fix(forwarder): cap number of concurrency SQS tasks

The forwarder function processes SQS tasks concurrently. By default this
value was previously unbounded, which would cause significant delay when
redriving a Dead Letter Queue.

This commit changes the default to be set to the runtime number of CPUs,
which allows the behaviour to scale with the amount of memory allocated
to the lambda function.

pkg/handler/forwarder/config.go

@@ -15,14 +15,14 @@ var (
 )
 
 type Config struct {
-	DestinationURI    string // S3 URI to write messages and copy files to
-	MaxFileSize       int64  // maximum file size in bytes for the files to be processed
-	SourceBucketNames []string
-	SourceObjectKeys  []string
-	Override          Override
-	S3Client          S3Client
-	GetTime           func() *time.Time
-	MaxConcurrency    int // fan out limit
+	DestinationURI     string // S3 URI to write messages and copy files to
+	MaxFileSize        int64  // maximum file size in bytes for the files to be processed
+	SourceBucketNames  []string
+	SourceObjectKeys   []string
+	Override           Override
+	S3Client           S3Client
+	GetTime            func() *time.Time
+	MaxConcurrentTasks int // fan out limit
 }
 
 func (c *Config) Validate() error {

pkg/handler/forwarder/handler.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"io"
 	"net/url"
+	"runtime"
 	"strings"
 	"time"
 
@@ -36,13 +37,14 @@ type Override interface {
 
 type Handler struct {
 	handler.Mux
-	MaxFileSize    int64
-	DestinationURI *url.URL
-	S3Client       S3Client
-	Override       Override
-	ObjectPolicy   interface{ Allow(string) bool }
-	Now            func() time.Time
-	MaxConcurrency int
+
+	MaxFileSize        int64
+	DestinationURI     *url.URL
+	S3Client           S3Client
+	Override           Override
+	ObjectPolicy       interface{ Allow(string) bool }
+	Now                func() time.Time
+	MaxConcurrentTasks int
 }
 
 // GetCopyObjectInput constructs the input struct for CopyObject.
@@ -169,8 +171,8 @@ func (h *Handler) Handle(ctx context.Context, request events.SQSEvent) (response
 		releaseToken = func() {}
 	)
 
-	if h.MaxConcurrency > 0 {
-		limitCh := make(chan struct{}, h.MaxConcurrency)
+	if h.MaxConcurrentTasks > 0 {
+		limitCh := make(chan struct{}, h.MaxConcurrentTasks)
 		defer close(limitCh)
 		acquireToken = func() { limitCh <- struct{}{} }
 		releaseToken = func() { <-limitCh }
@@ -217,16 +219,21 @@ func New(cfg *Config) (h *Handler, err error) {
 
 	u, _ := url.ParseRequestURI(cfg.DestinationURI)
 
+	maxConcurrentTasks := cfg.MaxConcurrentTasks
+	if maxConcurrentTasks == 0 {
+		maxConcurrentTasks = runtime.NumCPU()
+	}
+
 	objectFilter, _ := NewObjectFilter(cfg.SourceBucketNames, cfg.SourceObjectKeys)
 
 	h = &Handler{
-		DestinationURI: u,
-		S3Client:       cfg.S3Client,
-		MaxFileSize:    cfg.MaxFileSize,
-		Override:       cfg.Override,
-		ObjectPolicy:   objectFilter,
-		Now:            time.Now,
-		MaxConcurrency: cfg.MaxConcurrency,
+		DestinationURI:     u,
+		S3Client:           cfg.S3Client,
+		MaxFileSize:        cfg.MaxFileSize,
+		Override:           cfg.Override,
+		ObjectPolicy:       objectFilter,
+		Now:                time.Now,
+		MaxConcurrentTasks: maxConcurrentTasks,
 	}
 
 	return h, nil

pkg/handler/forwarder/handler_test.go

@@ -275,7 +275,7 @@ func TestHandler(t *testing.T) {
 						return nil, errSentinel
 					},
 				},
-				MaxConcurrency: 1, // ensure ordering
+				MaxConcurrentTasks: 1, // ensure ordering
 			},
 			ExpectedCopyCalls: 3, // Expect three unsuccessful calls to CopyObjectFunc
 			ExpectResponse: events.SQSEventResponse{

pkg/lambda/forwarder/lambda.go

@@ -38,6 +38,7 @@ type Config struct {
 	PresetOverrides      []string         `env:"PRESET_OVERRIDES,default=aws/v1,infer/v1"`
 	SourceBucketNames    []string         `env:"SOURCE_BUCKET_NAMES"`
 	SourceObjectKeys     []string         `env:"SOURCE_OBJECT_KEYS"`
+	MaxConcurrentTasks   int              `env:"MAX_CONCURRENT_TASKS"`
 
 	Logging *logging.Config
 
@@ -136,12 +137,13 @@ func New(ctx context.Context, cfg *Config) (*Lambda, error) {
 	}
 
 	f, err := forwarder.New(&forwarder.Config{
-		DestinationURI:    cfg.DestinationURI,
-		MaxFileSize:       cfg.MaxFileSize,
-		S3Client:          s3Client,
-		Override:          append(override.Sets{customOverrides}, presets...),
-		SourceBucketNames: cfg.SourceBucketNames,
-		SourceObjectKeys:  cfg.SourceObjectKeys,
+		DestinationURI:     cfg.DestinationURI,
+		MaxFileSize:        cfg.MaxFileSize,
+		S3Client:           s3Client,
+		Override:           append(override.Sets{customOverrides}, presets...),
+		SourceBucketNames:  cfg.SourceBucketNames,
+		SourceObjectKeys:   cfg.SourceObjectKeys,
+		MaxConcurrentTasks: cfg.MaxConcurrentTasks,
 	})
 	if err != nil {
 		return nil, fmt.Errorf("failed to create handler: %w", err)