MINOR: Add retry mechanism to EOS example (#15561)

In the initial EOS example, a retry logic was implemented within the resetToLastCommittedPositions method. During refactoring, this logic was removed becasue a poison pill prevented the example from reaching the final phase of consuming from the output topic.

In this change, I suggest to add it back, but with a retry limit defined as MAX_RETRIES. Once this limit is reached, the problematic batch will be logged and skipped, allowing the processor to move on and process remaining records. If some records are skipped, the example will still hit the hard timeout (2 minutes), but after consuming all processed records.

Reviewers: Luke Chen <showuon@gmail.com>

examples/src/main/java/kafka/examples/Consumer.java

@@ -109,7 +109,7 @@ public class Consumer extends Thread implements ConsumerRebalanceListener {
                 }
             }
         } catch (Throwable e) {
-            Utils.printOut("Unhandled exception");
+            Utils.printErr("Unhandled exception");
             e.printStackTrace();
         }
         Utils.printOut("Fetched %d records", numRecords - remainingRecords);

examples/src/main/java/kafka/examples/ExactlyOnceMessageProcessor.java

@@ -36,6 +36,7 @@ import org.apache.kafka.common.errors.UnsupportedVersionException;
 
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Optional;
@@ -49,6 +50,8 @@ import static java.util.Collections.singleton;
  * This class implements a read-process-write application.
  */
 public class ExactlyOnceMessageProcessor extends Thread implements ConsumerRebalanceListener, AutoCloseable {
+    private static final int MAX_RETRIES = 5;
+    
     private final String bootstrapServers;
     private final String inputTopic;
     private final String outputTopic;
@@ -103,19 +106,21 @@ public class ExactlyOnceMessageProcessor extends Thread implements ConsumerRebal
 
     @Override
     public void run() {
+        int retries = 0;
         int processedRecords = 0;
         long remainingRecords = Long.MAX_VALUE;
+
         // it is recommended to have a relatively short txn timeout in order to clear pending offsets faster
         int transactionTimeoutMs = 10_000;
         // consumer must be in read_committed mode, which means it won't be able to read uncommitted data
         boolean readCommitted = true;
+
         try (KafkaProducer<Integer, String> producer = new Producer("processor-producer", bootstrapServers, outputTopic,
                 true, transactionalId, true, -1, transactionTimeoutMs, null).createKafkaProducer();
              KafkaConsumer<Integer, String> consumer = new Consumer("processor-consumer", bootstrapServers, inputTopic,
                  "processor-group", Optional.of(groupInstanceId), readCommitted, -1, null).createKafkaConsumer()) {
             // called first and once to fence zombies and abort any pending transaction
             producer.initTransactions();
-
             consumer.subscribe(singleton(inputTopic), this);
 
             Utils.printOut("Processing new records");
@@ -140,6 +145,7 @@ public class ExactlyOnceMessageProcessor extends Thread implements ConsumerRebal
                         // commit the transaction including offsets
                         producer.commitTransaction();
                         processedRecords += records.count();
+                        retries = 0;
                     }
                 } catch (AuthorizationException | UnsupportedVersionException | ProducerFencedException
                          | FencedInstanceIdException | OutOfOrderSequenceException | SerializationException e) {
@@ -151,18 +157,21 @@ public class ExactlyOnceMessageProcessor extends Thread implements ConsumerRebal
                     Utils.printOut("Invalid or no offset found, using latest");
                     consumer.seekToEnd(emptyList());
                     consumer.commitSync();
+                    retries = 0;
                 } catch (KafkaException e) {
-                    // abort the transaction and try to continue
+                    // abort the transaction
-                    Utils.printOut("Aborting transaction: %s", e);
+                    Utils.printOut("Aborting transaction: %s", e.getMessage());
                     producer.abortTransaction();
+                    retries = maybeRetry(retries, consumer);
                 }
+
                 remainingRecords = getRemainingRecords(consumer);
                 if (remainingRecords != Long.MAX_VALUE) {
                     Utils.printOut("Remaining records: %d", remainingRecords);
                 }
             }
         } catch (Throwable e) {
-            Utils.printOut("Unhandled exception");
+            Utils.printErr("Unhandled exception");
             e.printStackTrace();
         }
         Utils.printOut("Processed %d records", processedRecords);
@@ -215,6 +224,44 @@ public class ExactlyOnceMessageProcessor extends Thread implements ConsumerRebal
         }).sum();
     }
 
+    /**
+     * When we get a generic {@code KafkaException} while processing records, we retry up to {@code MAX_RETRIES} times.
+     * If we exceed this threshold, we log an error and move on to the next batch of records.
+     * In a real world application you may want to to send these records to a dead letter topic (DLT) for further processing.
+     * 
+     * @param retries Current number of retries
+     * @param consumer Consumer instance
+     * @return Updated number of retries
+     */
+    private int maybeRetry(int retries, KafkaConsumer<Integer, String> consumer) {
+        if (retries < 0) {
+            Utils.printErr("The number of retries must be greater than zero");
+            shutdown();
+        }
+        
+        if (retries < MAX_RETRIES) {
+            // retry: reset fetch offset
+            // the consumer fetch position needs to be restored to the committed offset before the transaction started
+            Map<TopicPartition, OffsetAndMetadata> committed = consumer.committed(consumer.assignment());
+            consumer.assignment().forEach(tp -> {
+                OffsetAndMetadata offsetAndMetadata = committed.get(tp);
+                if (offsetAndMetadata != null) {
+                    consumer.seek(tp, offsetAndMetadata.offset());
+                } else {
+                    consumer.seekToBeginning(Collections.singleton(tp));
+                }
+            });
+            retries++;
+        } else {
+            // continue: skip records
+            // the consumer fetch position needs to be committed as if records were processed successfully
+            Utils.printErr("Skipping records after %d retries", MAX_RETRIES);
+            consumer.commitSync();
+            retries = 0;
+        }
+        return retries;
+    }
+
     @Override
     public void close() throws Exception {
         if (producer != null) {

examples/src/main/java/kafka/examples/Producer.java

@@ -89,7 +89,7 @@ public class Producer extends Thread {
                 sentRecords++;
             }
         } catch (Throwable e) {
-            Utils.printOut("Unhandled exception");
+            Utils.printErr("Unhandled exception");
             e.printStackTrace();
         }
         Utils.printOut("Sent %d records", sentRecords);