CQRS Event Sourcing: Validate UserName uniqueness

Question

Let\'s take a simple \"Account Registration\" example, here is the flow:

• User visit website
• Click \"Register\" button and fill form, click \"Save\" button

Answer 1

I think for such cases, we can use a mechanism like "advisory lock with expiration".

Sample execution:

• Check username exists or not in eventually consistent read model
• If not exists; by using a redis-couchbase like keyvalue storage or cache; try to push the username as key field with some expiration.
• If successful; then raise userRegisteredEvent.
• If either username exists in read model or cache storage, inform visitor that username has taken.

Even you can use an sql database; insert username as a primary key of some lock table; and then a scheduled job can handle expirations.