Add generic jsonendpoint adapter

components/jsonendpoint/endpoint.go

@@ -0,0 +1,122 @@
+/*
+
+Package jsonendpoint provides a generic adapter that converts a validated JSON request into a [message.Message] published to a certain topic. Use together with an HTTP router to build gateways to any [message.Publisher].
+
+# Usage Example
+
+Imagine a fairly standard situation of collecting newsletter signups from a Restful API:
+
+	type NewsletterSignup struct {
+		Name  string
+		Email string
+	}
+
+    // Validate satisfies [Validator] interface.
+	func (n *NewsletterSignup) Validate() error {
+		if n.Name == "" || n.Email == "" {
+			return errors.New("newsletter signup requires both name and email address")
+		}
+		return nil
+	}
+
+	func main() {
+		pubSub := gochannel.NewGoChannel(gochannel.Config{
+			OutputChannelBuffer: 100,
+			Persistent:          true,
+		},
+			watermill.NewStdLogger(true, true),
+		)
+
+		endpoint := New(
+			1024*1024, // HTTP readLimit
+			func(m *NewsletterSignup) (*message.Message, error) { // converter
+				payload, err := json.Marshal(m)
+				if err != nil {
+					return nil, fmt.Errorf("failed to encode: %w", err)
+				}
+				return message.NewMessage(watermill.NewUUID(), payload), nil
+			},
+			"newsletter/signup", // Watermill topic
+			pubSub)
+
+		// ... setup HTTP server and router
+		router.Post("/api/v1/newsletter/signup", endpoint)
+	}
+
+JSON HTTP post requests that hit "/api/v1/newsletter/signup" will get parsed, validated, and converted into a [message.Message].
+*/
+package jsonendpoint
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+
+	"github.com/ThreeDotsLabs/watermill/message"
+)
+
+// Validatable is a generic interface that requires type T to be a pointer and implement the Validate method. It complements the adapter definitions. See <https://stackoverflow.com/questions/72090387/what-is-the-generic-type-for-a-pointer-that-implements-an-interface>.
+type Validatable[T any] interface {
+	*T
+	Validate() error
+}
+
+// New creates an adapter that converts an HTTP request to a [message.Message] and sends it to the [message.Publisher] topic. Enforces message validation on the generic type.
+func New[T any, P Validatable[T]](readLimit int64, converter func(P) (*message.Message, error), topic string, p message.Publisher) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		var (
+			in  = new(T) //*IN
+			err error
+		)
+		defer func() {
+			if err != nil {
+				w.WriteHeader(http.StatusInternalServerError)
+				_ = json.NewEncoder(w).Encode(map[string]string{
+					"Error": err.Error(),
+				})
+			}
+		}()
+
+		if r.Method != http.MethodPost {
+			err = fmt.Errorf("request method %q is not supported", r.Method)
+			return
+		}
+
+		reader := http.MaxBytesReader(w, r.Body, readLimit)
+		defer reader.Close()
+
+		err = json.NewDecoder(reader).Decode(in)
+		if err != nil {
+			r.Body.Close()
+			err = errors.New("JSON decoding failure: " + err.Error())
+			return
+		}
+		r.Body.Close()
+
+		if in == nil {
+			err = errors.New("no post data provided")
+			return
+		}
+
+		if err = P(in).Validate(); err != nil {
+			err = fmt.Errorf("failed to validate: %w", err)
+			return
+		}
+
+		message, err := converter(in)
+		if err != nil {
+			err = fmt.Errorf("failed to construct a message: %w", err)
+			return
+		}
+
+		if err = p.Publish(topic, message); err != nil {
+			err = fmt.Errorf("publisher rejected the message: %w", err)
+			return
+		}
+
+		io.WriteString(w, "nil")
+	}
+}

components/jsonendpoint/endpoint_test.go

@@ -0,0 +1,103 @@
+package jsonendpoint
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/ThreeDotsLabs/watermill"
+	"github.com/ThreeDotsLabs/watermill/message"
+	"github.com/ThreeDotsLabs/watermill/pubsub/gochannel"
+)
+
+const testTopic = "testTopic"
+
+type testMessage struct {
+	Title  string
+	Number int
+}
+
+func (m *testMessage) ToBytes(t *testing.T) []byte {
+	result, err := json.Marshal(m)
+	if err != nil {
+		t.Fatalf("failed to encode to bytes: %v", err)
+	}
+	return result
+}
+
+func (m *testMessage) Validate() error {
+	if m.Title == "" {
+		return errors.New("title is required")
+	}
+	if m.Number == 0 {
+		return errors.New("number is required")
+	}
+	return nil
+}
+
+func TestEndpointCreationIntegration(t *testing.T) {
+	pubSub := gochannel.NewGoChannel(gochannel.Config{
+		OutputChannelBuffer: 100,
+		Persistent:          false,
+	},
+		watermill.NewStdLogger(true, true),
+	)
+
+	m := &testMessage{
+		Title:  "title",
+		Number: 9,
+	}
+
+	// err := pubSub.Publish(testTopic, m)
+	// if err != nil {
+	// 	t.Fatalf("cannot publish the test message: %v", err)
+	// }
+
+	messages, err := pubSub.Subscribe(context.Background(), testTopic)
+	if err != nil {
+		t.Fatalf("failed to subscribe: %v", err)
+	}
+
+	endpoint := New(999999, func(m *testMessage) (*message.Message, error) {
+		payload, err := json.Marshal(m)
+		if err != nil {
+			return nil, fmt.Errorf("failed to encode: %w", err)
+		}
+		return message.NewMessage(watermill.NewUUID(), payload), nil
+	}, testTopic, pubSub)
+
+	request := httptest.NewRequest(
+		http.MethodPost,
+		"/url/path",
+		bytes.NewReader(m.ToBytes(t)),
+	)
+	w := httptest.NewRecorder()
+	endpoint(w, request)
+	res := w.Result()
+	defer res.Body.Close()
+
+	data, err := ioutil.ReadAll(res.Body)
+	if res.StatusCode != http.StatusOK {
+		t.Logf("Data: %s", data)
+		t.Fatalf("HTTP test request failed: status code is not OK: %d", res.StatusCode)
+	}
+
+	replayedMessage := <-messages
+	var rm *testMessage
+	if err = json.Unmarshal(replayedMessage.Payload, &rm); err != nil {
+		t.Fatalf("cannot decode replayed message: %v", err)
+	}
+
+	if rm.Title != m.Title {
+		t.Fatalf("title mismatch: %q vs %q", rm.Title, rm.Title)
+	}
+	if rm.Number != m.Number {
+		t.Fatalf("number mismatch: %q vs %q", rm.Number, rm.Number)
+	}
+}

go.mod

@@ -1,6 +1,6 @@
 module github.com/ThreeDotsLabs/watermill
 
-go 1.17
+go 1.18
 
 require (
 	github.com/cenkalti/backoff/v3 v3.2.2