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

Have you considered using a "working" cache as sort of an RSVP? It's hard to explain because it works in a bit of a cycle, but basically, when a new username is "claimed" (that is, the command was issued to create it), you place the username in the cache with a short expiration (long enough to account for another request getting through the queue and denormalized into the read model). If it's one service instance, then in memory would probably work, otherwise centralize it with Redis or something.

Then while the next user is filling out the form (assuming there's a front end), you asynchronously check the read model for availability of the username and alert the user if it's already taken. When the command is submitted, you check the cache (not the read model) in order to validate the request before accepting the command (before returning 202); if the name is in the cache, don't accept the command, if it's not then you add it to the cache; if adding it fails (duplicate key because some other process beat you to it), then assume the name is taken -- then respond to the client appropriately. Between the two things, I don't think there'll be much opportunity for a collision.

If there's no front end, then you can skip the async look up or at least have your API provide the endpoint to look it up. You really shouldn't be allowing the client to speak directly to the command model anyway, and placing an API in front of it would allow you to have the API to act as a mediator between the command and read hosts.