Middleware

Middleware in Dew provides a powerful mechanism to intercept and augment the processing of actions and queries within the command bus. Middleware functions can perform a variety of tasks such as logging, validation, transaction management, and more, either before or after the main command handling logic.

Middleware Types

Dew categorizes middleware into different types based on the operation they are associated with:

ACTION: Middleware that is executed when an action is processed.
QUERY: Middleware that is executed when a query is processed.
ALL: Middleware that is applied to both actions and queries.

Also, middleware can be set for the dispatch of actions and queries:

Dispatch Middleware: Middleware that is executed before the action is dispatched to the handler.
Query Middleware: Middleware that is executed before the query is dispatched to the handler.

Usage

To use middleware in Dew, you attach it to the bus using the Use, UseDispatch, or UseQuery methods depending on whether you want it to apply to actions, queries, or both.

Adding Middleware to the Bus

You can add middleware to the bus at the time of bus creation, before registering any handlers:

func main() {
    bus := dew.New()

    // Register middleware
    bus.Use(dew.ALL, LoggingMiddleware)
    bus.Use(dew.ACTION, ValidationMiddleware)
    bus.Use(dew.QUERY, AuditMiddleware)

    // Continue setting up bus and handlers
}

Defining Middleware

A middleware in Dew is defined as a function that takes a dew.Middleware and returns another dew.Middleware. Here’s an example of a simple logging middleware:

func LoggingMiddleware(next dew.Middleware) dew.Middleware {
    return dew.MiddlewareFunc(func(ctx dew.Context) error {
        log.Println("Before executing command or query")
        err := next.Handle(ctx)
        log.Println("After executing command or query")
        return err
    })
}

Applying Middleware

You apply middleware to specific operations using the Use, UseDispatch, and UseQuery methods of the bus. This design allows for targeted middleware application, enhancing flexibility and control over command and query processing.

Grouping Handlers and Middleware

Dew allows for grouping of handlers and applying middleware to, allowing granular control just like http routers.

func main() {
    bus := dew.New()

    // Group middleware and handlers
    bus.Group(func(group dew.Bus) {
        group.Use(dew.ACTION, TransactionMiddleware)
        group.Register(new(UserHandler))
    })
}