Introduce Client Interface

Testing the components of the consumer where proving difficult because
the consumer code was so tightly coupled with the Kinesis client
library.

* Extract the concept of Client interface
* Create default client w/ kinesis connection
* Test with fake client to avoid round trip to kinesis

client.go

@@ -0,0 +1,123 @@
+package consumer
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/aws/session"
+	"github.com/aws/aws-sdk-go/service/kinesis"
+)
+
+// NewClient returns a new client with kinesis client
+func NewClient() *client {
+	svc := kinesis.New(session.New(aws.NewConfig()))
+	return &client{svc}
+}
+
+// Client acts as wrapper around Kinesis client
+type client struct {
+	svc *kinesis.Kinesis
+}
+
+// GetShardIDs returns shard ids in a given stream
+func (c *client) GetShardIDs(streamName string) ([]string, error) {
+	resp, err := c.svc.DescribeStream(
+		&kinesis.DescribeStreamInput{
+			StreamName: aws.String(streamName),
+		},
+	)
+	if err != nil {
+		return nil, fmt.Errorf("describe stream error: %v", err)
+	}
+
+	ss := []string{}
+	for _, shard := range resp.StreamDescription.Shards {
+		ss = append(ss, *shard.ShardId)
+	}
+	return ss, nil
+}
+
+// GetRecords returns a chan Record from a Shard of the Stream
+func (c *client) GetRecords(ctx context.Context, streamName, shardID, lastSeqNum string) (<-chan *Record, <-chan error, error) {
+	shardIterator, err := c.getShardIterator(streamName, shardID, lastSeqNum)
+	if err != nil {
+		return nil, nil, fmt.Errorf("get shard iterator error: %v", err)
+	}
+
+	var (
+		recc = make(chan *Record, 10000)
+		errc = make(chan error, 1)
+	)
+
+	go func() {
+		defer func() {
+			close(recc)
+			close(errc)
+		}()
+
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			default:
+				resp, err := c.svc.GetRecords(
+					&kinesis.GetRecordsInput{
+						ShardIterator: shardIterator,
+					},
+				)
+
+				if err != nil {
+					shardIterator, err = c.getShardIterator(streamName, shardID, lastSeqNum)
+					if err != nil {
+						errc <- fmt.Errorf("get shard iterator error: %v", err)
+						return
+					}
+					continue
+				}
+
+				for _, r := range resp.Records {
+					select {
+					case <-ctx.Done():
+						return
+					case recc <- r:
+						lastSeqNum = *r.SequenceNumber
+					}
+				}
+
+				if resp.NextShardIterator == nil || shardIterator == resp.NextShardIterator {
+					shardIterator, err = c.getShardIterator(streamName, shardID, lastSeqNum)
+					if err != nil {
+						errc <- fmt.Errorf("get shard iterator error: %v", err)
+						return
+					}
+				} else {
+					shardIterator = resp.NextShardIterator
+				}
+			}
+		}
+	}()
+
+	return recc, errc, nil
+}
+
+func (c *client) getShardIterator(streamName, shardID, lastSeqNum string) (*string, error) {
+	params := &kinesis.GetShardIteratorInput{
+		ShardId:    aws.String(shardID),
+		StreamName: aws.String(streamName),
+	}
+
+	if lastSeqNum != "" {
+		params.ShardIteratorType = aws.String("AFTER_SEQUENCE_NUMBER")
+		params.StartingSequenceNumber = aws.String(lastSeqNum)
+	} else {
+		params.ShardIteratorType = aws.String("TRIM_HORIZON")
+	}
+
+	resp, err := c.svc.GetShardIterator(params)
+	if err != nil {
+		return nil, err
+	}
+
+	return resp.ShardIterator, nil
+}

consumer.go

@@ -7,13 +7,18 @@ import (
 	"log"
 	"sync"
 
-	"github.com/aws/aws-sdk-go/aws"
-	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/aws/aws-sdk-go/service/kinesis"
 )
 
+// Record is an alias of record returned from kinesis library
 type Record = kinesis.Record
 
+// Client interface is used for interacting with kinesis stream
+type Client interface {
+	GetShardIDs(string) ([]string, error)
+	GetRecords(ctx context.Context, streamName, shardID, lastSeqNum string) (<-chan *Record, <-chan error, error)
+}
+
 // Counter interface is used for exposing basic metrics from the scanner
 type Counter interface {
 	Add(string, int64)
@@ -61,39 +66,44 @@ func WithCounter(counter Counter) Option {
 	}
 }
 
+// WithClient overrides the default client
+func WithClient(client Client) Option {
+	return func(c *Consumer) error {
+		c.client = client
+		return nil
+	}
+}
+
 // New creates a kinesis consumer with default settings. Use Option to override
 // any of the optional attributes.
-func New(stream string, opts ...Option) (*Consumer, error) {
-	if stream == "" {
+func New(streamName string, opts ...Option) (*Consumer, error) {
+	if streamName == "" {
 		return nil, fmt.Errorf("must provide stream name")
 	}
 
+	// new consumer with no-op checkpoint, counter, and logger
 	c := &Consumer{
-		streamName: stream,
+		streamName: streamName,
 		checkpoint: &noopCheckpoint{},
 		counter:    &noopCounter{},
 		logger:     log.New(ioutil.Discard, "", log.LstdFlags),
+		client:     NewClient(),
 	}
 
-	// set options
+	// override defaults
 	for _, opt := range opts {
 		if err := opt(c); err != nil {
 			return nil, err
 		}
 	}
 
-	// provide a default kinesis client
-	if c.client == nil {
-		c.client = kinesis.New(session.New(aws.NewConfig()))
-	}
-
 	return c, nil
 }
 
 // Consumer wraps the interaction with the Kinesis stream
 type Consumer struct {
 	streamName string
-	client     *kinesis.Kinesis
+	client     Client
 	logger     *log.Logger
 	checkpoint Checkpoint
 	counter    Counter
@@ -101,32 +111,27 @@ 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(*kinesis.Record) bool) error {
-	ctx, cancel := context.WithCancel(ctx)
-	defer cancel()
-
-	// grab the stream details
-	resp, err := c.client.DescribeStream(
-		&kinesis.DescribeStreamInput{
-			StreamName: aws.String(c.streamName),
-		},
-	)
+func (c *Consumer) Scan(ctx context.Context, fn func(*Record) bool) error {
+	shardIDs, err := c.client.GetShardIDs(c.streamName)
 	if err != nil {
-		return fmt.Errorf("describe stream error: %v", err)
+		return fmt.Errorf("get shards error: %v", err)
 	}
 
-	if len(resp.StreamDescription.Shards) == 0 {
+	if len(shardIDs) == 0 {
 		return fmt.Errorf("no shards available")
 	}
 
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
 	var (
 		wg   sync.WaitGroup
 		errc = make(chan error, 1)
 	)
-	wg.Add(len(resp.StreamDescription.Shards))
+	wg.Add(len(shardIDs))
 
-	// launch goroutine to process each of the shards
-	for _, shard := range resp.StreamDescription.Shards {
+	// process each shard in goroutine
+	for _, shardID := range shardIDs {
 		go func(shardID string) {
 			defer wg.Done()
 
@@ -139,9 +144,8 @@ func (c *Consumer) Scan(ctx context.Context, fn func(*kinesis.Record) bool) erro
 				}
 			}
 
-			c.logger.Println("exiting", shardID)
 			cancel()
-		}(*shard.ShardId)
+		}(shardID)
 	}
 
 	wg.Wait()
@@ -152,100 +156,34 @@ func (c *Consumer) Scan(ctx context.Context, fn func(*kinesis.Record) bool) erro
 // 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(*kinesis.Record) bool) error {
-	c.logger.Println("scanning", shardID)
-
+func (c *Consumer) ScanShard(ctx context.Context, shardID string, fn func(*Record) bool) (err error) {	
 	lastSeqNum, err := c.checkpoint.Get(c.streamName, shardID)
 	if err != nil {
 		return fmt.Errorf("get checkpoint error: %v", err)
 	}
 
-	shardIterator, err := c.getShardIterator(shardID, lastSeqNum)
+	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 shard iterator error: %v", err)
+		return fmt.Errorf("get records error: %v", err)
 	}
 
-	for {
-		select {
-		case <-ctx.Done():
-			return nil
-		default:
-			resp, err := c.client.GetRecords(
-				&kinesis.GetRecordsInput{
-					ShardIterator: shardIterator,
-				},
-			)
+	// loop records
+	for r := range recc {
+		if ok := fn(r); !ok {
+			break
+		}
 
-			if err != nil {
-				shardIterator, err = c.getShardIterator(shardID, lastSeqNum)
-				if err != nil {
-					return fmt.Errorf("get shard iterator error: %v", err)
-				}
-				continue
-			}
+		c.counter.Add("records", 1)
 
-			if len(resp.Records) > 0 {
-				for _, r := range resp.Records {
-					select {
-					case <-ctx.Done():
-						return nil
-					default:
-						lastSeqNum = *r.SequenceNumber
-						c.counter.Add("records", 1)
-
-						if ok := fn(r); !ok {
-							if err := c.setCheckpoint(shardID, lastSeqNum); err != nil {
-								return fmt.Errorf("set checkpoint error: %v", err)
-							}
-							return nil
-						}
-					}
-				}
-
-				if err := c.setCheckpoint(shardID, lastSeqNum); err != nil {
-					return fmt.Errorf("set checkpoint error: %v", err)
-				}
-			}
-
-			if resp.NextShardIterator == nil || shardIterator == resp.NextShardIterator {
-				shardIterator, err = c.getShardIterator(shardID, lastSeqNum)
-				if err != nil {
-					return fmt.Errorf("get shard iterator error: %v", err)
-				}
-			} else {
-				shardIterator = resp.NextShardIterator
-			}
+		err := c.checkpoint.Set(c.streamName, shardID, *r.SequenceNumber)
+		if err != nil {
+			return fmt.Errorf("set checkpoint error: %v", err)
 		}
 	}
-}
-
-func (c *Consumer) setCheckpoint(shardID, lastSeqNum string) error {
-	err := c.checkpoint.Set(c.streamName, shardID, lastSeqNum)
-	if err != nil {
-		return err
-	}
-	c.logger.Println("checkpoint", shardID)
-	c.counter.Add("checkpoints", 1)
-	return nil
-}
-
-func (c *Consumer) getShardIterator(shardID, lastSeqNum string) (*string, error) {
-	params := &kinesis.GetShardIteratorInput{
-		ShardId:    aws.String(shardID),
-		StreamName: aws.String(c.streamName),
-	}
-
-	if lastSeqNum != "" {
-		params.ShardIteratorType = aws.String("AFTER_SEQUENCE_NUMBER")
-		params.StartingSequenceNumber = aws.String(lastSeqNum)
-	} else {
-		params.ShardIteratorType = aws.String("TRIM_HORIZON")
-	}
-
-	resp, err := c.client.GetShardIterator(params)
-	if err != nil {
-		return nil, err
-	}
-
-	return resp.ShardIterator, nil
+
+	c.logger.Println("exiting", shardID)
+	return <-errc
 }

consumer_test.go

@@ -0,0 +1,135 @@
+package consumer
+
+import (
+	"context"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"sync"
+	"testing"
+
+	"github.com/aws/aws-sdk-go/aws"
+)
+
+func TestNew(t *testing.T) {
+	_, err := New("myStreamName")
+	if err != nil {
+		t.Fatalf("new consumer error: %v", err)
+	}
+}
+
+func TestScanShard(t *testing.T) {
+	var (
+		ckp    = &fakeCheckpoint{cache: map[string]string{}}
+		ctr    = &fakeCounter{}
+		client = newFakeClient(
+			&Record{
+				Data:           []byte("firstData"),
+				SequenceNumber: aws.String("firstSeqNum"),
+			},
+			&Record{
+				Data:           []byte("lastData"),
+				SequenceNumber: aws.String("lastSeqNum"),
+			},
+		)
+	)
+
+	c := &Consumer{
+		streamName: "myStreamName",
+		client:     client,
+		checkpoint: ckp,
+		counter:    ctr,
+		logger:     log.New(ioutil.Discard, "", log.LstdFlags),
+	}
+
+	// callback fn simply appends the record data to result string
+	var (
+		resultData string
+		fn         = func(r *Record) bool {
+			resultData += string(r.Data)
+			return true
+		}
+	)
+
+	// scan shard
+	err := c.ScanShard(context.Background(), "myShard", fn)
+	if err != nil {
+		t.Fatalf("scan shard error: %v", err)
+	}
+
+	// increments counter
+	if val := ctr.counter; val != 2 {
+		t.Fatalf("counter error expected %d, got %d", 2, val)
+	}
+
+	// sets checkpoint
+	val, err := ckp.Get("myStreamName", "myShard")
+	if err != nil && val != "lastSeqNum" {
+		t.Fatalf("checkout error expected %s, got %s", "lastSeqNum", val)
+	}
+
+	// calls callback func
+	if resultData != "firstDatalastData" {
+		t.Fatalf("callback error expected %s, got %s", "firstDatalastData", val)
+	}
+}
+
+func newFakeClient(rs ...*Record) *fakeClient {
+	fc := &fakeClient{
+		recc: make(chan *Record, len(rs)),
+		errc: make(chan error),
+	}
+
+	for _, r := range rs {
+		fc.recc <- r
+	}
+
+	close(fc.errc)
+	close(fc.recc)
+
+	return fc
+}
+
+type fakeClient struct {
+	shardIDs []string
+	recc     chan *Record
+	errc     chan error
+}
+
+func (fc *fakeClient) GetShardIDs(string) ([]string, error) {
+	return fc.shardIDs, nil
+}
+
+func (fc *fakeClient) GetRecords(ctx context.Context, streamName, shardID, lastSeqNum string) (<-chan *Record, <-chan error, error) {
+	return fc.recc, fc.errc, nil
+}
+
+type fakeCheckpoint struct {
+	cache map[string]string
+	mu    sync.Mutex
+}
+
+func (fc *fakeCheckpoint) Set(streamName, shardID, sequenceNumber string) error {
+	fc.mu.Lock()
+	defer fc.mu.Unlock()
+
+	key := fmt.Sprintf("%s-%s", streamName, shardID)
+	fc.cache[key] = sequenceNumber
+	return nil
+}
+
+func (fc *fakeCheckpoint) Get(streamName, shardID string) (string, error) {
+	fc.mu.Lock()
+	defer fc.mu.Unlock()
+
+	key := fmt.Sprintf("%s-%s", streamName, shardID)
+	return fc.cache[key], nil
+}
+
+type fakeCounter struct {
+	counter int64
+}
+
+func (fc *fakeCounter) Add(streamName string, count int64) {
+	fc.counter += count
+}