This is a Swift implementation of the dotted version vector as described in the paper Dotted Version Vectors: Efficient Causality Tracking for Distributed Key-Value Stores by Gonçalves R, Almeida PS, Moreno CB, Fonte V, Preguiça N. (2012). This code used the reference implementation in Erlang for guidance (and test cases).
The basic class is
DVV which contains a version vector and a
Dot which is just a named counter. The version vection itself is
simply an ordered collection of
Dot instances, but with slightly different semantics. The
Dot instance in the
DVV refers to a
write counter for a given server, while the
Dot instances in the version history record the relationships between versions.
The Erlang reference implementation incorporates a timestamp in the
Dot defintion that is used to order
Dot instances when the
server or replica key is the same. This Swift implementation does not do this – it assumes that the server or replica managing the
Dot counter can guarantee that it is properly incremented at every update.
Just like the Erlang implementation, a DVV is immutable. The DVV API will generate a new DVV instance instead of changing a value in-place.