I have my text validation messages in a properties file. How would you consider getting the message from the key? A domain service interface implemented by the infraestructure? Or would the implementation live in the domain too?
In the red book, text messages are literal, when passed to exceptions for example. They belong to domain.
But what if we deal with keys of a messages.properties file? How would you do it?
Thank you.
It is unlikely that the messages are part of the domain itself. Think about who will drive the changes to the messages. Will it be domain experts or marketing/UX people indicating that the particular message should be altered to convey the meaning better to customers?
If you had to create, for example, a new B2B client, do you anticipate that some messages would have to be changed? Does it mean that the domain has changed?
I would advise keeping the exceptions in the domain as independent from the external representation as possible. So, instead of saying throw new PaymentProcessingException("Insufficient funds") consider throw new InsufficientFundsPaymentProcessingException(). Then you could translate the specific exception into a proper message using the infrastructure service at the representational edge of your application.