#202 adds parallel processing option

consumer.go

@@ -43,6 +43,7 @@ func New(streamName string, opts ...Option) (*Consumer, error) {
 		scanInterval:             250 * time.Millisecond,
 		maxRecords:               10000,
 		metricRegistry:           nil,
+		numWorkers:               1,
 	}
 
 	// override defaults
@@ -93,6 +94,8 @@ type Consumer struct {
 	maxRecords         int64
 	isAggregated       bool
 	shardClosedHandler ShardClosedHandler
+	numWorkers         int
+	workerPool         WorkerPool
 }
 
 // ScanFunc is the type of the function called for each message read
@@ -115,25 +118,25 @@ func (c *Consumer) Scan(ctx context.Context, fn ScanFunc) error {
 	defer cancel()
 
 	var (
-		errc   = make(chan error, 1)
+		errC   = make(chan error, 1)
-		shardc = make(chan types.Shard, 1)
+		shardC = make(chan types.Shard, 1)
 	)
 
 	go func() {
-		c.group.Start(ctx, shardc)
+		c.group.Start(ctx, shardC)
 		<-ctx.Done()
-		close(shardc)
+		close(shardC)
 	}()
 
 	wg := new(sync.WaitGroup)
 	// process each of the shards
-	for shard := range shardc {
+	for shard := range shardC {
 		wg.Add(1)
 		go func(shardID string) {
 			defer wg.Done()
 			if err := c.ScanShard(ctx, shardID, fn); err != nil {
 				select {
-				case errc <- fmt.Errorf("shard %s error: %w", shardID, err):
+				case errC <- fmt.Errorf("shard %s error: %w", shardID, err):
 					// first error to occur
 					cancel()
 				default:
@@ -145,15 +148,19 @@ func (c *Consumer) Scan(ctx context.Context, fn ScanFunc) error {
 
 	go func() {
 		wg.Wait()
-		close(errc)
+		close(errC)
 	}()
 
-	return <-errc
+	return <-errC
 }
 
 // ScanShard loops over records on a specific shard, calls the callback func
 // for each record and checkpoints the progress of scan.
 func (c *Consumer) ScanShard(ctx context.Context, shardID string, fn ScanFunc) error {
+	wp := NewWorkerPool(c.streamName, c.numWorkers, fn)
+	wp.Start(ctx)
+	defer wp.Stop()
+
 	// get last seq number from checkpoint
 	lastSeqNum, err := c.group.GetCheckpoint(ctx, c.streamName, shardID)
 	if err != nil {
@@ -211,7 +218,14 @@ func (c *Consumer) ScanShard(ctx context.Context, shardID string, fn ScanFunc) e
 				case <-ctx.Done():
 					return nil
 				default:
-					err := fn(&Record{r, shardID, resp.MillisBehindLatest})
+					record := Record{r, shardID, resp.MillisBehindLatest}
+					wp.Submit(record)
+
+					res := wp.Result()
+					var err error
+					if res != nil && res.Err != nil {
+						err = res.Err
+					}
 
 					secondsBehindLatest := float64(time.Duration(*resp.MillisBehindLatest)*time.Millisecond) / float64(time.Second)
 					collectorMillisBehindLatest.

examples/consumer-redis/main.go

@@ -46,6 +46,7 @@ func main() {
 		consumer.WithClient(client),
 		consumer.WithStore(checkpointStore),
 		consumer.WithLogger(slog.Default()),
+		consumer.WithParallelProcessing(2),
 	)
 	if err != nil {
 		slog.Error("consumer error", slog.String("error", err.Error()))

options.go

@@ -91,6 +91,13 @@ func WithAggregation(a bool) Option {
 	}
 }
 
+// WithParallelProcessing sets the size of the Worker Pool that processes incoming requests. Defaults to 1
+func WithParallelProcessing(numWorkers int) Option {
+	return func(c *Consumer) {
+		c.numWorkers = numWorkers
+	}
+}
+
 // WithShardClosedHandler defines a custom handler for closed shards.
 func WithShardClosedHandler(h ShardClosedHandler) Option {
 	return func(c *Consumer) {

worker.go

@@ -1 +1,101 @@
 package consumer
+
+import (
+	"context"
+	"fmt"
+)
+
+// Result is the output of the worker. It contains the ID of the worker that processed it, the record itself (mainly to
+// maintain the offset that the record has and the error of processing to propagate up.
+type Result struct {
+	Record
+	WorkerName string
+	Err        error
+}
+
+// WorkerPool allows to parallel process records
+type WorkerPool struct {
+	name       string
+	numWorkers int
+	fn         ScanFunc
+	recordC    chan Record
+	resultC    chan Result
+}
+
+// NewWorkerPool returns an instance of WorkerPool
+func NewWorkerPool(name string, numWorkers int, fn ScanFunc) *WorkerPool {
+	return &WorkerPool{
+		name:       fmt.Sprintf("wp-%s", name),
+		numWorkers: numWorkers,
+		fn:         fn,
+		recordC:    make(chan Record, 1),
+		resultC:    make(chan Result, 1),
+	}
+}
+
+// Start spawns the amount of workers specified in numWorkers and starts them.
+func (wp *WorkerPool) Start(ctx context.Context) {
+	// How do I reopen workers if one fails?
+	for i := range wp.numWorkers {
+		name := fmt.Sprintf("%s-worker-%d", wp.name, i)
+		w := newWorker(name, wp.fn, wp.recordC, wp.resultC)
+		w.start(ctx)
+	}
+}
+
+// Stop stops the WorkerPool by closing the channels used for processing.
+func (wp *WorkerPool) Stop() {
+	close(wp.recordC)
+	close(wp.resultC)
+}
+
+// Submit a new Record for processing
+func (wp *WorkerPool) Submit(r Record) {
+	wp.recordC <- r
+}
+
+// Result returns the Result of the Submit-ed Record after it has been processed.
+func (wp *WorkerPool) Result() *Result {
+	select {
+	case r := <-wp.resultC:
+		return &r
+	default:
+		return nil
+	}
+}
+
+type worker struct {
+	name    string
+	fn      ScanFunc
+	recordC chan Record
+	resultC chan Result
+}
+
+func newWorker(name string, fn ScanFunc, recordC chan Record, resultC chan Result) *worker {
+	return &worker{
+		name:    name,
+		fn:      fn,
+		recordC: recordC,
+		resultC: resultC,
+	}
+}
+
+func (w *worker) start(ctx context.Context) {
+	go func(ctx context.Context) {
+		for r := range w.recordC {
+			select {
+			case <-ctx.Done():
+				return
+			default:
+				err := w.fn(&r)
+				res := Result{
+					Record:     r,
+					WorkerName: w.name,
+					Err:        err,
+				}
+
+				w.resultC <- res
+			}
+		}
+	}(ctx)
+}