spring-projectsGH-2172: Expose Retry Topic Chain at Runtime

Resolves spring-projects#2172

* Add methods to `DestinationTopicContainer` interface
* Reduce memory footprint of `DefaultDestinationTopicResolver`
* Add documentation

Author: tomazfernandes

spring-kafka-docs/src/main/asciidoc/retrytopic.adoc

@@ -848,6 +848,18 @@ protected Consumer<RetryTopicConfigurer> configureRetryTopicConfigurer() {
 ----
 ====
 
+[[access-topic-info-runtime]]
+==== Accessing Topics' Information at Runtime
+
+Since 2.9, you can access information regarding the topic chain at runtime by injecting the provided `DestinationTopicContainer` bean.
+This interface provides methods to look up the next topic in the chain or the DLT for a topic if configured, as well as useful properties such as the topic's name, delay and type.
+
+As a real-world use-case example, you can use such information so a console application can resend a record from the DLT to the first retry topic in the chain after the cause of the failed processing, e.g. bug / inconsistent state, has been resolved.
+
+IMPORTANT: The `DestinationTopic` provided by the `DestinationTopicContainer#getNextDestinationTopicFor()` method corresponds to the next topic registered in the chain for the input topic.
+The actual topic the message will be forwarded to may differ due to different factors such as exception classification, number of attempts or single-topic fixed-delay strategies.
+Use the `DestinationTopicResolver` interface if you need to weigh in these factors.
+
 [[change-kboe-logging-level]]
 ==== Changing KafkaBackOffException Logging Level
 

spring-kafka/src/main/java/org/springframework/kafka/retrytopic/DefaultDestinationTopicResolver.java

@@ -34,11 +34,12 @@
 import org.springframework.kafka.listener.ExceptionClassifier;
 import org.springframework.kafka.listener.ListenerExecutionFailedException;
 import org.springframework.kafka.listener.TimestampedException;
+import org.springframework.lang.Nullable;
 
 
 /**
  *
- * Default implementation of the DestinationTopicResolver interface.
+ * Default implementation of the {@link DestinationTopicResolver} interface.
  * The container is closed when a {@link ContextRefreshedEvent} is received
  * and no more destinations can be added after that.
  *
@@ -58,8 +59,6 @@ public class DefaultDestinationTopicResolver extends ExceptionClassifier
 
 	private final Map<String, DestinationTopicHolder> sourceDestinationsHolderMap;
 
-	private final Map<String, DestinationTopic> destinationsTopicMap;
-
 	private final Clock clock;
 
 	private ApplicationContext applicationContext;
@@ -81,7 +80,6 @@ public DefaultDestinationTopicResolver(Clock clock, ApplicationContext applicati
 	public DefaultDestinationTopicResolver(Clock clock) {
 		this.clock = clock;
 		this.sourceDestinationsHolderMap = new HashMap<>();
-		this.destinationsTopicMap = new HashMap<>();
 		this.contextRefreshed = false;
 	}
 
@@ -103,7 +101,7 @@ public DestinationTopic resolveDestinationTopic(String topic, Integer attempt, E
 						&& isNotFatalException(e)
 						&& !isPastTimout(originalTimestamp, destinationTopicHolder)
 					? resolveRetryDestination(destinationTopicHolder)
-					: resolveDltOrNoOpsDestination(topic);
+					: getDltOrNoOpsDestination(topic);
 	}
 
 	private Boolean isNotFatalException(Exception e) {
@@ -140,18 +138,28 @@ private DestinationTopic resolveRetryDestination(DestinationTopicHolder destinat
 
 	@Override
 	public DestinationTopic getDestinationTopicByName(String topic) {
-		return Objects.requireNonNull(this.destinationsTopicMap.get(topic),
-				() -> "No topic found for " + topic);
+		return Objects.requireNonNull(this.sourceDestinationsHolderMap.get(topic),
+				() -> "No DestinationTopic found for " + topic).getSourceDestination();
+	}
+
+	@Nullable
+	@Override
+	public DestinationTopic getDltFor(String topicName) {
+		DestinationTopic destination = getDltOrNoOpsDestination(topicName);
+		return destination.isNoOpsTopic()
+				? null
+				: destination;
 	}
 
-	private DestinationTopic resolveDltOrNoOpsDestination(String topic) {
-		DestinationTopic destination = getDestinationFor(topic);
+	private DestinationTopic getDltOrNoOpsDestination(String topic) {
+		DestinationTopic destination = getNextDestinationTopicFor(topic);
 		return destination.isDltTopic() || destination.isNoOpsTopic()
 				? destination
-				: resolveDltOrNoOpsDestination(destination.getDestinationName());
+				: getDltOrNoOpsDestination(destination.getDestinationName());
 	}
 
-	private DestinationTopic getDestinationFor(String topic) {
+	@Override
+	public DestinationTopic getNextDestinationTopicFor(String topic) {
 		return getDestinationHolderFor(topic).getNextDestination();
 	}
 
@@ -179,9 +187,6 @@ public void addDestinationTopics(List<DestinationTopic> destinationsToAdd) {
 					+ DefaultDestinationTopicResolver.class.getSimpleName() + " is already refreshed.");
 		}
 		synchronized (this.sourceDestinationsHolderMap) {
-			this.destinationsTopicMap.putAll(destinationsToAdd
-					.stream()
-					.collect(Collectors.toMap(destination -> destination.getDestinationName(), destination -> destination)));
 			this.sourceDestinationsHolderMap.putAll(correlatePairSourceAndDestinationValues(destinationsToAdd));
 		}
 	}

spring-kafka/src/main/java/org/springframework/kafka/retrytopic/DestinationTopicContainer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2021 the original author or authors.
+ * Copyright 2017-2022 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.
@@ -18,6 +18,8 @@
 
 import java.util.List;
 
+import org.springframework.lang.Nullable;
+
 /**
  *
  * Provides methods to store and retrieve {@link DestinationTopic} instances.
@@ -34,10 +36,33 @@ public interface DestinationTopicContainer {
 	void addDestinationTopics(List<DestinationTopic> destinationTopics);
 
 	/**
-	 * Returns the DestinationTopic instance registered for that topic.
+	 * Returns the {@link DestinationTopic} instance registered for that topic.
 	 * @param topicName the topic name of the DestinationTopic to be returned.
 	 * @return the DestinationTopic instance registered for that topic.
 	 */
 	DestinationTopic getDestinationTopicByName(String topicName);
 
+	/**
+	 * Returns the {@link DestinationTopic} instance registered as the next
+	 * destination topic in the chain for the given topic.
+	 * Note that this might not correspond to the actual next topic a message will
+	 * be forwarded to, since that depends on different factors.
+	 *
+	 * If you need to find out the exact next topic for a message use the
+	 * {@link DestinationTopicResolver#resolveDestinationTopic(String, Integer, Exception, long)}
+	 * method instead.
+	 * @param topicName the topic name of the DestinationTopic to be returned.
+	 * @return the next DestinationTopic in the chain registered for that topic.
+	 */
+	DestinationTopic getNextDestinationTopicFor(String topicName);
+
+	/**
+	 * Returns the {@link DestinationTopic} instance registered as
+	 * DLT for the given topic, or null if none is found.
+	 * @param topicName the topic name for which to look the DLT for
+	 * @return The {@link DestinationTopic} instance corresponding to the DLT.
+	 */
+	@Nullable
+	DestinationTopic getDltFor(String topicName);
+
 }