Scanerror signals the consumer if we should continue scanning for next record & whether to checkpoint. (#54)

* remove ValidateCheckpoint

* update for checkpoint can not customize retryer

* implement the scan error as in PR 44

* at least log if record processor has error

* mistakenly removed this line

* propage error up. ignore invalid state

README.md

@@ -38,9 +38,13 @@ func main() {
 	}
 
 	// start
-	err = c.Scan(context.TODO(), func(r *consumer.Record) bool {
+	err = c.Scan(context.TODO(), func(r *consumer.Record) consumer.ScanError {
 		fmt.Println(string(r.Data))
-		return true // continue scanning
+        // continue scanning
+        return consumer.ScanError{
+            StopScan:       false,  // true to stop scan 
+            SkipCheckpoint: false,  // true to skip checkpoint
+        }
 	})
 	if err != nil {
 		log.Fatalf("scan error: %v", err)
@@ -53,7 +57,7 @@ func main() {
 
 ## Checkpoint
 
-To record the progress of the consumer in the stream we use a checkpoint to store the last sequence number the consumer has read from a particular shard.
+To record the progress of the consumer in the stream we use a checkpoint to store the last sequence number the consumer has read from a particular shard. The boolean value SkipCheckpoint of consumer.ScanError determines if checkpoint will be activated. ScanError is returned by the record processing callback.
 
 This will allow consumers to re-launch and pick up at the position in the stream where they left off.
 

consumer.go

@@ -10,6 +10,14 @@ import (
 	"github.com/aws/aws-sdk-go/service/kinesis"
 )
 
+// ScanError signals the consumer if we should continue scanning for next record
+// and whether to checkpoint.
+type ScanError struct {
+	Error          error
+	StopScan       bool
+	SkipCheckpoint bool
+}
+
 // Record is an alias of record returned from kinesis library
 type Record = kinesis.Record
 
@@ -111,7 +119,7 @@ type Consumer struct {
 
 // Scan scans each of the shards of the stream, calls the callback
 // func with each of the kinesis records.
-func (c *Consumer) Scan(ctx context.Context, fn func(*Record) bool) error {
+func (c *Consumer) Scan(ctx context.Context, fn func(*Record) ScanError) error {
 	shardIDs, err := c.client.GetShardIDs(c.streamName)
 	if err != nil {
 		return fmt.Errorf("get shards error: %v", err)
@@ -156,33 +164,41 @@ func (c *Consumer) Scan(ctx context.Context, fn func(*Record) bool) error {
 // ScanShard loops over records on a specific shard, calls the callback func
 // for each record and checkpoints the progress of scan.
 // Note: Returning `false` from the callback func will end the scan.
-func (c *Consumer) ScanShard(ctx context.Context, shardID string, fn func(*Record) bool) (err error) {
+func (c *Consumer) ScanShard(ctx context.Context, shardID string, fn func(*Record) ScanError) (err error) {
 	lastSeqNum, err := c.checkpoint.Get(c.streamName, shardID)
 	if err != nil {
 		return fmt.Errorf("get checkpoint error: %v", err)
 	}
 
 	c.logger.Println("scanning", shardID, lastSeqNum)
-
 	// get records
 	recc, errc, err := c.client.GetRecords(ctx, c.streamName, shardID, lastSeqNum)
 	if err != nil {
 		return fmt.Errorf("get records error: %v", err)
 	}
-
 	// loop records
 	for r := range recc {
-		if ok := fn(r); !ok {
+		scanError := fn(r)
+		// It will be nicer if this can be reported with checkpoint error
+		err = scanError.Error
+
+		// Skip invalid state
+		if scanError.StopScan && scanError.SkipCheckpoint {
+			continue
+		}
+
+		if scanError.StopScan {
 			break
 		}
 
+		if !scanError.SkipCheckpoint {
 			c.counter.Add("records", 1)
-
 			err := c.checkpoint.Set(c.streamName, shardID, *r.SequenceNumber)
 			if err != nil {
 				return fmt.Errorf("set checkpoint error: %v", err)
 			}
 		}
+	}
 
 	c.logger.Println("exiting", shardID)
 	return <-errc

consumer_test.go

@@ -45,9 +45,14 @@ func TestScanShard(t *testing.T) {
 	// callback fn simply appends the record data to result string
 	var (
 		resultData string
-		fn         = func(r *Record) bool {
+		fn         = func(r *Record) ScanError {
 			resultData += string(r.Data)
-			return true
+			err := errors.New("some error happened")
+			return ScanError{
+				Error:          err,
+				StopScan:       false,
+				SkipCheckpoint: false,
+			}
 		}
 	)
 

examples/consumer/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"errors"
 	"expvar"
 	"flag"
 	"fmt"
@@ -89,9 +90,15 @@ func main() {
 	}()
 
 	// scan stream
-	err = c.Scan(ctx, func(r *consumer.Record) bool {
+	err = c.Scan(ctx, func(r *consumer.Record) consumer.ScanError {
 		fmt.Println(string(r.Data))
-		return true // continue scanning
+		err := errors.New("some error happened")
+		// continue scanning
+		return consumer.ScanError{
+			Error:          err,
+			StopScan:       true,
+			SkipCheckpoint: false,
+		}
 	})
 	if err != nil {
 		log.Fatalf("scan error: %v", err)