GH-80: Add DeDuplication Listener Adapters

Resolves #80

CheckStyle Fixes

Polishing - PR Comments

Upgrade to Gradle 2.13

.travis.yml

@@ -1,2 +1,11 @@
 language: java
 jdk: oraclejdk8
+install: true
+before_cache:
+  - rm -f $HOME/.gradle/caches/modules-2/modules-2.lock
+cache:
+  directories:
+    - $HOME/.gradle/caches/
+    - $HOME/.gradle/wrapper/
+script:
+  - ./gradlew check --no-daemon

gradle.properties

@@ -1 +1,2 @@
 version=1.0.0.BUILD-SNAPSHOT
+org.gradle.daemon=true

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
-#Wed Mar 30 18:31:11 EDT 2016
+#Mon May 16 18:24:00 EDT 2016
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-2.12-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-2.13-bin.zip

gradlew

@@ -6,12 +6,30 @@
 ##
 ##############################################################################
 
-# Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
-DEFAULT_JVM_OPTS=""
+# Attempt to set APP_HOME
+# Resolve links: $0 may be a link
+PRG="$0"
+# Need this for relative symlinks.
+while [ -h "$PRG" ] ; do
+    ls=`ls -ld "$PRG"`
+    link=`expr "$ls" : '.*-> \(.*\)$'`
+    if expr "$link" : '/.*' > /dev/null; then
+        PRG="$link"
+    else
+        PRG=`dirname "$PRG"`"/$link"
+    fi
+done
+SAVED="`pwd`"
+cd "`dirname \"$PRG\"`/" >/dev/null
+APP_HOME="`pwd -P`"
+cd "$SAVED" >/dev/null
 
 APP_NAME="Gradle"
 APP_BASE_NAME=`basename "$0"`
 
+# Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+DEFAULT_JVM_OPTS=""
+
 # Use the maximum available, or set MAX_FD != -1 to use that value.
 MAX_FD="maximum"
 
@@ -30,6 +48,7 @@ die ( ) {
 cygwin=false
 msys=false
 darwin=false
+nonstop=false
 case "`uname`" in
   CYGWIN* )
     cygwin=true
@@ -40,26 +59,11 @@ case "`uname`" in
   MINGW* )
     msys=true
     ;;
+  NONSTOP* )
+    nonstop=true
+    ;;
 esac
 
-# Attempt to set APP_HOME
-# Resolve links: $0 may be a link
-PRG="$0"
-# Need this for relative symlinks.
-while [ -h "$PRG" ] ; do
-    ls=`ls -ld "$PRG"`
-    link=`expr "$ls" : '.*-> \(.*\)$'`
-    if expr "$link" : '/.*' > /dev/null; then
-        PRG="$link"
-    else
-        PRG=`dirname "$PRG"`"/$link"
-    fi
-done
-SAVED="`pwd`"
-cd "`dirname \"$PRG\"`/" >/dev/null
-APP_HOME="`pwd -P`"
-cd "$SAVED" >/dev/null
-
 CLASSPATH=$APP_HOME/gradle/wrapper/gradle-wrapper.jar
 
 # Determine the Java command to use to start the JVM.
@@ -85,7 +89,7 @@ location of your Java installation."
 fi
 
 # Increase the maximum file descriptors if we can.
-if [ "$cygwin" = "false" -a "$darwin" = "false" ] ; then
+if [ "$cygwin" = "false" -a "$darwin" = "false" -a "$nonstop" = "false" ] ; then
     MAX_FD_LIMIT=`ulimit -H -n`
     if [ $? -eq 0 ] ; then
         if [ "$MAX_FD" = "maximum" -o "$MAX_FD" = "max" ] ; then

gradlew.bat

@@ -8,14 +8,14 @@
 @rem Set local scope for the variables with windows NT shell
 if "%OS%"=="Windows_NT" setlocal
 
-@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
-set DEFAULT_JVM_OPTS=
-
 set DIRNAME=%~dp0
 if "%DIRNAME%" == "" set DIRNAME=.
 set APP_BASE_NAME=%~n0
 set APP_HOME=%DIRNAME%
 
+@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+set DEFAULT_JVM_OPTS=
+
 @rem Find java.exe
 if defined JAVA_HOME goto findJavaFromJavaHome
 

spring-kafka/src/main/java/org/springframework/kafka/annotation/KafkaListener.java

@@ -24,53 +24,57 @@ import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
 import org.springframework.kafka.listener.MessageListener;
-import org.springframework.kafka.listener.adapter.MessagingMessageListenerAdapter;
 import org.springframework.messaging.handler.annotation.MessageMapping;
 
 /**
- * Annotation that marks a method to be the target of a Kafka message
- * listener on the specified topics.
+ * Annotation that marks a method to be the target of a Kafka message listener on the
+ * specified topics.
  *
- * The {@link #containerFactory()}
- * identifies the {@link org.springframework.kafka.config.KafkaListenerContainerFactory
+ * The {@link #containerFactory()} identifies the
+ * {@link org.springframework.kafka.config.KafkaListenerContainerFactory
  * KafkaListenerContainerFactory} to use to build the Kafka listener container. If not
- * set, a <em>default</em> container factory is assumed to be available with a bean
- * name of {@code kafkaListenerContainerFactory} unless an explicit default has been
- * provided through configuration.
+ * set, a <em>default</em> container factory is assumed to be available with a bean name
+ * of {@code kafkaListenerContainerFactory} unless an explicit default has been provided
+ * through configuration.
  *
- * <p>Processing of {@code @KafkaListener} annotations is performed by
- * registering a {@link KafkaListenerAnnotationBeanPostProcessor}. This can be
- * done manually or, more conveniently, through {@link EnableKafka} annotation.
+ * <p>
+ * Processing of {@code @KafkaListener} annotations is performed by registering a
+ * {@link KafkaListenerAnnotationBeanPostProcessor}. This can be done manually or, more
+ * conveniently, through {@link EnableKafka} annotation.
  *
- * <p>Annotated methods are allowed to have flexible signatures similar to what
+ * <p>
+ * Annotated methods are allowed to have flexible signatures similar to what
  * {@link MessageMapping} provides, that is
  * <ul>
- * <li>{@link org.apache.kafka.clients.consumer.ConsumerRecord} to
- * access to the raw Kafka message</li>
+ * <li>{@link org.apache.kafka.clients.consumer.ConsumerRecord} to access to the raw Kafka
+ * message</li>
  * <li>{@link org.springframework.kafka.support.Acknowledgment} to manually ack</li>
- * <li>{@link org.springframework.messaging.handler.annotation.Payload @Payload}-annotated method
- * arguments including the support of validation</li>
- * <li>{@link org.springframework.messaging.handler.annotation.Header @Header}-annotated method
- * arguments to extract a specific header value, defined by
+ * <li>{@link org.springframework.messaging.handler.annotation.Payload @Payload}-annotated
+ * method arguments including the support of validation</li>
+ * <li>{@link org.springframework.messaging.handler.annotation.Header @Header}-annotated
+ * method arguments to extract a specific header value, defined by
  * {@link org.springframework.kafka.support.KafkaHeaders KafkaHeaders}</li>
  * <li>{@link org.springframework.messaging.handler.annotation.Headers @Headers}-annotated
- * argument that must also be assignable to {@link java.util.Map} for getting access to all
- * headers.</li>
+ * argument that must also be assignable to {@link java.util.Map} for getting access to
+ * all headers.</li>
  * <li>{@link org.springframework.messaging.MessageHeaders MessageHeaders} arguments for
  * getting access to all headers.</li>
- * <li>{@link org.springframework.messaging.support.MessageHeaderAccessor MessageHeaderAccessor}
- * for convenient access to all method arguments.</li>
+ * <li>{@link org.springframework.messaging.support.MessageHeaderAccessor
+ * MessageHeaderAccessor} for convenient access to all method arguments.</li>
  * </ul>
  *
- * <p>When defined at the method level, a listener container is created for each method. The
- * {@link MessageListener} is a {@link MessagingMessageListenerAdapter}, configured with a
- * {@link org.springframework.kafka.config.MethodKafkaListenerEndpoint}.
+ * <p>When defined at the method level, a listener container is created for each method.
+ * The {@link MessageListener} is a
+ * {@link org.springframework.kafka.listener.adapter.MessagingMessageListenerAdapter},
+ * configured with a {@link org.springframework.kafka.config.MethodKafkaListenerEndpoint}.
  *
- * <p>When defined at the class level, a single message listener container is used to service
- * all methods annotated with {@code @KafkaHandler}. Method signatures of such annotated
- * methods must not cause any ambiguity such that a single method can be resolved for a
- * particular inbound message. The {@link MessagingMessageListenerAdapter} is configured with
- * a {@link org.springframework.kafka.config.MultiMethodKafkaListenerEndpoint}.
+ * <p>When defined at the class level, a single message listener container is used to
+ * service all methods annotated with {@code @KafkaHandler}. Method signatures of such
+ * annotated methods must not cause any ambiguity such that a single method can be
+ * resolved for a particular inbound message. The
+ * {@link org.springframework.kafka.listener.adapter.MessagingMessageListenerAdapter} is
+ * configured with a
+ * {@link org.springframework.kafka.config.MultiMethodKafkaListenerEndpoint}.
  *
  * @author Gary Russell
  *

spring-kafka/src/main/java/org/springframework/kafka/config/AbstractKafkaListenerContainerFactory.java

@@ -23,6 +23,7 @@ import org.springframework.kafka.core.ConsumerFactory;
 import org.springframework.kafka.listener.AbstractMessageListenerContainer;
 import org.springframework.kafka.listener.AbstractMessageListenerContainer.AckMode;
 import org.springframework.kafka.listener.ErrorHandler;
+import org.springframework.kafka.listener.adapter.DeDuplicationStrategy;
 import org.springframework.kafka.support.converter.MessageConverter;
 
 /**
@@ -57,6 +58,8 @@ public abstract class AbstractKafkaListenerContainerFactory<C extends AbstractMe
 
 	private MessageConverter messageConverter;
 
+	private DeDuplicationStrategy<K, V> deDuplicationStrategy;
+
 	/**
 	 * Specify a {@link ConsumerFactory} to use.
 	 * @param consumerFactory The consumer factory.
@@ -140,6 +143,11 @@ public abstract class AbstractKafkaListenerContainerFactory<C extends AbstractMe
 		this.messageConverter = messageConverter;
 	}
 
+	public void setDeDuplicationStrategy(DeDuplicationStrategy<K, V> deDuplicationStrategy) {
+		this.deDuplicationStrategy = deDuplicationStrategy;
+	}
+
+	@SuppressWarnings("unchecked")
 	@Override
 	public C createListenerContainer(KafkaListenerEndpoint endpoint) {
 		C instance = createContainerInstance(endpoint);
@@ -169,6 +177,9 @@ public abstract class AbstractKafkaListenerContainerFactory<C extends AbstractMe
 			instance.setPollTimeout(this.pollTimeout);
 		}
 
+		if (this.deDuplicationStrategy != null && endpoint instanceof AbstractKafkaListenerEndpoint) {
+			((AbstractKafkaListenerEndpoint<K, V>) endpoint).setDeDuplicationStrategy(this.deDuplicationStrategy);
+		}
 		endpoint.setupListenerContainer(instance, this.messageConverter);
 		initializeContainer(instance);
 

spring-kafka/src/main/java/org/springframework/kafka/config/AbstractKafkaListenerEndpoint.java

@@ -33,6 +33,7 @@ import org.springframework.beans.factory.config.BeanExpressionResolver;
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.kafka.listener.MessageListener;
 import org.springframework.kafka.listener.MessageListenerContainer;
+import org.springframework.kafka.listener.adapter.DeDuplicationStrategy;
 import org.springframework.kafka.support.converter.MessageConverter;
 import org.springframework.util.Assert;
 
@@ -67,6 +68,8 @@ public abstract class AbstractKafkaListenerEndpoint<K, V>
 
 	private String group;
 
+	private DeDuplicationStrategy<K, V> deDuplicationStrategy;
+
 
 	@Override
 	public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
@@ -193,6 +196,18 @@ public abstract class AbstractKafkaListenerEndpoint<K, V>
 		}
 	}
 
+	protected DeDuplicationStrategy<K, V> getDeDuplicationStrategy() {
+		return this.deDuplicationStrategy;
+	}
+
+	/**
+	 * Set a {@link DeDuplicationStrategy} implementation.
+	 * @param deDuplicationStrategy the strategy implementation.
+	 */
+	public void setDeDuplicationStrategy(DeDuplicationStrategy<K, V> deDuplicationStrategy) {
+		this.deDuplicationStrategy = deDuplicationStrategy;
+	}
+
 	@Override
 	public void setupListenerContainer(MessageListenerContainer listenerContainer, MessageConverter messageConverter) {
 		setupMessageListener(listenerContainer, messageConverter);

spring-kafka/src/main/java/org/springframework/kafka/config/MethodKafkaListenerEndpoint.java

@@ -94,6 +94,9 @@ public class MethodKafkaListenerEndpoint<K, V> extends AbstractKafkaListenerEndp
 		Assert.state(this.messageHandlerMethodFactory != null,
 				"Could not create message listener - MessageHandlerMethodFactory not set");
 		MessagingMessageListenerAdapter<K, V> messageListener = createMessageListenerInstance();
+		if (getDeDuplicationStrategy() != null) {
+			messageListener.setDeDuplicationStrategy(getDeDuplicationStrategy());
+		}
 		messageListener.setHandlerMethod(configureListenerAdapter(messageListener));
 		if (messageConverter != null) {
 			messageListener.setMessageConverter(messageConverter);

spring-kafka/src/main/java/org/springframework/kafka/listener/KafkaMessageListenerContainer.java

@@ -458,8 +458,8 @@ public class KafkaMessageListenerContainer<K, V> extends AbstractMessageListener
 								ackImmediate(record);
 							}
 							else {
-								throw new IllegalStateException("AckMode must be MANUAL or MANUAL_IMMEDIATE "
-										+ "for manual acks");
+								throw new IllegalStateException("AckMode must be MANUAL, "
+										+ "MANUAL_IMMEDIATE, or MANUAL_IMMEDIATE_SYNC for manual acks");
 							}
 						}
 

spring-kafka/src/main/java/org/springframework/kafka/listener/adapter/AbstractDeDuplicatingMessageListener.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2016 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
+ *
+ *      http://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.kafka.listener.adapter;
+
+import org.apache.kafka.clients.consumer.ConsumerRecord;
+
+import org.springframework.util.Assert;
+
+/**
+ * An abstract message listener adapter that implements de-duplication logic
+ * via a {@link DeDuplicationStrategy}.
+ *
+ * @param <K> the key type.
+ * @param <V> the value type.
+ *
+ * @author Gary Russell
+ *
+ */
+public abstract class AbstractDeDuplicatingMessageListener<K, V> {
+
+	private final DeDuplicationStrategy<K, V> deDupStrategy;
+
+	protected AbstractDeDuplicatingMessageListener(DeDuplicationStrategy<K, V> deDupStrategy) {
+		Assert.notNull(deDupStrategy, "'deDupStrategy' cannot be null");
+		this.deDupStrategy = deDupStrategy;
+	}
+
+	protected boolean isDuplicate(ConsumerRecord<K, V> consumerRecord) {
+		return this.deDupStrategy.isDuplicate(consumerRecord);
+	}
+
+}

spring-kafka/src/main/java/org/springframework/kafka/listener/adapter/DeDuplicatingMessageListenerAdapter.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2016 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
+ *
+ *      http://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.kafka.listener.adapter;
+
+import org.apache.kafka.clients.consumer.ConsumerRecord;
+
+import org.springframework.kafka.listener.MessageListener;
+
+/**
+ * A {@link MessageListener} adapter that implements de-duplication logic
+ * via a DeDuplicationStrategy.
+ *
+ * @param <K> the key type.
+ * @param <V> the value type.
+ *
+ * @author Gary Russell
+ *
+ */
+public class DeDuplicatingMessageListenerAdapter<K, V> extends AbstractDeDuplicatingMessageListener<K, V>
+		implements MessageListener<K, V> {
+
+	private final MessageListener<K, V> delegate;
+
+	public DeDuplicatingMessageListenerAdapter(DeDuplicationStrategy<K, V> deDupStrategy,
+			MessageListener<K, V> delegate) {
+		super(deDupStrategy);
+		this.delegate = delegate;
+	}
+
+	@Override
+	public void onMessage(ConsumerRecord<K, V> consumerRecord) {
+		if (!isDuplicate(consumerRecord)) {
+			this.delegate.onMessage(consumerRecord);
+		}
+	}
+
+}

spring-kafka/src/main/java/org/springframework/kafka/listener/adapter/DeDuplicationStrategy.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2016 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
+ *
+ *      http://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.kafka.listener.adapter;
+
+import org.apache.kafka.clients.consumer.ConsumerRecord;
+
+/**
+ * Implementations of this interface can signal that a message about
+ * to be delivered to a message listener is a duplicate.
+ *
+ * @param <K> the key type.
+ * @param <V> the value type.
+ *
+ * @author Gary Russell
+ *
+ */
+public interface DeDuplicationStrategy<K, V> {
+
+	/**
+	 * Return true if the record is a duplicate and should be discarded.
+	 * @param consumerRecord the record.
+	 * @return true to discard.
+	 */
+	boolean isDuplicate(ConsumerRecord<K, V> consumerRecord);
+
+}

spring-kafka/src/main/java/org/springframework/kafka/listener/adapter/MessagingMessageListenerAdapter.java

@@ -59,6 +59,8 @@ public class MessagingMessageListenerAdapter<K, V> extends AbstractAdaptableMess
 
 	private MessageConverter messageConverter = new MessagingMessageConverter();
 
+	private DeDuplicationStrategy<K, V> deDuplicationStrategy;
+
 
 	public MessagingMessageListenerAdapter(Method method) {
 		this.inferredType = determineInferredType(method);
@@ -101,6 +103,18 @@ public class MessagingMessageListenerAdapter<K, V> extends AbstractAdaptableMess
 		invokeHandler(record, acknowledgment, message);
 	}
 
+	protected DeDuplicationStrategy<K, V> getDeDuplicationStrategy() {
+		return this.deDuplicationStrategy;
+	}
+
+	/**
+	 * Set a {@link DeDuplicationStrategy} implementation.
+	 * @param deDuplicationStrategy the strategy implementation.
+	 */
+	public void setDeDuplicationStrategy(DeDuplicationStrategy<K, V> deDuplicationStrategy) {
+		this.deDuplicationStrategy = deDuplicationStrategy;
+	}
+
 	protected Message<?> toMessagingMessage(ConsumerRecord<K, V> record, Acknowledgment acknowledgment) {
 		return getMessageConverter().toMessage(record, acknowledgment, this.inferredType);
 	}
@@ -114,6 +128,9 @@ public class MessagingMessageListenerAdapter<K, V> extends AbstractAdaptableMess
 	 * @return the result of invocation.
 	 */
 	private Object invokeHandler(ConsumerRecord<K, V> record, Acknowledgment acknowledgment, Message<?> message) {
+		if (this.deDuplicationStrategy != null && this.deDuplicationStrategy.isDuplicate(record)) {
+			return null;
+		}
 		try {
 			return this.handlerMethod.invoke(message, record, acknowledgment);
 		}

spring-kafka/src/test/java/org/springframework/kafka/annotation/EnableKafkaIntegrationTests.java

@@ -47,6 +47,7 @@ import org.springframework.kafka.core.ProducerFactory;
 import org.springframework.kafka.listener.AbstractMessageListenerContainer.AckMode;
 import org.springframework.kafka.listener.ConcurrentMessageListenerContainer;
 import org.springframework.kafka.listener.MessageListenerContainer;
+import org.springframework.kafka.listener.adapter.DeDuplicationStrategy;
 import org.springframework.kafka.support.Acknowledgment;
 import org.springframework.kafka.support.KafkaHeaders;
 import org.springframework.kafka.support.converter.StringJsonMessageConverter;
@@ -95,6 +96,9 @@ public class EnableKafkaIntegrationTests {
 	@Autowired
 	public KafkaListenerEndpointRegistry registry;
 
+	@Autowired
+	private DeDupImpl deDup;
+
 	@Test
 	public void testSimple() throws Exception {
 		template.send("annotated1", 0, "foo");
@@ -130,6 +134,7 @@ public class EnableKafkaIntegrationTests {
 		template.flush();
 		assertThat(this.listener.latch7.await(10, TimeUnit.SECONDS)).isTrue();
 
+		assertThat(this.deDup.called).isTrue();
 	}
 
 	@Test
@@ -195,9 +200,15 @@ public class EnableKafkaIntegrationTests {
 			ConcurrentKafkaListenerContainerFactory<Integer, String> factory =
 					new ConcurrentKafkaListenerContainerFactory<>();
 			factory.setConsumerFactory(consumerFactory());
+			factory.setDeDuplicationStrategy(deDup());
 			return factory;
 		}
 
+		@Bean
+		public DeDupImpl deDup() {
+			return new DeDupImpl();
+		}
+
 		@Bean
 		public KafkaListenerContainerFactory<ConcurrentMessageListenerContainer<Integer, String>>
 				kafkaJsonListenerContainerFactory() {
@@ -462,4 +473,16 @@ public class EnableKafkaIntegrationTests {
 
 	}
 
+	public static class DeDupImpl implements DeDuplicationStrategy<Integer, String> {
+
+		private boolean called;
+
+		@Override
+		public boolean isDuplicate(ConsumerRecord<Integer, String> consumerRecord) {
+			called = true;
+			return false;
+		}
+
+	}
+
 }

src/reference/asciidoc/kafka.adoc

@@ -330,3 +330,23 @@ public void listen(@Payload String foo,
     ...
 }
 ----
+
+===== Handling Duplicates
+
+In certain scenarios, such as rebalancing, a message may be redelivered that has already been processed.
+The framework cannot know whether such a message has been processed or not, that is an application-level
+function.
+This is known as the http://www.enterpriseintegrationpatterns.com/patterns/messaging/IdempotentReceiver.html[Idempotent
+Receiver] pattern and Spring Integration provides an
+http://docs.spring.io/spring-integration/reference/html/messaging-endpoints-chapter.html#idempotent-receiver
+[implementation thereof].
+
+The Spring for Apache Kafka project also provides some assistance by means of the `DeDuplicatingMessageListenerAdapter`
+classe, which can wrap your `MessageListener`.
+This class takes an implementation of `DeDuplicationStrategy` where you implement the `isDuplicate` method to signal
+that a message is a duplicate and should be discarded.
+
+Similarly, when using `@KafkaListener`, the `DeDuplicationStrategy` can be injected into the container factory.
+
+A wrapper is not provided for the `AcknowledgingMessageListener` because you might need to acknowledge the duplicated
+message.