In DDD you should never let your entities enter an invalid state. That being said, how do you handle the validation of a unique constraint? The creation of an entity is not a real problem. But let say you have an entity that must have a unique name and there is a thousand instances of this entity type - they are not in memory but stored in a database. Now let say you want to rename an instance. You can't just use a setter... the object could enter an invalid state - you have to validate against the database. How do you handle this scenario in a web environment?