.high` does **not** match `order.eu.*`, because `*` matches one word and `west.high`
+  is two. Regex intuition says otherwise.
+- `order.high` **does** match `order.#.high`, because `#` matches zero words. So a binding you
+  wrote to mean "something in the middle" also matches "nothing in the middle".
+
+Design routing keys most-general-to-most-specific (`order.eu.west.high`, not
+`high.order.eu.west`), because `#` and `*` work left to right and a hierarchy you can bind
+usefully is one that starts broad.
+
+## Headers — match a map, ignore the routing key
+
+`x-match=all` requires every named header to be present **and equal**. `x-match=any` requires
+one. From `RoutingTest`:
+
+| Message headers | `x-match=all` queue | `x-match=any` queue |
+|---|---|---|
+| `priority=high, region=eu` | yes | yes |
+| `priority=high` | no | yes |
+| `priority=high, region=us` | no | yes |
+
+The third row is the one to remember: `all` matches on **value**, not on presence. A header that
+is there with the wrong value fails the same way a missing one does.
+
+Headers exchanges are slower than topic exchanges and much less common. Reach for them when the
+routing criteria are genuinely multi-dimensional and do not compose into a hierarchy.
+
+## The default exchange
+
+Publishing to the empty exchange name `""` routes by **queue name**, using the routing key as the
+queue name. Every queue is implicitly bound to it. That is how
+`convertAndSend("", "orders.work", message)` works, and it is the one piece of AMQP that behaves
+like a magic constant.
+
+[The silent drop →](03-the-silent-drop.md)