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.

Answer 2

I think you are yet to have the mindset shift to eventual consistency and the nature of event sourcing. I had the same problem. Specifically I refused to accept that you should trust commands from the client that, using your example, say "Place this order with 10% discount" without the domain validating that the discount should go ahead. One thing that really hit home for me was something that Udi himself said to me (check the comments of the accepted answer).

Basically I came to realise that there is no reason not to trust the client; everything on the read side has been produced from the domain model, so there is no reason not to accept the commands. Whatever in the read side that says the customer qualifies for discount has been put there by the domain.

BTW: In the requirement, if the entered UserName is already taken, the website should show error message "Sorry, the user name XXX is not available" to the visitor. It's not acceptable to show a message, say, "We are creating your account, please wait, we will send the registration result to you via Email later", to the visitor.

If you are going to adopt event sourcing & eventual consistency, you will need to accept that sometimes it will not be possible to show error messages instantly after submitting a command. With the unique username example the chances of this happening are so slim (given that you check the read side before sending the command) its not worth worrying about too much, but a subsequent notification would need to be sent for this scenario, or perhaps ask them for a different username the next time they log on. The great thing about these scenarios is that it gets you thinking about business value & what's really important.

UPDATE : Oct 2015

Just wanted to add, that in actual fact, where public facing websites are concerned - indicating that an email is already taken is actually against security best practices. Instead, the registration should appear to have gone through successfully informing the user that a verification email has been sent, but in the case where the username exists, the email should inform them of this and prompt them to login or reset their password. Although this only works when using email addresses as the username, which I think is advisable for this reason.