Improve rebalance example by simulating

a user transaction an random failures
to show how it must behave

examples/consumer_rebalance_example/consumer_rebalance_example.go

@@ -16,16 +16,24 @@
 
 // Example high-level Apache Kafka consumer demonstrating use of rebalance
 // callback along with manual commit.
+// It processes a batch of messages in parallel and pairs Kafka commits with
+// a simulated user transaction. Random failures are present
+// to show how it must behave in each case.
 package main
 
 import (
 	"fmt"
+	"math/rand"
 	"os"
 	"os/signal"
+	"syscall"
+	"time"
 
 	"github.com/confluentinc/confluent-kafka-go/v2/kafka"
 )
 
+var processing []chan error
+
 func main() {
 	if len(os.Args) < 4 {
 		fmt.Fprintf(os.Stderr, "Usage: %s <bootstrap-servers> <group> <topics..>\n",
@@ -37,9 +45,10 @@ func main() {
 	bootstrapServers := os.Args[1]
 	group := os.Args[2]
 	topics := os.Args[3:]
+	exitCode := 0
 
 	sigchan := make(chan os.Signal, 1)
-	signal.Notify(sigchan, os.Interrupt)
+	signal.Notify(sigchan, syscall.SIGINT, syscall.SIGTERM)
 
 	c, err := kafka.NewConsumer(&kafka.ConfigMap{
 		"bootstrap.servers": bootstrapServers,
@@ -50,25 +59,38 @@ func main() {
 		// for this group.
 		"auto.offset.reset": "earliest",
 		// Whether or not we commit offsets automatically.
-		"enable.auto.commit": false,
+		"enable.auto.commit":   false,
+		"enable.partition.eof": true,
 	})
+	defer func() {
+		fmt.Printf("%% Closing consumer\n")
+		c.Close()
+		os.Exit(exitCode)
+	}()
 
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "Failed to create consumer: %s\n", err)
-		os.Exit(1)
+		exitCode = 1
+		return
 	}
 
 	fmt.Printf("%% Created Consumer %v\n", c)
 
 	// Subscribe to topics, call the rebalanceCallback on assignment/revoke.
 	// The rebalanceCallback can be triggered from c.Poll() and c.Close().
 	err = c.SubscribeTopics(topics, rebalanceCallback)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed subscribing to topics: %s\n", err)
+		exitCode = 1
+		return
+	}
 
 	run := true
-	for run == true {
+	for run {
 		select {
 		case sig := <-sigchan:
 			fmt.Printf("%% Caught signal %v: terminating\n", sig)
+			commit(c)
 			run = false
 		default:
 			ev := c.Poll(100)
@@ -79,12 +101,8 @@ func main() {
 			if err = processEvent(c, ev); err != nil {
 				fmt.Fprintf(os.Stderr, "Failed to process event: %s\n", err)
 			}
-
 		}
 	}
-
-	fmt.Printf("%% Closing consumer\n")
-	c.Close()
 }
 
 // processEvent processes the message/error received from the kafka Consumer's
@@ -94,12 +112,20 @@ func processEvent(c *kafka.Consumer, ev kafka.Event) error {
 
 	case *kafka.Message:
 		fmt.Printf("%% Message on %s:\n%s\n", e.TopicPartition, string(e.Value))
+		processMessage(e)
 
 		// Handle manual commit since enable.auto.commit is unset.
 		if err := maybeCommit(c, e.TopicPartition); err != nil {
 			return err
 		}
 
+	case kafka.PartitionEOF:
+		fmt.Printf("%% EOF for topic %s partition %d, committing\n", *e.Topic,
+			e.Partition)
+		if err := commit(c); err != nil {
+			return err
+		}
+
 	case kafka.Error:
 		// Errors should generally be considered informational, the client
 		// will try to automatically recover.
@@ -113,6 +139,42 @@ func processEvent(c *kafka.Consumer, ev kafka.Event) error {
 	return nil
 }
 
+// processMessage starts parallel processing of a message and
+// appends the returned channel to the processing slice.
+func processMessage(message *kafka.Message) error {
+	processing = append(processing, parallelProcessMessage(message))
+	return nil
+}
+
+// parallelProcessMessage starts parallel processing of a message and
+// returns a channel were the corresponding error will be produced.
+func parallelProcessMessage(message *kafka.Message) chan error {
+	channel := make(chan error)
+	go func() {
+		time.Sleep(100 * time.Millisecond)
+		channel <- randomFailure()
+		close(channel)
+	}()
+	return channel
+}
+
+// completeProcessing awaits all processing tasks and
+// returns an error if at least one of them failed.
+func completeProcessing() error {
+	fmt.Printf("%% Complete pending tasks\n")
+	var err error = nil
+	// Awaits a result from all the processing tasks
+	for _, channel := range processing {
+		currentErr := <-channel
+		if err == nil {
+			err = currentErr
+		}
+	}
+	// Clear the processing slice
+	processing = processing[:0]
+	return err
+}
+
 // maybeCommit is called for each message we receive from a Kafka topic.
 // This method can be used to apply some arbitary logic/processing to the
 // offsets, write the offsets into some external storage, and finally, to
@@ -126,7 +188,32 @@ func maybeCommit(c *kafka.Consumer, topicPartition kafka.TopicPartition) error {
 		return nil
 	}
 
-	commitedOffsets, err := c.Commit()
+	fmt.Printf("%% maybeCommit: do commit\n")
+	return commit(c)
+}
+
+// commit completes current parallel processing,
+// commits user transaction and offsets on Kafka.
+// If something fails, calls abort to abort the transaction.
+func commit(c *kafka.Consumer) error {
+	fmt.Printf("%% Committing transaction\n")
+
+	var err error
+	err = completeProcessing()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Processing tasks failed, aborting\n")
+		abort(c)
+		return err
+	}
+
+	err = userTransactionCommit()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "User commit failed, aborting\n")
+		abort(c)
+		return err
+	}
+
+	committedOffsets, err := c.Commit()
 
 	// ErrNoOffset occurs when there are no stored offsets to commit. This
 	// can happen if we haven't stored anything since the last commit.
@@ -135,10 +222,89 @@ func maybeCommit(c *kafka.Consumer, topicPartition kafka.TopicPartition) error {
 	// handling is illustrative of how to handle it in cases we call Commit()
 	// in another way, for example, every N seconds.
 	if err != nil && err.(kafka.Error).Code() != kafka.ErrNoOffset {
+		fmt.Fprintf(os.Stderr, "Fatal: Committed offsets to user transaction but not to Kafka: %s\n", err.Error())
 		return err
 	}
 
-	fmt.Printf("%% Commited offsets to Kafka: %v\n", commitedOffsets)
+	fmt.Printf("%% Committed offsets to Kafka: %v\n", committedOffsets)
+	return nil
+}
+
+// userTransactionCommit models committing a user transaction, that can be a
+// DBMS transaction of something else.
+// Returns a random failure
+func userTransactionCommit() error {
+	fmt.Printf("%% Committing user transaction\n")
+	return randomFailure()
+}
+
+// userTransactionAbort models aborting a user transaction, that can be a
+// DBMS transaction of something else.
+// Returns a random failure
+func userTransactionAbort() error {
+	fmt.Printf("%% Aborting user transaction\n")
+	return randomFailure()
+}
+
+// Returns an error with probability 0.05 or no error with probability 0.95
+func randomFailure() error {
+	if rand.Float64() < 0.05 {
+		return fmt.Errorf("random failure")
+	}
+	return nil
+}
+
+// abort completes current parallel processing,
+// aborts user transaction and rewinds consumer to committed offsets.
+func abort(c *kafka.Consumer) {
+	fmt.Printf("%% Aborting transaction\n")
+	completeProcessing()
+	// Ignore error, transaction is aborting anyway
+
+	var err error
+	// Continue retrying, if it cannot abort a transaction or seek assigned
+	// partitions, probably it cannot communicate with one of the two components,
+	// so it's not worth trying anything else.
+	for {
+		err = userTransactionAbort()
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "userTransactionAbort failed, retry: %s\n", err.Error())
+			continue
+		}
+		err = rewindConsumerPosition(c)
+		if err == nil {
+			return
+		}
+		fmt.Fprintf(os.Stderr, "rewindConsumerPosition failed, retry: %s\n", err.Error())
+		// Pause between retries
+		time.Sleep(3 * time.Second)
+	}
+}
+
+// rewindConsumerPosition Rewinds consumer's position to the
+// previous committed offset
+func rewindConsumerPosition(c *kafka.Consumer) error {
+	fmt.Printf("%% Rewind to committed offsets\n")
+	assignment, err := c.Assignment()
+	if err != nil {
+		return err
+	}
+
+	committed, err := c.Committed(assignment, 30*1000 /* 30s */)
+	if err != nil {
+		return err
+	}
+
+	for _, tp := range committed {
+		if tp.Offset < 0 {
+			tp.Offset = kafka.OffsetBeginning
+			tp.LeaderEpoch = nil
+		}
+		err := c.Seek(tp, 1)
+		if err != nil {
+			return err
+		}
+	}
 	return nil
 }
 
@@ -175,6 +341,11 @@ func rebalanceCallback(c *kafka.Consumer, event kafka.Event) error {
 		fmt.Printf("%% %s rebalance: %d partition(s) revoked: %v\n",
 			c.GetRebalanceProtocol(), len(ev.Partitions), ev.Partitions)
 
+		if c.IsClosed() {
+			// This is last revoke before closing, cannot commit here
+			// must be done before closing.
+			return nil
+		}
 		// Usually, the rebalance callback for `RevokedPartitions` is called
 		// just before the partitions are revoked. We can be certain that a
 		// partition being revoked is not yet owned by any other consumer.
@@ -183,28 +354,20 @@ func rebalanceCallback(c *kafka.Consumer, event kafka.Event) error {
 		// However, there can be cases where the assignment is lost
 		// involuntarily. In this case, the partition might already be owned
 		// by another consumer, and operations including committing
-		// offsets may not work.
+		// offsets may not work. We abort user transaction too in this case.
 		if c.AssignmentLost() {
 			// Our consumer has been kicked out of the group and the
 			// entire assignment is thus lost.
-			fmt.Fprintln(os.Stderr, "Assignment lost involuntarily, commit may fail")
-		}
-
-		// Since enable.auto.commit is unset, we need to commit offsets manually
-		// before the partition is revoked.
-		commitedOffsets, err := c.Commit()
-
-		if err != nil && err.(kafka.Error).Code() != kafka.ErrNoOffset {
-			fmt.Fprintf(os.Stderr, "Failed to commit offsets: %s\n", err)
-			return err
+			abort(c)
+		} else {
+			return commit(c)
 		}
-		fmt.Printf("%% Commited offsets to Kafka: %v\n", commitedOffsets)
 
 		// Similar to Assign, client automatically calls Unassign() unless the
 		// callback has already called that method. Here, we don't call it.
 
 	default:
-		fmt.Fprintf(os.Stderr, "Unxpected event type: %v\n", event)
+		fmt.Fprintf(os.Stderr, "Unexpected event type: %v\n", event)
 	}
 
 	return nil