Add FanOut proposal

message/router.go

@@ -34,6 +34,11 @@ type HandlerFunc func(msg *Message) ([]*Message, error)
 // NoPublishHandlerFunc is HandlerFunc alternative, which doesn't produce any messages.
 type NoPublishHandlerFunc func(msg *Message) error
 
+// PassthroughHandler is a handler that passes the message unchanged from the subscriber to the publisher.
+var PassthroughHandler HandlerFunc = func(msg *Message) ([]*Message, error) {
+	return []*Message{msg}, nil
+}
+
 // HandlerMiddleware allows us to write something like decorators to HandlerFunc.
 // It can execute something before handler (for example: modify consumed message)
 // or after (modify produced messages, ack/nack on consumed message, handle errors, logging, etc.).

pubsub/gochannel/fanout.go

@@ -0,0 +1,99 @@
+package gochannel
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"sync"
+
+	"github.com/ThreeDotsLabs/watermill"
+	"github.com/ThreeDotsLabs/watermill/message"
+)
+
+// FanOut is a component that receives messages from the subscriber and passes them
+// to all publishers. In effect, messages are "multiplied".
+//
+// A typical use case for using FanOut is having one external subscription and multiple workers
+// inside the process.
+//
+// You need to call AddSubscription method for all topics that you want to listen to.
+// This needs to be done *before* the router is started.
+//
+// FanOut exposes the standard Subscriber interface.
+type FanOut struct {
+	internalPubSub *GoChannel
+
+	router     *message.Router
+	subscriber message.Subscriber
+
+	logger watermill.LoggerAdapter
+
+	subscribedTopics map[string]struct{}
+	subscribedLock   sync.Mutex
+}
+
+// NewFanOut creates new FanOut.
+// The passed router should not be running yet.
+func NewFanOut(
+	router *message.Router,
+	subscriber message.Subscriber,
+	logger watermill.LoggerAdapter,
+) (*FanOut, error) {
+	if router == nil {
+		return nil, errors.New("missing router")
+	}
+	if subscriber == nil {
+		return nil, errors.New("missing subscriber")
+	}
+	if logger == nil {
+		logger = watermill.NopLogger{}
+	}
+
+	return &FanOut{
+		internalPubSub: NewGoChannel(Config{}, logger),
+
+		router:     router,
+		subscriber: subscriber,
+
+		logger: logger,
+
+		subscribedTopics: map[string]struct{}{},
+	}, nil
+}
+
+// AddSubscription add an internal subscription for the given topic.
+// You need to call this method with all topics that you want to listen to, before the router is started.
+// AddSubscription is idempotent.
+func (f *FanOut) AddSubscription(topic string) {
+	f.subscribedLock.Lock()
+	defer f.subscribedLock.Unlock()
+
+	_, ok := f.subscribedTopics[topic]
+	if ok {
+		// Subscription already exists
+		return
+	}
+
+	f.logger.Trace("Adding fan-out subscription for topic", watermill.LogFields{
+		"topic": topic,
+	})
+
+	f.router.AddHandler(
+		fmt.Sprintf("fanout-%s", topic),
+		topic,
+		f.subscriber,
+		topic,
+		f.internalPubSub,
+		message.PassthroughHandler,
+	)
+
+	f.subscribedTopics[topic] = struct{}{}
+}
+
+func (f *FanOut) Subscribe(ctx context.Context, topic string) (<-chan *message.Message, error) {
+	return f.internalPubSub.Subscribe(ctx, topic)
+}
+
+func (f *FanOut) Close() error {
+	return f.internalPubSub.Close()
+}

pubsub/gochannel/fanout_test.go

@@ -0,0 +1,71 @@
+package gochannel_test
+
+import (
+	"context"
+	"fmt"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/ThreeDotsLabs/watermill"
+	"github.com/ThreeDotsLabs/watermill/message"
+	"github.com/ThreeDotsLabs/watermill/pubsub/gochannel"
+)
+
+func TestFanOut(t *testing.T) {
+	logger := watermill.NopLogger{}
+
+	upstreamPubSub := gochannel.NewGoChannel(gochannel.Config{}, logger)
+	upstreamTopic := "upstream-topic"
+
+	router, err := message.NewRouter(message.RouterConfig{}, logger)
+	require.NoError(t, err)
+
+	fanout, err := gochannel.NewFanOut(router, upstreamPubSub, logger)
+	require.NoError(t, err)
+
+	fanout.AddSubscription(upstreamTopic)
+
+	var counter uint64
+
+	workersCount := 10
+	messagesCount := 100
+
+	for i := 0; i < workersCount; i++ {
+		router.AddNoPublisherHandler(
+			fmt.Sprintf("worker-%v", i),
+			upstreamTopic,
+			fanout,
+			func(msg *message.Message) error {
+				atomic.AddUint64(&counter, 1)
+				return nil
+			},
+		)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second*1)
+	defer cancel()
+
+	go func() {
+		err := router.Run(ctx)
+		require.NoError(t, err)
+	}()
+
+	<-router.Running()
+
+	go func() {
+		for i := 0; i < messagesCount; i++ {
+			msg := message.NewMessage(watermill.NewUUID(), nil)
+			err := upstreamPubSub.Publish(upstreamTopic, msg)
+			if err != nil {
+				panic(err)
+			}
+		}
+	}()
+
+	<-ctx.Done()
+
+	require.Equal(t, uint64(workersCount*messagesCount), counter)
+}