ApplicationError

Overview

As discussed in the domain error component you already have a good abstraction for meaningful errors for your problem space, but what happens with unexpected errors caused by the environment of your system?

In a similar logic you need a different wrapper that helps you distinguish those exceptions from your domain errors. This abstraction is named application error since those types of problems are caused during a use case execution (application layer) where interaction with external parts is initiated.

Syntax

ApplicationError DatabaseError(error: string) {
    message: `the following ${error} happened in the db`,
    errorId: 'd0cc35f0-37f7-11ed-a261-0242ac120002',
  }

The syntax is similar to the domain error syntax since as discussed, the purpose of this component is to help you have better separation between those main categories of errors.

Usage Example

CommandHandler CreateBookingCommandHandler () {
  execute(command: CreateBookingCommand):
  (OK(void), Errors(DomainErrors.InvalidBooking, ApplicationErrors.UndefinedName))
  {
    ...
    booking = db.Booking.save({ name, date })
    return ok();
  }
}

Consider a use case where you want to create a booking for a hotel. As showed in the example above there will eventually come a point that you will want to interact with the infrastructure layer of your system and save your result in the database. With the use of the appropriate application error you can properly define a possible malfunction of this external process while still keeping it separate from your domain errors.