Initial support for Outbox pattern

 - Provide repostiroy that represents the database that stores the business entities and message outbox.
 - Provide event abstraction and event sender service.
 - Default implementation for the event sender service that converts the events into entities and persist them into the outbox db.

 Related to spring-projects#8698

build.gradle

@@ -788,9 +788,11 @@ project('spring-integration-jpa') {
         api 'org.springframework:spring-orm'
         optionalApi "jakarta.persistence:jakarta.persistence-api:$jpaApiVersion"
 
-        testImplementation('org.springframework.data:spring-data-jpa') {
+        api('org.springframework.data:spring-data-jpa') {
             exclude group: 'org.springframework'
         }
+        api 'com.fasterxml.jackson.core:jackson-databind'
+
         testImplementation "com.h2database:h2:$h2Version"
         testImplementation "org.hibernate.orm:hibernate-core:$hibernateVersion"
     }

spring-integration-jpa/src/main/java/org/springframework/integration/jpa/outbox/DefaultOutboxEventSender.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2023 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.integration.jpa.outbox;
+
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+/**
+ * {@link OutboxEventSender} interface implementation that converts the {@link ExportEvent} into {@link OutboxEvent}
+ * entity and persists it in the outbox table via the {@link OutboxEventRepository} repository.
+ *
+ * @author Christian Tzolov
+ *
+ * @since 6.2
+ */
+public class DefaultOutboxEventSender implements OutboxEventSender {
+
+	private final OutboxEventRepository outboxEventRepository;
+
+	/**
+	 * Whether the outbox entry is removed after having been inserted. The removal of the entry does not impact the
+	 * Debezium connector from being able to emit CDC events. This is used as a way to keep the table’s underlying
+	 * storage from growing over time.
+	 */
+	private final boolean removeEventAfterInsert;
+
+	public DefaultOutboxEventSender(OutboxEventRepository outboxEventRepository, boolean removeEventAfterInsert) {
+		this.outboxEventRepository = outboxEventRepository;
+		this.removeEventAfterInsert = removeEventAfterInsert;
+	}
+
+	@Override
+	public void fire(ExportedEvent<?, ?> event) {
+
+		// Create an OutboxEvent object based on the ExportedEvent interface
+		final OutboxEvent outboxEvent = new OutboxEvent(
+				event.getAggregateType(),
+				"" + event.getAggregateId(),
+				event.getType(),
+				payloadAsString(event.getPayload()),
+				event.getTimestamp().toEpochMilli());
+
+		// We want the events table to remain empty; however this triggers both an INSERT and DELETE
+		// in the database transaction log which is sufficient for Debezium to process the event.
+		OutboxEvent savedOutboxEvent = this.outboxEventRepository.save(outboxEvent);
+		if (this.removeEventAfterInsert) {
+			this.outboxEventRepository.delete(savedOutboxEvent);
+		}
+	}
+
+	private static String payloadAsString(Object jsonNode) {
+		try {
+			return new ObjectMapper().writeValueAsString(jsonNode);
+		}
+		catch (JsonProcessingException e) {
+			throw new IllegalArgumentException(e);
+		}
+	}
+}

spring-integration-jpa/src/main/java/org/springframework/integration/jpa/outbox/ExportedEvent.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2023 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.integration.jpa.outbox;
+
+import java.time.Instant;
+
+/**
+ * Describes an event that should be exported via the "outbox" table.
+ *
+ * The {@link ExportedEvent} interface is parameterized to allow the application to designate the Java types used by
+ * several attributes emitted by the event.
+ *
+ * It’s important that for a given Spring application, all implementations of the {@link ExportedEvent} interface use
+ * the same parameter types or a build failure will be thrown since all events will use the same underlying database
+ * table.
+ *
+ * @param <I> Specifies the aggregateId Java type.
+ * @param <P> Specifies the event's payload type.
+ *
+ * @author Christian Tzolov
+ *
+ * @since 6.2
+ */
+public interface ExportedEvent<I, P> {
+
+	/**
+	 * The id of the aggregate affected by a given event. For example, the order id in case of events relating to an
+	 * order, or order lines of that order. This is used to ensure ordering of events within an aggregate type.
+	 */
+	I getAggregateId();
+
+	/**
+	 * The type of the aggregate affected by the event. For example, "order" in case of events relating to an order, or
+	 * order lines of that order. This is used as the topic name.
+	 */
+	String getAggregateType();
+
+	/**
+	 * The type of an event. For example, "Order Created" or "Order Line Cancelled" for events that belong to an given
+	 * aggregate type such as "order".
+	 */
+	String getType();
+
+	/**
+	 * The timestamp at which the event occurred.
+	 */
+	Instant getTimestamp();
+
+	/**
+	 * The event payload.
+	 */
+	P getPayload();
+}

spring-integration-jpa/src/main/java/org/springframework/integration/jpa/outbox/OutboxEvent.java

@@ -0,0 +1,116 @@
+/*
+ * Copyright 2023 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.integration.jpa.outbox;
+
+import java.util.UUID;
+
+// import javax.validation.constraints.NotNull;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.GeneratedValue;
+import jakarta.persistence.Id;
+
+/**
+ * The outbox event entity.
+ *
+ * The contents of any {@link ExportedEvent} is converted into a {@link OutboxEvent} entity definition and persisted to
+ * the database in order for Debezium to capture the event.
+ *
+ * @author Christian Tzolov
+ *
+ * @since 6.2
+ */
+@Entity
+public class OutboxEvent {
+	@Id
+	@GeneratedValue
+	private UUID id;
+
+	// @NotNull
+	private String aggregateType;
+
+	// @NotNull
+	private String aggregateId;
+
+	// @NotNull
+	private String type;
+
+	// @NotNull
+	private Long timestamp;
+
+	// @NotNull
+	private String payload;
+
+	public OutboxEvent() {
+	}
+
+	public OutboxEvent(String aggregateType, String aggregateId, String type, String payload, Long timestamp) {
+		this.aggregateType = aggregateType;
+		this.aggregateId = aggregateId;
+		this.type = type;
+		this.payload = payload;
+		this.timestamp = timestamp;
+	}
+
+	public UUID getId() {
+		return this.id;
+	}
+
+	public void setId(UUID id) {
+		this.id = id;
+	}
+
+	public String getAggregateType() {
+		return this.aggregateType;
+	}
+
+	public void setAggregateType(String aggregateType) {
+		this.aggregateType = aggregateType;
+	}
+
+	public String getAggregateId() {
+		return this.aggregateId;
+	}
+
+	public void setAggregateId(String aggregateId) {
+		this.aggregateId = aggregateId;
+	}
+
+	public String getType() {
+		return this.type;
+	}
+
+	public void setType(String type) {
+		this.type = type;
+	}
+
+	public Long getTimestamp() {
+		return this.timestamp;
+	}
+
+	public void setTimestamp(Long timestamp) {
+		this.timestamp = timestamp;
+	}
+
+	public String getPayload() {
+		return this.payload;
+	}
+
+	public void setPayload(String payload) {
+		this.payload = payload;
+	}
+}

spring-integration-jpa/src/main/java/org/springframework/integration/jpa/outbox/OutboxEventRepository.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2023 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.integration.jpa.outbox;
+
+import java.util.UUID;
+
+import org.springframework.data.jpa.repository.JpaRepository;
+
+/**
+ * Defines the Outbox table for the {@link OutboxEvent}s.
+ *
+ * @author Christian Tzolov
+ *
+ * @since 6.2
+ */
+public interface OutboxEventRepository extends JpaRepository<OutboxEvent, UUID> {
+}

spring-integration-jpa/src/main/java/org/springframework/integration/jpa/outbox/OutboxEventSender.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2023 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.integration.jpa.outbox;
+
+/**
+ * Implementations of this interface are responsible to persist the {@link ExportEvent} in the Outbox table.
+ *
+ * @author Christian Tzolov
+ *
+ * @since 6.2
+ */
+public interface OutboxEventSender {
+
+	/**
+	 * Persist the event in the outbox table as {@link OutboxEvent} entity.
+	 *
+	 * @param event to be persisted in the outbox table.
+	 */
+	void fire(ExportedEvent<?, ?> event);
+}