GH-859 - Avoid creating a transaction for event externalization.

We now use the ability to override transaction propagation introduce for GH-858 to tweak the application module listener declaration on DelegatingEventListener so that we do not create a transaction by default. This avoids a database connection being acquired as the listener is very likely to interact with a non-transactional resource anyway.

spring-modulith-events/spring-modulith-events-core/src/main/java/org/springframework/modulith/events/support/DelegatingEventExternalizer.java

@@ -22,6 +22,7 @@ import org.springframework.modulith.events.ApplicationModuleListener;
 import org.springframework.modulith.events.EventExternalizationConfiguration;
 import org.springframework.modulith.events.RoutingTarget;
 import org.springframework.stereotype.Component;
+import org.springframework.transaction.annotation.Propagation;
 import org.springframework.util.Assert;
 
 /**
@@ -59,7 +60,7 @@ public class DelegatingEventExternalizer extends EventExternalizationSupport {
 	 * @see org.springframework.modulith.events.support.EventExternalizationSupport#externalize(java.lang.Object)
 	 */
 	@Override
-	@ApplicationModuleListener
+	@ApplicationModuleListener(propagation = Propagation.SUPPORTS)
 	public CompletableFuture<?> externalize(Object event) {
 		return super.externalize(event);
 	}

spring-modulith-events/spring-modulith-events-core/src/test/java/org/springframework/modulith/events/support/DelegatingEventExternalizerUnitTests.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.modulith.events.support;
+
+import static org.assertj.core.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.core.annotation.AnnotatedElementUtils;
+import org.springframework.transaction.annotation.Propagation;
+import org.springframework.transaction.annotation.Transactional;
+
+/**
+ * Unit tests for {@link DelegatingEventExternalizer}.
+ *
+ * @author Oliver Drotbohm
+ * @since 1.3
+ */
+class DelegatingEventExternalizerUnitTests {
+
+	@Test // GH-859
+	void doesNotRequireATransactionForExternalization() throws Exception {
+
+		var method = DelegatingEventExternalizer.class.getMethod("externalize", Object.class);
+		var annotation = AnnotatedElementUtils.getMergedAnnotation(method, Transactional.class);
+
+		assertThat(annotation.propagation()).isEqualTo(Propagation.SUPPORTS);
+	}
+}