fix: Enable parallel requests in the http output

internal/impl/io/output_http_client.go

@@ -2,7 +2,10 @@ package io
 
 import (
 	"context"
+	"errors"
+	"fmt"
 
+	"github.com/benthosdev/benthos/v4/internal/component"
 	"github.com/benthosdev/benthos/v4/internal/component/output"
 	"github.com/benthosdev/benthos/v4/internal/httpclient"
 	"github.com/benthosdev/benthos/v4/public/service"
@@ -27,6 +30,9 @@ It's possible to propagate the response from each HTTP request back to the input
 			service.NewBoolField("batch_as_multipart").
 				Description("Send message batches as a single request using [RFC1341](https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html). If disabled messages in batches will be sent as individual requests.").
 				Advanced().Default(false),
+			service.NewBoolField("parallel").
+				Description("When processing batched messages and `batch_as_multipart` is false, whether to send messages of the batch in parallel, otherwise they are sent serially.").
+				Advanced().Default(false),
 			service.NewBoolField("propagate_response").
 				Description("Whether responses from the server should be [propagated back](/docs/guides/sync_responses) to the input.").
 				Advanced().Default(false),
@@ -79,6 +85,7 @@ type httpClientWriter struct {
 	logURL           string
 	propResponse     bool
 	batchAsMultipart bool
+	parallel         bool
 }
 
 func newHTTPClientOutputFromParsed(conf *service.ParsedConfig, mgr *service.Resources) (*httpClientWriter, error) {
@@ -123,12 +130,18 @@ func newHTTPClientOutputFromParsed(conf *service.ParsedConfig, mgr *service.Reso
 		return nil, err
 	}
 
+	parallel, err := conf.FieldBool("parallel")
+	if err != nil {
+		return nil, err
+	}
+
 	return &httpClientWriter{
 		client:           client,
 		log:              mgr.Logger(),
 		logURL:           logURL,
 		propResponse:     propResponse,
 		batchAsMultipart: batchAsMultipart,
+		parallel:         parallel,
 	}, nil
 }
 
@@ -139,12 +152,15 @@ func (h *httpClientWriter) Connect(ctx context.Context) error {
 
 func (h *httpClientWriter) WriteBatch(ctx context.Context, msg service.MessageBatch) error {
 	if len(msg) > 1 && !h.batchAsMultipart {
-		for _, v := range msg {
-			if err := h.WriteBatch(ctx, service.MessageBatch{v}); err != nil {
-				return err
+		if !h.parallel {
+			for _, v := range msg {
+				if err := h.WriteBatch(ctx, service.MessageBatch{v}); err != nil {
+					return err
+				}
 			}
 		}
-		return nil
+		// Hard, need to do parallel requests limited by max parallelism.
+		return h.handleParallelRequests(msg)
 	}
 
 	resultMsg, err := h.client.Send(ctx, msg)
@@ -175,6 +191,58 @@ func (h *httpClientWriter) WriteBatch(ctx context.Context, msg service.MessageBa
 	return err
 }
 
+func (h *httpClientWriter) handleParallelRequests(msg service.MessageBatch) error {
+	results := make(service.MessageBatch, len(msg))
+	for i, p := range msg {
+		results[i] = p.Copy()
+	}
+	reqChan, resChan := make(chan int), make(chan error)
+
+	for i := 0; i < len(msg); i++ {
+		go func() {
+			for index := range reqChan {
+				tmpMsg := service.MessageBatch{msg[index]}
+				result, err := h.client.Send(context.Background(), tmpMsg)
+				if err == nil && len(result) != 1 {
+					err = fmt.Errorf("unexpected response size: %v", len(result))
+				}
+				if err == nil {
+					mBytes, _ := result[0].AsBytes()
+					results[index].SetBytes(mBytes)
+					_ = result[0].MetaWalkMut(func(k string, v any) error {
+						results[index].MetaSetMut(k, v)
+						return nil
+					})
+				} else {
+					var hErr component.ErrUnexpectedHTTPRes
+					if ok := errors.As(err, &hErr); ok {
+						results[index].MetaSetMut("http_status_code", hErr.Code)
+					}
+					results[index].SetError(err)
+				}
+				resChan <- err
+			}
+		}()
+	}
+	go func() {
+		for i := 0; i < len(msg); i++ {
+			reqChan <- i
+		}
+	}()
+	for i := 0; i < len(msg); i++ {
+		if err := <-resChan; err != nil {
+			h.log.Errorf("HTTP parallel request to '%v' failed: %v", h.logURL, err)
+		}
+	}
+
+	close(reqChan)
+
+	if len(results) < 1 {
+		return fmt.Errorf("HTTP response from '%v' was empty", h.logURL)
+	}
+	return nil
+}
+
 func (h *httpClientWriter) Close(ctx context.Context) error {
 	return h.client.Close(ctx)
 }