Uniqueness in CQRS

00 Old specs.txt

When a user registers with a unique username
Then that registration should be approved 

Given a user registration with a certain username was approved
When a new user registers with the same username
Then that registration should be rejected

01 New specs.txt

When a user registers with a certain username
Then that registration should be pending

Given a user registration with a certain username was approved
When a new user registers with the same username
Then that registration should be pending

03 The saga specs

Given a user registration with a certain username is pending
When the information is processed
Then it will approve the registration with that username

Given a user registration with a certain username has been approved
Given a user registration with the same username is pending
When the information is processed
Then it will reject the registration with that username

04 Saga specs template

Given [something happened]
When [all the sagas have processed the events]
Then [a command should be issued]

05 Example saga.cs

public class UserRegistrationSaga
{
  IIndexStore indexstore;
  IRunCommand bus;
  
  public UserRegistrationSaga(IRunCommand bus,IIndexStore indexstore )
  {
    this.indexstore= indexstore;
    this.bus = bus;
  }

  public void OnUserRegistrationPending(string UserRegistrationId,string Username)
  {
    if (indexstore.ContainsValue<string>("RegistrationUsername",Username))
    {
      bus.RunCommand(new RejectUserRegistration { UserRegistrationId = UserRegistrationId,Reason = "Username "+Username+" is allready in use"});
    }
    else
    {
       indexstore.Add<string>("RegistrationUsername",Username);
       bus.RunCommand(new ApproveUserRegistration { UserRegistrationId = UserRegistrationId, Username=Username });
    }
  }

  public void OnUserRegistrationApproved(string UserRegistrationId,string Username)
  {
    if (!indexstore.ContainsValue<string>("RegistrationUsername",Username))
    {
       indexstore.Add<string>("RegistrationUsername",Username);
    }
  }

  public void OnUserRegistrationRejected(string UserRegistrationId,string Username)
  {
    if (indexstore.ContainsValue<string>("RegistrationUsername",Username))
    {
       indexstore.Remove<string>("RegistrationUsername",Username);
    }
  }
}