Since the beginning, Riak has supported high write-availability using Dynamo-style multi-valued keys – also known as conflicts or siblings. The tradeoff for this type of availability is that the application must include logic to resolve conflicting updates. While it is convenient to say that the application can reason best about conflicts, ad hoc resolution is error-prone and can result in surprising anomalies, like the reappearing item problem in Dynamo’s shopping cart.
What is needed is a more formal and general approach to the problem of conflict resolution for complex data structures. Luckily, there are some formal strategies in recent literature, including Conflict-Free Replicated Data Types (CRDTs) and BloomL lattices. We’ll review these strategies and cover some recent work we’ve done toward adding automatically-convergent data structures to Riak