Add the rabbitmq module

rabbitmq/src/test/java/com/ankurm/rabbit/TopologyTrapsTest.java

@@ -0,0 +1,100 @@
+package com.ankurm.rabbit;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.amqp.AmqpIOException;
+import org.springframework.amqp.core.Queue;
+import org.springframework.amqp.core.QueueBuilder;
+import org.springframework.amqp.core.ReturnedMessage;
+import org.springframework.amqp.rabbit.core.RabbitAdmin;
+import org.springframework.amqp.rabbit.core.RabbitTemplate;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+
+/**
+ * Two ways a RabbitMQ topology loses your work quietly.
+ *
+ * @see <a href="../../../../../docs/03-the-silent-drop.md">docs/03-the-silent-drop.md</a>
+ * @see <a href="../../../../../docs/06-changing-your-mind.md">docs/06-changing-your-mind.md</a>
+ */
+@SpringBootTest
+class TopologyTrapsTest {
+
+	@Autowired
+	RabbitTemplate template;
+
+	@Autowired
+	RabbitAdmin admin;
+
+	@Test
+	void anUnroutableMessageIsDiscardedUnlessYouAskForItBack() throws Exception {
+		List<ReturnedMessage> returned = new CopyOnWriteArrayList<>();
+		this.template.setReturnsCallback(returned::add);
+
+		// "amend" matches no binding on orders.direct. The broker has nowhere to put it.
+		this.template.convertAndSend(Topology.DIRECT, "amend", OrderMessage.of("o-1"));
+		for (int i = 0; i < 100 && returned.isEmpty(); i++) {
+			Thread.sleep(20);
+		}
+
+		// spring.rabbitmq.template.mandatory=true is what turns a silent discard into a return.
+		assertThat(this.template.isMandatoryFor(null)).isTrue();
+		assertThat(returned).hasSize(1);
+		ReturnedMessage message = returned.get(0);
+		System.out.println("=== returned message ===");
+		System.out.println("  replyCode    " + message.getReplyCode());
+		System.out.println("  replyText    " + message.getReplyText());
+		System.out.println("  exchange     " + message.getExchange());
+		System.out.println("  routingKey   " + message.getRoutingKey());
+
+		assertThat(message.getReplyCode()).isEqualTo(312);
+		assertThat(message.getReplyText()).isEqualTo("NO_ROUTE");
+		assertThat(message.getRoutingKey()).isEqualTo("amend");
+		// Note what did NOT happen: convertAndSend returned normally. Publishing is fire and
+		// forget at the protocol level, so even with mandatory=true the failure arrives
+		// asynchronously on another thread. Nothing throws.
+	}
+
+	@Test
+	void redeclaringAQueueWithDifferentArgumentsIsAPreconditionFailure() {
+		// orders.ttl already exists with x-message-ttl=1500. Same name, different argument.
+		Queue conflicting = QueueBuilder.durable(Topology.Q_TTL)
+				.ttl(9999)
+				.deadLetterExchange(Topology.DLX)
+				.deadLetterRoutingKey("failed")
+				.build();
+
+		assertThatExceptionOfType(AmqpIOException.class)
+				.isThrownBy(() -> this.admin.declareQueue(conflicting))
+				.satisfies((ex) -> {
+					String detail = rootMessage(ex);
+					System.out.println("=== redeclaring orders.ttl with x-message-ttl=9999 ===");
+					System.out.println("  " + detail);
+					assertThat(detail).contains("PRECONDITION_FAILED")
+							.contains("inequivalent arg 'x-message-ttl'");
+				});
+
+		// Queue arguments are immutable. There is no ALTER QUEUE. Changing a TTL, a max-length
+		// or a dead-letter exchange on an existing queue means: declare a new queue, move the
+		// consumers, drain the old one, delete it. Plan the rename into the change.
+	}
+
+	private static String rootMessage(Throwable throwable) {
+		// The useful text is on the cause. AmqpIOException's own message is just
+		// "java.io.IOException", which is why this failure is so often reported as "IOException"
+		// with no further detail.
+		Throwable current = throwable;
+		StringBuilder all = new StringBuilder();
+		while (current != null) {
+			all.append(current.getMessage()).append(" | ");
+			current = current.getCause();
+		}
+		return all.toString();
+	}
+
+}