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go-mediatr
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This package is a Mediator Pattern implementation in golang, and inspired by great jbogard/mediatr library in .Net.

For decoupling some objects in a system we could use Mediator object as an interface, for decrease coupling between the objects. Mostly I uses this pattern when I use CQRS in my system.

There are some samples for using this package here, also I used this packages widely in this microservices sample

🧰 Installation

go get github.com/mehdihadeli/go-mediatr

🔥 Features

✅ Handling Request/Response message for delivering message to only one handler (Commands, Queries)

✅ Handling Notification message for delivering message to multiple handlers (Events)

✅ Pipelenes Behaviours for handling some cross cutting concerns before or after executing handlers

🛡️ Strategies

Mediatr has two strategies for dispatching messages:

  1. Request/Response messages, dispatched to a single handler.
  2. Notification messages, dispatched to all (multiple) handlers and they don't have any response.

Request/Response Strategy

The request/response message, has just one handler, and can handle both command and query scenarios in CQRS Pattern.

Creating a Request/Response Message

For creating a request (command or query) that has just one handler, we could create a command message or query message as a request like this:

// Command (Request)
type CreateProductCommand struct {
    ProductID   uuid.UUID `validate:"required"`
    Name        string    `validate:"required,gte=0,lte=255"`
    Description string    `validate:"required,gte=0,lte=5000"`
    Price       float64   `validate:"required,gte=0"`
    CreatedAt   time.Time `validate:"required"`
}

// Query (Request)
type GetProdctByIdQuery struct {
    ProductID uuid.UUID `validate:"required"`
}

And for response of these requests, we could create response messages as a response like this:

// Command (Response)
type CreateProductCommandResponse struct {
    ProductID uuid.UUID `json:"productId"`
}

// Query (Response)
type GetProductByIdQueryResponse struct {
    ProductID   uuid.UUID `json:"productId"`
    Name        string    `json:"name"`
    Description string    `json:"description"`
    Price       float64   `json:"price"`
    CreatedAt   time.Time `json:"createdAt"`
}

Creating Request Handler

For handling our requests, we should create a single request handler for each request. Each handler should implement the RequestHandler interface.

type RequestHandler[TRequest any, TResponse any] interface {
	Handle(ctx context.Context, request TRequest) (TResponse, error)
}

Here we Create request handler (command handler and query handler) for our requests, that implements above interface:

// Command Handler
type CreateProductCommandHandler struct {
	productRepository *repository.InMemoryProductRepository
}

func NewCreateProductCommandHandler(productRepository *repository.InMemoryProductRepository) *CreateProductCommandHandler {
	return &CreateProductCommandHandler{productRepository: productRepository}
}

func (c *CreateProductCommandHandler) Handle(ctx context.Context, command *CreateProductCommand) (*creatingProductDtos.CreateProductCommandResponse, error) {

	product := &models.Product{
		ProductID:   command.ProductID,
		Name:        command.Name,
		Description: command.Description,
		Price:       command.Price,
		CreatedAt:   command.CreatedAt,
	}

	createdProduct, err := c.productRepository.CreateProduct(ctx, product)
	if err != nil {
		return nil, err
	}

	response := &creatingProductDtos.CreateProductCommandResponse{ProductID: createdProduct.ProductID}

	return response, nil
}
// Query Handler
type GetProductByIdQueryHandler struct {
    productRepository *repository.InMemoryProductRepository
}

func NewGetProductByIdQueryHandler(productRepository *repository.InMemoryProductRepository) *GetProductByIdQueryHandler {
    return &GetProductByIdQueryHandler{productRepository: productRepository}
}

func (c *GetProductByIdQueryHandler) Handle(ctx context.Context, query *GetProductByIdQuery) (*gettingProductDtos.GetProdctByIdQueryResponse, error) {

    product, err := c.productRepository.GetProductById(ctx, query.ProductID)
    if err != nil {
        return nil, err
    }

    response := &gettingProductDtos.GetProdctByIdQueryResponse{
        ProductID:   product.ProductID,
        Name:        product.Name,
        Description: product.Description,
        Price:       product.Price,
        CreatedAt:   product.CreatedAt,
    }

    return response, nil
}

Note: In the cases we don't need a response from our request handler, we can use Unit type, that actually is an empty struct:.

Registering Request Handler to the MediatR

Before sending or dispatching our requests, we should register our request handlers to the MediatR.

Here we register our request handlers (command handler and query handler) to the MediatR:

// Registering `createProductCommandHandler` request handler for `CreateProductCommand` request to the MediatR
mediatr.RegisterHandler[*creatingProduct.CreateProductCommand, *creatingProductsDtos.CreateProductCommandResponse](createProductCommandHandler)

// Registering `getProductByIdQueryHandler` request handler for `GetProductByIdQuery` request to the MediatR
mediatr.RegisterHandler[*gettingProduct.GetProductByIdQuery, *gettingProductDtos.GetProdctByIdQueryResponse](getProductByIdQueryHandler)

Sending Request to the MediatR

Finally, send a message through the mediator.

Here we send our requests to the MediatR for dispatching them to the request handlers (command handler and query handler):

// Sending `CreateProductCommand` request to mediatr for dispatching to the `CreateProductCommandHandler` request handler
command := &CreateProductCommand{
    ProductID:   uuid.NewV4(),
    Name:        request.name,
    Description: request.description,
    Price:       request.price,
    CreatedAt:   time.Now(),
}

mediatr.Send[*CreateProductCommand, *creatingProductsDtos.CreateProductCommandResponse](ctx, command)
// Sending `GetProductByIdQuery` request to mediatr for dispatching to the `GetProductByIdQueryHandler` request handler
query := &GetProdctByIdQuery{
    ProductID:   uuid.NewV4()
}

mediatr.Send[*GetProductByIdQuery, *gettingProductsDtos.GetProductByIdQueryResponse](ctx, query)

Notification Strategy

The notification message, can have multiple handlers and doesn't have any response, and it can handle an event notification or notification in event driven architecture.

Creating a Notification Message

For creating a notification (event), that has multiple handlers and doesn't have any response, we could create an event notification as a notification like this:

// Event (Notification)
type ProductCreatedEvent struct {
    ProductID uuid.UUID   `json:"productId"`
    Name        string    `json:"name"`
    Description string    `json:"description"`
    Price       float64   `json:"price"`
    CreatedAt   time.Time `json:"createdAt"`
}

This event doesn't have any response.

Creating Notification Handlers

For handling our notification, we can create multiple notification handlers for each notification event. Each handler should implement the NotificationHandler interface.

type NotificationHandler[TNotification any] interface {
    Handle(ctx context.Context, notification TNotification) error
}

Here we Create multiple notification event handler for our notification, that implements above interface:

// Notification Event Handler1
type ProductCreatedEventHandler1 struct {
}

func (c *ProductCreatedEventHandler1) Handle(ctx context.Context, event *ProductCreatedEvent) error {
//Do something with the event here !
    return nil
}
// Notification Event Handler2
type ProductCreatedEventHandler2 struct {
}

func (c *ProductCreatedEventHandler2) Handle(ctx context.Context, event *ProductCreatedEvent) error {
//Do something with the event here !
    return nil
}

Registering Notification Handlers to the MediatR

Before publishing our notifications, we should register our notification handlers to the MediatR.

Here we register our notification handlers to the MediatR:

// Registering `notificationHandler1`, `notificationHandler2` notification handler for `ProductCreatedEvent` notification event to the MediatR
notificationHandler1 := &ProductCreatedEventHandler1{}
notificationHandler2 := &ProductCreatedEventHandler2{}

mediatr.RegisterNotificationHandlers[*events.ProductCreatedEvent](notificationHandler1, notificationHandler2)

Publishing Notification to the MediatR

Finally, publish a notification event through the mediator.

Here we publish our notification to the MediatR for dispatching them to the notification handlers:

// Publishing `ProductCreatedEvent` notification to mediatr for dispatching to the `ProductCreatedEventHandler1`, `ProductCreatedEventHandler2` notification handlers
productCreatedEvent := 	&ProductCreatedEvent {
    ProductID:   createdProduct.ProductID,
    Name:        createdProduct.Name,
    Price:       createdProduct.Price,
    CreatedAt:   createdProduct.CreatedAt,
    Description: createdProduct.Description,
}
	
mediatr.Publish[*events.ProductCreatedEvent](ctx, productCreatedEvent)

⚒️ Using Pipeline Behaviors

Sometimes we need to add some cross-cutting concerns before after running our request handlers like logging, metrics, circuit breaker, retry, etc. In this case we can use PipelineBehavior. It is actually is like a middleware or decorator pattern.

These behaviors will execute before or after running our request handlers with calling Send method for a request on the mediatr.

Creating Pipeline Behavior

For creating a pipeline behaviour we should implement the PipelineBehavior interface:

type PipelineBehavior interface {
	Handle(ctx context.Context, request interface{}, next RequestHandlerFunc) (interface{}, error)
}

The request parameter is the request object passed in through Send method of mediatr, while the next parameter is a continuation for the next action in the behavior chain and its type is RequestHandlerFunc.

Here is an example of a pipeline behavior:

type RequestLoggerBehaviour struct {
}

func (r *RequestLoggerBehaviour) Handle(ctx context.Context, request interface{}, next mediatr.RequestHandlerFunc) (interface{}, error) {
	log.Printf("logging some stuff before handling the request")

	response, err := next()
	if err != nil {
		return nil, err
	}

	log.Println("logging some stuff after handling the request")

	return response, nil
}

In our defined behavior, we need to call next parameter that call next action in the behavior chain, if there aren't any other behaviours next will call our actual request handler and return the response. We can do something before of after of calling next action in the behavior chain.

Registering Pipeline Behavior to the MediatR

For registering our pipeline behavior to the MediatR, we should use RegisterPipelineBehaviors method:

loggerPipeline := &behaviours.RequestLoggerBehaviour{}
err = mediatr.RegisterRequestPipelineBehaviors(loggerPipeline)