Interfaces RFC: Register

[mweidner037]

Interface

https://github.com/composablesys/compoventuals/blob/master/client/src/crdt/register/interfaces.ts

Planned implementations (already complete)

• AggregateRegister<T>: a wrapper around a multi-value register that applies a given "aggregation" function to the set of conflicting (causally maximal) values, in order to return the actual value. The aggregation is allowed to look at the values as well as metadata for each value, namely, its sender, the sender's vector clock entry, and the real-world time it was set.
  • E.g., in the whiteboard, we could choose to aggregate conflicting colors by averaging them, instead of picking one.
• LwwRegister<T>: a standard LWW register. It is an AggregateRegister where the aggregation function chooses the value with the greatest real-world time.
• FwwRegister<T>: same, but the first writer wins instead of the last.

Questions

• Should MultiValueRegister be exported? I have left it out so far because it wouldn't actually implement the Register interface. In principle, it is not needed because you can use a LwwRegister and use the conflicts() method to see all conflicting values.
• Should FwwRegister be included by default, or is it not useful enough? (A user who really wants it can easily implement it using AggregateRegister.)
• AggregateRegister metadata: currently, it is fixed to (sender, sender's vector clock entry, time). In principle, someone might want to add more metadata, or omit the time as an optimization; should this be supported?
• More implementations? E.g., a register where there is a total order on values, the value can only increase under this order, and in case of conflicts, you pick the greatest element. (This should be doable with a subclass of AggregateRegister.)

[mweidner037]

Fine as-is, but perhaps the name should be changed (now CRegister). "Register" suggests a low-level processor register, and is probably just historical at this point. Perhaps CValue or CVariable?

[mweidner037]

Implementations: move the MVR implementation within AggregateRegister.

[mweidner037]

Done by https://github.com/composablesys/compoventuals/pull/138