snapshot of https://github.com/ThreeDotsLabs/watermill.git (master, 2026-09-15; blobs > 131072B omitted)

_examples/real-world-examples/exactly-once-delivery-counter/worker/main.go

@@ -0,0 +1,148 @@
+package main
+
+import (
+	"context"
+	stdSQL "database/sql"
+	"encoding/json"
+	"os"
+	"os/signal"
+	"syscall"
+
+	"github.com/ThreeDotsLabs/watermill/message"
+	driver "github.com/go-sql-driver/mysql"
+	"github.com/pkg/errors"
+
+	"github.com/ThreeDotsLabs/watermill"
+	"github.com/ThreeDotsLabs/watermill-sql/v4/pkg/sql"
+)
+
+const topic = "counter"
+
+func main() {
+	db := createDB()
+	logger := watermill.NewStdLogger(false, false)
+
+	go runWatermillRouter(db, logger)
+
+	sigs := make(chan os.Signal, 1)
+	signal.Notify(sigs, syscall.SIGINT, syscall.SIGTERM)
+
+	// no graceful shutdown, to increase chance of problems :-)
+	<-sigs
+}
+
+type messagePayload struct {
+	CounterUUID string `json:"counter_uuid"`
+}
+
+func runWatermillRouter(db *stdSQL.DB, logger watermill.LoggerAdapter) {
+	subscriber, err := sql.NewSubscriber(
+		sql.BeginnerFromStdSQL(db),
+		sql.SubscriberConfig{
+			SchemaAdapter:    sql.DefaultMySQLSchema{},
+			OffsetsAdapter:   sql.DefaultMySQLOffsetsAdapter{},
+			InitializeSchema: true,
+		},
+		logger,
+	)
+	if err != nil {
+		panic(err)
+	}
+
+	router, err := message.NewRouter(message.RouterConfig{}, logger)
+	if err != nil {
+		panic(err)
+	}
+
+	router.AddConsumerHandler(
+		"counter",
+		topic,
+		subscriber,
+		processMessage,
+	)
+
+	if err := router.Run(context.Background()); err != nil {
+		panic(err)
+	}
+}
+
+func processMessage(msg *message.Message) error {
+	tx, ok := sql.TxFromContext(msg.Context())
+	if !ok {
+		return errors.New("tx not found in message context")
+	}
+
+	payload := messagePayload{}
+	err := json.Unmarshal(msg.Payload, &payload)
+	if err != nil {
+		return errors.Wrap(err, "unable to unmarshal payload")
+	}
+
+	// let's do it more fragile, let's get the value from DB instead of simple increment
+	counterValue, err := dbCounterValue(msg.Context(), tx, payload.CounterUUID)
+	if err != nil {
+		return err
+	}
+
+	counterValue += 1
+
+	if err := updateDbCounter(msg.Context(), tx, payload.CounterUUID, counterValue); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func updateDbCounter(ctx context.Context, tx sql.Tx, counterUUD string, counterValue int) error {
+	_, err := tx.ExecContext(
+		ctx,
+		"INSERT INTO counter (id, value) VALUES (?, ?) ON DUPLICATE KEY UPDATE value = ?",
+		counterUUD,
+		counterValue,
+		counterValue,
+	)
+	if err != nil {
+		return errors.Wrap(err, "can't update counter value")
+	}
+
+	return nil
+}
+
+func dbCounterValue(ctx context.Context, tx sql.Tx, counterUUID string) (int, error) {
+	var counterValue int
+	rows, err := tx.QueryContext(ctx, "SELECT value from counter WHERE id = ?", counterUUID)
+	if err != nil {
+		return 0, errors.Wrap(err, "can't get counter value")
+	}
+
+	if !rows.Next() {
+		return 0, nil
+	}
+
+	err = rows.Scan(&counterValue)
+	if err != nil {
+		return 0, errors.Wrap(err, "can't get counter value")
+	}
+
+	return counterValue, nil
+}
+
+func createDB() *stdSQL.DB {
+	conf := driver.NewConfig()
+	conf.Net = "tcp"
+	conf.User = "root"
+	conf.Addr = "mysql"
+	conf.DBName = "example"
+
+	db, err := stdSQL.Open("mysql", conf.FormatDSN())
+	if err != nil {
+		panic(err)
+	}
+
+	err = db.Ping()
+	if err != nil {
+		panic(err)
+	}
+
+	return db
+}