I am working on a system based on Apache Cassandra and written using Akka and I am learning to live without an RDBMS. However, I have a singular problem that I would like to have feedback on how to implement it. Basically I have learned to work without ACID and have come to the conclusion that even banking systems can be done in a database so long as it has the (A)tomic in the ACID. Purchases for products, inventory management and so on are solvable and I have solved them. The question, then, revolves around a simple activity, namely registering users.
When a user registered the constraints are that a user may not be registered with a username used by another user nor an email used by another user. However this is a global constraint and cassandra gets in my way. I can't first check the DB for the username and then write the user in a distributed implementation because I lack the (I)isolated aspect inside of Cassandra. For most other things I can spin up an actor to handle a single product or vendor or customer and handle messages in those actors and enforce constraints there but in this case I am dealing with a global resource.
To solve the problem I have resorted to a cluster singleton to implement the registration process and all registration is funneled through this actor which enforces the constraints but having a single point of failure makes me uneasy.
So that leaves me to the question. Using only Cassandra and Akka, do people have any suggestions for how I could implement user registration, while keeping the global constraints, in a distributed environment without a cluster singleton. This kind of implementation should be applicable other other global resources in the system as well.
I eagerly look forward to your ideas.
