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

_examples/basic/6-cqrs-ordered-events/message.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"fmt"
+	"log/slog"
+
+	"github.com/ThreeDotsLabs/watermill/components/cqrs"
+	"github.com/ThreeDotsLabs/watermill/message"
+	"google.golang.org/protobuf/types/known/timestamppb"
+)
+
+func GenerateMessageMetadata(partitionKey string) *MessageMetadata {
+	return &MessageMetadata{
+		PartitionKey: partitionKey,
+		CreatedAt:    timestamppb.Now(),
+	}
+}
+
+type CqrsMarshalerDecorator struct {
+	cqrs.ProtoMarshaler
+}
+
+const PartitionKeyMetadataField = "partition_key"
+
+func (c CqrsMarshalerDecorator) Marshal(v interface{}) (*message.Message, error) {
+	msg, err := c.ProtoMarshaler.Marshal(v)
+	if err != nil {
+		return nil, err
+	}
+
+	pm, ok := v.(ProtoMessage)
+	if !ok {
+		return nil, fmt.Errorf("%T does not implement ProtoMessage and can't be marshaled", v)
+	}
+
+	metadata := pm.GetMetadata()
+	if metadata == nil {
+		return nil, fmt.Errorf("%T.GetMetadata returned nil", v)
+	}
+
+	msg.Metadata.Set(PartitionKeyMetadataField, metadata.PartitionKey)
+	msg.Metadata.Set("created_at", metadata.CreatedAt.AsTime().String())
+
+	return msg, nil
+}
+
+type ProtoMessage interface {
+	GetMetadata() *MessageMetadata
+}
+
+// GenerateKafkaPartitionKey is a function that generates a partition key for Kafka messages.
+func GenerateKafkaPartitionKey(topic string, msg *message.Message) (string, error) {
+	slog.Debug("Setting partition key", "topic", topic, "msg_metadata", msg.Metadata)
+
+	return msg.Metadata.Get(PartitionKeyMetadataField), nil
+}