Object-oriented – Identifying Domain Services & Application Services when doing DDD

Architecturedesigndomain-driven-designobject-orientedservices

-I'm trying to figure out how to identify Application Services in my application. I think I can identify a Domain service by 2 things:

  1. It acts as a facade to the repository.
  2. It holds business logic that can't be encapsulated in a single entity.

So I thought about this simple use case that I'm currently working on:

An admin should be able to ban a certain user. The operation must be logged and an email must be sent to the banned user.

I have a UserRepository which has a function getUserById(). I have a User entity which has a ban() function.
I'll create a UserService like this:

class UserService{
    static void banUser(int userId){
          User user= UserRepository.getUserById(userId);
          user.ban();
          UserRepository.update(user);
    }
}

The userId is a POST variable that I'll be receiving in my Controller from a web form.

Now where does the logging go? I need to log the ban process twice before and after the operation ex:

Logger.log("Attempting to ban user with id:"+userId);
//user gets banned 
Logger.log("user banned successfully.");

Second, I need to send a mail to the user. where should the call go?

I thought about putting the logging and email is the UserService class itself, but I think there are better solutions.

Third, where do Application Services fit in all of this?

Best Answer

class UserService{
    public public UserService(IUserRepository repo, INotificationService notification)
    {
      this.UserRepository = repo;
      this.Notification = notification;
    }
    public void BanUser(int userId){
          User user= UserRepository.getUserById(userId);
          user.ban();
          Notification.NotifyBanned(user);
    }
}
When it comes to logging then you may choose a lot of solutions. Just log in BanUser method. Create logger per class (like log4j/log4net users do). Or you can inject ILogger in ctor. Or maybe it is some kind of Audit trail. Then maybe IUserAuditTrail would be better? 

public interface IUserAuditTrail
{
    void UserBanned(User user)
    void AccountCreated(User user)
}
or sth like that. Or you can add loggin AOP style. Decorate UserService with attribute and use PostSharp or use IoC container and configure interceptor to handle logging for you. Unless logging operations isn't business function and instead it's just for programmers I would go for interceptors. Otherwise IAuditTrail or sth like that.

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