A comprehensive study of Convergent and Commutative Replicated Data Types (PDF file) Marc Shapiro, Nuno M. Preguiça, Carlos Baquero, Marek Zawirski.
Abstract:
Eventual consistency aims to ensure that replicas of some mutable shared object converge without foreground synchronisation. Previous approaches to eventual consistency are ad-hoc and error-prone. We study a principled approach: to base the design of shared data types on some simple formal conditions that are sufficient to guarantee eventual consistency. We call these types Convergent or Commutative Replicated Data Types (CRDTs). This paper formalises asynchronous object replication, either state based or operation based, and provides a sufficient condition appropriate for each case. It describes several useful CRDTs, including container data types supporting both add and remove operations with clean semantics, and more complex types such as graphs, montonic DAGs, and sequences. It discusses some properties needed to implement non-trivial CRDTs.
I found this following a link in the readme for riak dt which said:
WHAT?
Currently under initial development, riak_dt is a platform for convergent data types. It’s built on riak core and deployed with riak. All of our current work is around supporting fast, replicated, eventually consistent counters (though more data types are in the repo, and on the way.) This work is based on the paper – A Comprehensive study of Convergent and Commutative Replicated Data Types – which you may find an interesting read.
WHY?
Riak’s current model for handling concurrent writes is to store sibling values and present them to the client for resolution on read. The client must encode the logic to merge these into a single, meaningful value, and then inform Riak by doing a further write. Convergent data types remove this burden from the client, as their structure guarantees they will deterministically converge to a single value. The simplest of these data types is a counter.
I haven’t thought of merging of subject representatives as a quest for “consistency” but that is one way to think about it.
The paper is forty-seven pages long and has forty-four references, most of which I suspect are necessary to fully appreciate the work.
Having said that, I suspect it will be well worth the effort.