// ...
// EventBus transports events to event handlers.
type EventBus struct {
// ...

// ...
type EventBusConfig struct {
	// GeneratePublishTopic is used to generate topic name for publishing event.
	GeneratePublishTopic GenerateEventPublishTopicFn

	// OnPublish is called before sending the event.
	// The *message.Message can be modified.
	//
	// This option is not required.
	OnPublish OnEventSendFn

	// Marshaler is used to marshal and unmarshal events.
	// It is required.
	Marshaler CommandEventMarshaler

	// Logger instance used to log.
	// If not provided, watermill.NopLogger is used.
	Logger watermill.LoggerAdapter
}

func (c *EventBusConfig) setDefaults() {
	if c.Logger == nil {
		c.Logger = watermill.NopLogger{}
	}
}
// ...

// ...
// EventProcessor determines which EventHandler should handle event received from event bus.
type EventProcessor struct {
// ...

// ...
type EventProcessorConfig struct {
	// GenerateSubscribeTopic is used to generate topic for subscribing to events.
	// If event processor is using handler groups, GenerateSubscribeTopic is used instead.
	GenerateSubscribeTopic EventProcessorGenerateSubscribeTopicFn

	// SubscriberConstructor is used to create subscriber for EventHandler.
	//
	// This function is called for every EventHandler instance.
	// If you want to re-use one subscriber for multiple handlers, use GroupEventProcessor instead.
	SubscriberConstructor EventProcessorSubscriberConstructorFn

	// OnHandle is called before handling event.
	// OnHandle works in a similar way to middlewares: you can inject additional logic before and after handling a event.
	//
	// Because of that, you need to explicitly call params.Handler.Handle() to handle the event.
	//
	//  func(params EventProcessorOnHandleParams) (err error) {
	//      // logic before handle
	//      //  (...)
	//
	//      err := params.Handler.Handle(params.Message.Context(), params.Event)
	//
	//      // logic after handle
	//      //  (...)
	//
	//      return err
	//  }
	//
	// This option is not required.
	OnHandle EventProcessorOnHandleFn

	// AckOnUnknownEvent is used to decide if message should be acked if event has no handler defined.
	AckOnUnknownEvent bool

	// Marshaler is used to marshal and unmarshal events.
	// It is required.
	Marshaler CommandEventMarshaler

	// Logger instance used to log.
	// If not provided, watermill.NopLogger is used.
	Logger watermill.LoggerAdapter

	// disableRouterAutoAddHandlers is used to keep backwards compatibility.
	// it is set when EventProcessor is created by NewEventProcessor.
	// Deprecated: please migrate to NewEventProcessorWithConfig.
	disableRouterAutoAddHandlers bool
}

func (c *EventProcessorConfig) setDefaults() {
	if c.Logger == nil {
		c.Logger = watermill.NopLogger{}
	}
}
// ...

// ...
// EventGroupProcessor determines which EventHandler should handle event received from event bus.
// Compared to EventProcessor, EventGroupProcessor allows to have multiple handlers that share the same subscriber instance.
type EventGroupProcessor struct {
// ...

// ...
type EventGroupProcessorConfig struct {
	// GenerateSubscribeTopic is used to generate topic for subscribing to events for handler groups.
	// This option is required for EventProcessor if handler groups are used.
	GenerateSubscribeTopic EventGroupProcessorGenerateSubscribeTopicFn

	// SubscriberConstructor is used to create subscriber for GroupEventHandler.
	// This function is called for every events group once - thanks to that it's possible to have one subscription per group.
	// It's useful, when we are processing events from one stream and we want to do it in order.
	SubscriberConstructor EventGroupProcessorSubscriberConstructorFn

	// OnHandle is called before handling event.
	// OnHandle works in a similar way to middlewares: you can inject additional logic before and after handling a event.
	//
	// Because of that, you need to explicitly call params.Handler.Handle() to handle the event.
	//
	//  func(params EventGroupProcessorOnHandleParams) (err error) {
	//      // logic before handle
	//      //  (...)
	//
	//      err := params.Handler.Handle(params.Message.Context(), params.Event)
	//
	//      // logic after handle
	//      //  (...)
	//
	//      return err
	//  }
	//
	// This option is not required.
	OnHandle EventGroupProcessorOnHandleFn

	// AckOnUnknownEvent is used to decide if message should be acked if event has no handler defined.
	AckOnUnknownEvent bool

	// Marshaler is used to marshal and unmarshal events.
	// It is required.
	Marshaler CommandEventMarshaler

	// Logger instance used to log.
	// If not provided, watermill.NopLogger is used.
	Logger watermill.LoggerAdapter
}

func (c *EventGroupProcessorConfig) setDefaults() {
	if c.Logger == nil {
		c.Logger = watermill.NopLogger{}
	}
}
// ...

// ...
// EventHandler receives events defined by NewEvent and handles them with its Handle method.
// If using DDD, CommandHandler may modify and persist the aggregate.
// It can also invoke a process manager, a saga or just build a read model.
//
// In contrast to CommandHandler, every Event can have multiple EventHandlers.
//
// One instance of EventHandler is used during handling messages.
// When multiple events are delivered at the same time, Handle method can be executed multiple times at the same time.
// Because of that, Handle method needs to be thread safe!
type EventHandler interface {
// ...

// ...
// CommandBus transports commands to command handlers.
type CommandBus struct {
// ...

// ...
type CommandBusConfig struct {
	// GeneratePublishTopic is used to generate topic for publishing command.
	GeneratePublishTopic CommandBusGeneratePublishTopicFn

	// OnSend is called before publishing the command.
	// The *message.Message can be modified.
	//
	// This option is not required.
	OnSend CommandBusOnSendFn

	// Marshaler is used to marshal and unmarshal commands.
	// It is required.
	Marshaler CommandEventMarshaler

	// Logger instance used to log.
	// If not provided, watermill.NopLogger is used.
	Logger watermill.LoggerAdapter
}

func (c *CommandBusConfig) setDefaults() {
	if c.Logger == nil {
		c.Logger = watermill.NopLogger{}
	}
}
// ...

// ...
// CommandProcessorSubscriberConstructorFn creates subscriber for CommandHandler.
// It allows you to create a separate customized Subscriber for every command handler.
type CommandProcessorSubscriberConstructorFn func(CommandProcessorSubscriberConstructorParams) (message.Subscriber, error)
// ...

// ...
type CommandProcessorConfig struct {
	// GenerateSubscribeTopic is used to generate topic for subscribing command.
	GenerateSubscribeTopic CommandProcessorGenerateSubscribeTopicFn

	// SubscriberConstructor is used to create subscriber for CommandHandler.
	SubscriberConstructor CommandProcessorSubscriberConstructorFn

	// OnHandle is called before handling command.
	// OnHandle works in a similar way to middlewares: you can inject additional logic before and after handling a command.
	//
	// Because of that, you need to explicitly call params.Handler.Handle() to handle the command.
	//  func(params CommandProcessorOnHandleParams) (err error) {
	//      // logic before handle
	//      //  (...)
	//
	//      err := params.Handler.Handle(params.Message.Context(), params.Command)
	//
	//      // logic after handle
	//      //  (...)
	//
	//      return err
	//  }
	//
	// This option is not required.
	OnHandle CommandProcessorOnHandleFn

	// Marshaler is used to marshal and unmarshal commands.
	// It is required.
	Marshaler CommandEventMarshaler

	// Logger instance used to log.
	// If not provided, watermill.NopLogger is used.
	Logger watermill.LoggerAdapter

	// If true, CommandProcessor will ack messages even if CommandHandler returns an error.
	// If RequestReplyBackend is not null and sending reply fails, the message will be nack-ed anyway.
	//
	// Warning: It's not recommended to use this option when you are using requestreply component
	// (requestreply.NewCommandHandler or requestreply.NewCommandHandlerWithResult), as it may ack the
	// command when sending reply failed.
	//
	// When you are using requestreply, you should use requestreply.PubSubBackendConfig.AckCommandErrors.
	AckCommandHandlingErrors bool

	// disableRouterAutoAddHandlers is used to keep backwards compatibility.
	// it is set when CommandProcessor is created by NewCommandProcessor.
	// Deprecated: please migrate to NewCommandProcessorWithConfig.
	disableRouterAutoAddHandlers bool
}

func (c *CommandProcessorConfig) setDefaults() {
	if c.Logger == nil {
		c.Logger = watermill.NopLogger{}
	}
}
// ...

// ...
// CommandHandler receives a command defined by NewCommand and handles it with the Handle method.
// If using DDD, CommandHandler may modify and persist the aggregate.
//
// In contrast to EventHandler, every Command must have only one CommandHandler.
//
// One instance of CommandHandler is used during handling messages.
// When multiple commands are delivered at the same time, Handle method can be executed multiple times at the same time.
// Because of that, Handle method needs to be thread safe!
type CommandHandler interface {
// ...

// ...
// CommandEventMarshaler marshals Commands and Events to Watermill's messages and vice versa.
// Payload of the command needs to be marshaled to []bytes.
type CommandEventMarshaler interface {
	// Marshal marshals Command or Event to Watermill's message.
	Marshal(v interface{}) (*message.Message, error)

	// Unmarshal unmarshals watermill's message to v Command or Event.
	Unmarshal(msg *message.Message, v interface{}) (err error)

	// Name returns the name of Command or Event.
	// Name is used to determine, that received command or event is event which we want to handle.
	Name(v interface{}) string

	// NameFromMessage returns the name of Command or Event from Watermill's message (generated by Marshal).
	//
	// When we have Command or Event marshaled to Watermill's message,
	// we should use NameFromMessage instead of Name to avoid unnecessary unmarshaling.
	NameFromMessage(msg *message.Message) string
}
// ...

// ...
		bookRoomCmd := &BookRoom{
			RoomId:    fmt.Sprintf("%d", i),
			GuestName: "John",
			StartDate: startDate,
			EndDate:   endDate,
		}
		if err := commandBus.Send(context.Background(), bookRoomCmd); err != nil {
			panic(err)
		}
// ...

// ...
// BookRoomHandler is a command handler, which handles BookRoom command and emits RoomBooked.
//
// In CQRS, one command must be handled by only one handler.
// When another handler with this command is added to command processor, error will be retuerned.
type BookRoomHandler struct {
	eventBus *cqrs.EventBus
}

func (b BookRoomHandler) Handle(ctx context.Context, cmd *BookRoom) error {
	// some random price, in production you probably will calculate in wiser way
	price := (rand.Int63n(40) + 1) * 10

	log.Printf(
		"Booked %s for %s from %s to %s",
		cmd.RoomId,
		cmd.GuestName,
		time.Unix(cmd.StartDate.Seconds, int64(cmd.StartDate.Nanos)),
		time.Unix(cmd.EndDate.Seconds, int64(cmd.EndDate.Nanos)),
	)

	// RoomBooked will be handled by OrderBeerOnRoomBooked event handler,
	// in future RoomBooked may be handled by multiple event handler
	if err := b.eventBus.Publish(ctx, &RoomBooked{
		ReservationId: watermill.NewUUID(),
		RoomId:        cmd.RoomId,
		GuestName:     cmd.GuestName,
		Price:         price,
		StartDate:     cmd.StartDate,
		EndDate:       cmd.EndDate,
	}); err != nil {
		return err
	}

	return nil
}

// OrderBeerOnRoomBooked is a event handler, which handles RoomBooked event and emits OrderBeer command.
// ...

// ...
// OrderBeerOnRoomBooked is a event handler, which handles RoomBooked event and emits OrderBeer command.
type OrderBeerOnRoomBooked struct {
	commandBus *cqrs.CommandBus
}

func (o OrderBeerOnRoomBooked) Handle(ctx context.Context, event *RoomBooked) error {
	orderBeerCmd := &OrderBeer{
		RoomId: event.RoomId,
		Count:  rand.Int63n(10) + 1,
	}

	return o.commandBus.Send(ctx, orderBeerCmd)
}

// OrderBeerHandler is a command handler, which handles OrderBeer command and emits BeerOrdered.
// ...

// ...
	err = eventProcessor.AddHandlersGroup(
		"events",
		cqrs.NewGroupEventHandler(OrderBeerOnRoomBooked{commandBus}.Handle),
		cqrs.NewGroupEventHandler(NewBookingsFinancialReport().Handle),
		cqrs.NewGroupEventHandler(func(ctx context.Context, event *BeerOrdered) error {
			logger.Info("Beer ordered", watermill.LogFields{
				"room_id": event.RoomId,
			})
			return nil
		}),
	)
	if err != nil {
// ...

// ...
		cqrs.NewGroupEventHandler(OrderBeerOnRoomBooked{commandBus}.Handle),
		cqrs.NewGroupEventHandler(NewBookingsFinancialReport().Handle),
		cqrs.NewGroupEventHandler(func(ctx context.Context, event *BeerOrdered) error {
			logger.Info("Beer ordered", watermill.LogFields{
				"room_id": event.RoomId,
			})
			return nil
		}),
// ...

// ...
// NewCommandHandler creates a new CommandHandler implementation based on provided function
// and command type inferred from function argument.
func NewCommandHandler[Command any](
// ...

// ...
// NewEventHandler creates a new EventHandler implementation based on provided function
// and event type inferred from function argument.
func NewEventHandler[T any](
// ...

// ...
// NewGroupEventHandler creates a new GroupEventHandler implementation based on provided function
// and event type inferred from function argument.
func NewGroupEventHandler[T any](handleFunc func(ctx context.Context, event *T) error) GroupEventHandler {
// ...

// ...
// BookingsFinancialReport is a read model, which calculates how much money we may earn from bookings.
// Like OrderBeerOnRoomBooked, it listens for RoomBooked event.
//
// This implementation is just writing to the memory. In production, you will probably will use some persistent storage.
type BookingsFinancialReport struct {
	handledBookings map[string]struct{}
	totalCharge     int64
	lock            sync.Mutex
}

func NewBookingsFinancialReport() *BookingsFinancialReport {
	return &BookingsFinancialReport{handledBookings: map[string]struct{}{}}
}

func (b *BookingsFinancialReport) Handle(ctx context.Context, event *RoomBooked) error {
	// Handle may be called concurrently, so it need to be thread safe.
	b.lock.Lock()
	defer b.lock.Unlock()

	// When we are using Pub/Sub which doesn't provide exactly-once delivery semantics, we need to deduplicate messages.
	// GoChannel Pub/Sub provides exactly-once delivery,
	// but let's make this example ready for other Pub/Sub implementations.
	if _, ok := b.handledBookings[event.ReservationId]; ok {
		return nil
	}
	b.handledBookings[event.ReservationId] = struct{}{}

	b.totalCharge += event.Price

	fmt.Printf(">>> Already booked rooms for $%d\n", b.totalCharge)
	return nil
}

var amqpAddress = "amqp://guest:guest@rabbitmq:5672/"

func main() {
// ...

// ...
func main() {
	logger := watermill.NewStdLogger(false, false)
	cqrsMarshaler := cqrs.ProtobufMarshaler{}

	// You can use any Pub/Sub implementation from here: https://watermill.io/pubsubs/
	// Detailed RabbitMQ implementation: https://watermill.io/pubsubs/amqp/
	// Commands will be send to queue, because they need to be consumed once.
	commandsAMQPConfig := amqp.NewDurableQueueConfig(amqpAddress)
	commandsPublisher, err := amqp.NewPublisher(commandsAMQPConfig, logger)
	if err != nil {
		panic(err)
	}
	commandsSubscriber, err := amqp.NewSubscriber(commandsAMQPConfig, logger)
	if err != nil {
		panic(err)
	}

	// Events will be published to PubSub configured Rabbit, because they may be consumed by multiple consumers.
	// (in that case BookingsFinancialReport and OrderBeerOnRoomBooked).
	eventsPublisher, err := amqp.NewPublisher(amqp.NewDurablePubSubConfig(amqpAddress, nil), logger)
	if err != nil {
		panic(err)
	}

	// CQRS is built on messages router. Detailed documentation: https://watermill.io/docs/messages-router/
	router, err := message.NewRouter(message.RouterConfig{}, logger)
	if err != nil {
		panic(err)
	}

	// Simple middleware which will recover panics from event or command handlers.
	// More about router middlewares you can find in the documentation:
	// https://watermill.io/docs/messages-router/#middleware
	//
	// List of available middlewares you can find in message/router/middleware.
	router.AddMiddleware(middleware.Recoverer)

	commandBus, err := cqrs.NewCommandBusWithConfig(commandsPublisher, cqrs.CommandBusConfig{
		GeneratePublishTopic: func(params cqrs.CommandBusGeneratePublishTopicParams) (string, error) {
			// we are using queue RabbitMQ config, so we need to have topic per command type
			return params.CommandName, nil
		},
		OnSend: func(params cqrs.CommandBusOnSendParams) error {
			logger.Info("Sending command", watermill.LogFields{
				"command_name": params.CommandName,
			})

			params.Message.Metadata.Set("sent_at", time.Now().String())

			return nil
		},
		Marshaler: cqrsMarshaler,
		Logger:    logger,
	})
	if err != nil {
		panic(err)
	}

	commandProcessor, err := cqrs.NewCommandProcessorWithConfig(
		router,
		cqrs.CommandProcessorConfig{
			GenerateSubscribeTopic: func(params cqrs.CommandProcessorGenerateSubscribeTopicParams) (string, error) {
				// we are using queue RabbitMQ config, so we need to have topic per command type
				return params.CommandName, nil
			},
			SubscriberConstructor: func(params cqrs.CommandProcessorSubscriberConstructorParams) (message.Subscriber, error) {
				// we can reuse subscriber, because all commands have separated topics
				return commandsSubscriber, nil
			},
			OnHandle: func(params cqrs.CommandProcessorOnHandleParams) error {
				start := time.Now()

				err := params.Handler.Handle(params.Message.Context(), params.Command)

				logger.Info("Command handled", watermill.LogFields{
					"command_name": params.CommandName,
					"duration":     time.Since(start),
					"err":          err,
				})

				return err
			},
			Marshaler: cqrsMarshaler,
			Logger:    logger,
		},
	)
	if err != nil {
		panic(err)
	}

	eventBus, err := cqrs.NewEventBusWithConfig(eventsPublisher, cqrs.EventBusConfig{
		GeneratePublishTopic: func(params cqrs.GenerateEventPublishTopicParams) (string, error) {
			// because we are using PubSub RabbitMQ config, we can use one topic for all events
			return "events", nil

			// we can also use topic per event type
			// return params.EventName, nil
		},

		OnPublish: func(params cqrs.OnEventSendParams) error {
			logger.Info("Publishing event", watermill.LogFields{
				"event_name": params.EventName,
			})

			params.Message.Metadata.Set("published_at", time.Now().String())

			return nil
		},

		Marshaler: cqrsMarshaler,
		Logger:    logger,
	})
	if err != nil {
		panic(err)
	}

	eventProcessor, err := cqrs.NewEventGroupProcessorWithConfig(
		router,
		cqrs.EventGroupProcessorConfig{
			GenerateSubscribeTopic: func(params cqrs.EventGroupProcessorGenerateSubscribeTopicParams) (string, error) {
				return "events", nil
			},
			SubscriberConstructor: func(params cqrs.EventGroupProcessorSubscriberConstructorParams) (message.Subscriber, error) {
				config := amqp.NewDurablePubSubConfig(
					amqpAddress,
					amqp.GenerateQueueNameTopicNameWithSuffix(params.EventGroupName),
				)

				return amqp.NewSubscriber(config, logger)
			},

			OnHandle: func(params cqrs.EventGroupProcessorOnHandleParams) error {
				start := time.Now()

				err := params.Handler.Handle(params.Message.Context(), params.Event)

				logger.Info("Event handled", watermill.LogFields{
					"event_name": params.EventName,
					"duration":   time.Since(start),
					"err":        err,
				})

				return err
			},

			Marshaler: cqrsMarshaler,
			Logger:    logger,
		},
	)
	if err != nil {
		panic(err)
	}

	err = commandProcessor.AddHandlers(
		cqrs.NewCommandHandler("BookRoomHandler", BookRoomHandler{eventBus}.Handle),
		cqrs.NewCommandHandler("OrderBeerHandler", OrderBeerHandler{eventBus}.Handle),
	)
	if err != nil {
		panic(err)
	}

	err = eventProcessor.AddHandlersGroup(
		"events",
		cqrs.NewGroupEventHandler(OrderBeerOnRoomBooked{commandBus}.Handle),
		cqrs.NewGroupEventHandler(NewBookingsFinancialReport().Handle),
		cqrs.NewGroupEventHandler(func(ctx context.Context, event *BeerOrdered) error {
			logger.Info("Beer ordered", watermill.LogFields{
				"room_id": event.RoomId,
			})
			return nil
		}),
	)
	if err != nil {
		panic(err)
	}

	// publish BookRoom commands every second to simulate incoming traffic
	go publishCommands(commandBus)

	// processors are based on router, so they will work when router will start
	if err := router.Run(context.Background()); err != nil {
		panic(err)
	}
}
// ...