stonecqrs/dispatcher.go

package stonecqrs

import (
	"context"
	"fmt"
	"sync"

	"gitstormr.dev/stone-utils/stoneerror"
)

// errCommandHandlerNotFound indicates a missing command handler error.
// errQueryHandlerNotFound indicates a missing query handler error.
// errEventHandlerNotFound indicates a missing event handler error.
var (
	errCommandHandlerNotFound = stoneerror.New(
		1001, "command handler not found",
	)
	errQueryHandlerNotFound = stoneerror.New(1002, "query handler not found")
	errEventHandlerNotFound = stoneerror.New(1003, "event handler not found")
)

// Command represents an action or operation to be handled by a dispatcher.
type Command interface{}

// Query represents a marker interface used to identify query types.
type Query interface{}

// Event represents a domain event within the application architecture.
type Event interface{}

// CommandHandler defines an interface for handling commands in a CQRS pattern.
// Implementations should process commands and return resulting events or errors.
// Handle processes the given command and returns any resulting events or error.
type CommandHandler interface {
	Handle(ctx context.Context, cmd Command) ([]Event, error)
}

// QueryHandler defines an interface for handling queries within a context.
type QueryHandler interface {
	Handle(ctx context.Context, query Query) (interface{}, error)
}

// EventHandler is an interface for handling specific types of events.
// It defines a single method Handle to process an Event with a context.
// Handle returns an error if the event handling fails.
type EventHandler interface {
	Handle(ctx context.Context, event Event) error
}

// Dispatcher is responsible for managing and dispatching handlers.
// It supports command, query, and event handlers concurrently.
// Commands, queries, and events are identified by their type names.
// Uses a read-write mutex to ensure thread-safe operations.
type Dispatcher struct {
	commandHandlers map[string]CommandHandler
	queryHandlers   map[string]QueryHandler
	eventHandlers   map[string][]EventHandler
	mutex           sync.RWMutex
}

// NewDispatcher creates and returns a new instance of Dispatcher.
func NewDispatcher() *Dispatcher {
	return &Dispatcher{
		commandHandlers: make(map[string]CommandHandler),
		queryHandlers:   make(map[string]QueryHandler),
		eventHandlers:   make(map[string][]EventHandler),
	}
}

// RegisterCommandHandler registers a handler for a specific command type.
func (d *Dispatcher) RegisterCommandHandler(
	cmd Command, handler CommandHandler,
) {
	d.mutex.Lock()
	defer d.mutex.Unlock()
	d.commandHandlers[typeName(cmd)] = handler
}

// RegisterQueryHandler registers a handler for the specified query type.
func (d *Dispatcher) RegisterQueryHandler(query Query, handler QueryHandler) {
	d.mutex.Lock()
	defer d.mutex.Unlock()
	d.queryHandlers[typeName(query)] = handler
}

// RegisterEventHandler registers a handler for a specific type of event.
func (d *Dispatcher) RegisterEventHandler(event Event, handler EventHandler) {
	d.mutex.Lock()
	defer d.mutex.Unlock()
	name := typeName(event)
	d.eventHandlers[name] = append(d.eventHandlers[name], handler)
}

// DispatchCommand processes a Command and dispatches the resulting Events.
// Returns the generated Events or an error if command handling fails.
func (d *Dispatcher) DispatchCommand(ctx context.Context, cmd Command) (
	[]Event, error,
) {
	handler, err := d.getCommandHandler(cmd)
	if err != nil {
		return nil, err
	}

	events, err := handler.Handle(ctx, cmd)
	if err != nil {
		return nil, err
	}

	if len(events) > 0 {
		if err = d.DispatchEvents(ctx, events...); err != nil {
			return events, stoneerror.Wrap(
				err, 1004, "failed to dispatch generated events",
			)
		}
	}

	return events, nil
}

// DispatchQuery dispatches a query to its corresponding handler.
// It retrieves the query handler and invokes its Handle method.
// Returns the result of the query handler or an error if not found.
func (d *Dispatcher) DispatchQuery(
	ctx context.Context, query Query,
) (interface{}, error) {
	handler, err := d.getQueryHandler(query)
	if err != nil {
		return nil, err
	}
	return handler.Handle(ctx, query)
}

// DispatchEvents dispatches multiple events to their registered handlers.
// It processes each event sequentially and stops on the first error.
// Returns an error if any event fails to dispatch.
func (d *Dispatcher) DispatchEvents(
	ctx context.Context, events ...Event,
) error {
	for _, event := range events {
		if err := d.DispatchEvent(ctx, event); err != nil {
			return err
		}
	}
	return nil
}

// DispatchEvent dispatches an event to all registered event handlers.
// It returns an error if any handler fails to process the event.
func (d *Dispatcher) DispatchEvent(ctx context.Context, event Event) error {
	handlers, err := d.getEventHandlers(event)
	if err != nil {
		return err
	}

	for _, handler := range handlers {
		if err = handler.Handle(ctx, event); err != nil {
			return stoneerror.Wrap(err, 1005, "event handling failed")
		}
	}

	return nil
}

// getCommandHandler retrieves the CommandHandler for the given Command.
// Returns an error if no handler is found.
func (d *Dispatcher) getCommandHandler(cmd Command) (CommandHandler, error) {
	d.mutex.RLock()
	defer d.mutex.RUnlock()

	handler, exists := d.commandHandlers[typeName(cmd)]
	if !exists {
		return nil, errCommandHandlerNotFound
	}
	return handler, nil
}

// getQueryHandler retrieves the handler for a specific query type.
// It returns an error if the handler is not found.
func (d *Dispatcher) getQueryHandler(query Query) (QueryHandler, error) {
	d.mutex.RLock()
	defer d.mutex.RUnlock()

	handler, exists := d.queryHandlers[typeName(query)]
	if !exists {
		return nil, errQueryHandlerNotFound
	}
	return handler, nil
}

// getEventHandlers retrieves all handlers for a specific event type.
// Returns an error if no handlers are registered for the event type.
func (d *Dispatcher) getEventHandlers(event Event) ([]EventHandler, error) {
	d.mutex.RLock()
	defer d.mutex.RUnlock()

	handlers, exists := d.eventHandlers[typeName(event)]
	if !exists {
		return nil, errEventHandlerNotFound
	}
	return handlers, nil
}

// typeName returns the name of the type of the given interface value.
func typeName(v interface{}) string {
	return fmt.Sprintf("%T", v)
}