How to Handle Set Based Consistency Validation in CQRS

This question keeps cropping up. How do you handle set based consistency validation in CQRS. In fact, one of my subscribers had this scenario:

“I have a user registration system with login/password. The login must be unique in the system. How do I implement such a requirement?”

Just in case you are not familiar with CQRS and ES, this problem arises because of something called ‘eventual consistency’.Continue reading