[hem777]

> that's not true in general

Can you elaborate what do you mean by this? I was arguing that it’s possible as the original argument was “this is not possible in a real system and is only theoretical”.

> provide no meaningful consistency guarantees to users, because they allows "committed" writes to be lost

If I set the (shared) value to green and you set it to blue, what is the expected observed value? What if you set it to green and I set it blue, what is the observed value? More importantly, what is the consistency that was lost?

[preseinger]

so there are multiple threads of conversation here

first, there is no straightforward way to module "a value" as a CRDT, precisely for the reason you point out: how to resolve conflicts between concurrent updates is not well-defined

concretely, if you set v=green and I set v=blue, then the expected observed value is undefined, without additional information

there are various ways to model values as CRDTs, each has different properties, the simplest way to model a value as a CRDT is with the LWW conflict resolution strategy, but this approach loses information

example: say your v=green is the last writer and wins, then I will observe (in my local replicas) that v=blue for a period of time until replication resolves the conflict, and sets (in my local replicas) v=green. when v changes from blue to green, the whole idea that v ever was blue is lost. after replication resolves the conflict, v was never blue. it was only ever green. but there was a period of time where i observed v as blue, right? that observation was invalid. that's a problem. consistency was lost there.

--

second

> almost any data structure can be turned into a (op-based) CRDT.

yes, in theory. but op-based CRDTs only work if message delivery between nodes is reliable. and no real-world network provides this property (while maintaining availability).

[hem777]

> there was a period of time where i observed v as blue, right? that observation was invalid.

Not invalid. The observation was correct at that time and place, meaning, in the partition that it was observed. This is the P in AP.

It seems to me that you’re talking about and arguing for synchronization, that is, consensus about the values (=state), which takes the system to CP as opposed to AP.

> there is no straightforward way to module "a value"

I would recommend to look into the “Register” (CR)data structure.