Applied Eventually Consistency and Race Conditions

Question

I have a question regarding the effect of eventually consistent (EC) microservice systems.

Imagine we have a booking system - a user-service A and booking-service B. Each service has its own database. Imagine the system does a concurrent booking of the same resource for distinct users at the same time. Lets assume we have a Runtime Verification System checking the concurrent booking.

Would it be possible that the monitor does not realize the concurrent booking at B, because the update in the database is done delayed because of the EC mechanism?

Answer 1

In your example, the Booking Service is the source of truth (presumably) for whether or not the resource is available to book. So, that service should be pretty clear on allowing the first booking request to happen and rejecting the second.

In a case like this, where "first come first served" is the requirement, you'd want an intermediate state that would wait for a response from the Booking Service and update the User Service only when a response has been received.

If your architecture is set up right, User Service shouldn't be calling Booking Service directly anyway - it should be communicating through a messaging plane. As such, when the User clicks "Book Now," you could generate a resourceBookingRequested message and submit it to the queue. You'd acknowledge this request has been queued to the user and update their UI to "Awaiting Booking Confirmation..." or something similar.

Once the booking is accepted, or rejected, the User Service subscribes to the resulting message and updates the UI (and/or takes other actions like sending an email) to let the user know their request succeeded or didn't.