Uniqueness validation when using CQRS and Event sourcing

Question

I\'m trying to implement my own CQRS infrastructure with Event Sourcing to learn it better. As a sample project I\'m implementing a blog engine, I know it might not be a perfect

Answer 1

I would go for an application service that is just responsible for generating unique ShortURL's. You can use a transactional DB to implement this behaviour. Typically this service would be used by the command handling part of the BlogPost aggregate. If there is a duplicate ShortURL, you can fire a DuplicateUrlErrorEvent. You can pre-catch this in the UI (but never 100%) by creating a thin query-model using the same data-source, so you can query whether a shortified URL is unique before submitting the post (as described by @RyanR's answer).