Just to make sure I understand correctly, would you agree that if clients always synchronously commit (i.e. wait until the MetaStore commit returns ok) before acknowledging any commit locally, the client will experience Strict Serializability?
Assuming you agree with that, what would be a more clear way to explain the tradeoffs and resulting consistency models in the event that a client desires to asynchronously commit?
I think I see the issue, but I'd love to hear your take on how to update the docs.
Firstly, it might be worth separating the concepts of read and write consistency. For example, in your system reads are eventually consistent (because syncing is not enforced) and the situation with writes is more complicated.
I think the key point of your design is flexibility, rather than any individual consistency properties. It might be better to emphasise this and try to explain, at the top-level, the concrete ways an application can interact with the storage and the different tradeoffs.
So you might have strong write consistency with forced global serialisation or weaker properties with less enforced sync policies. From the perspective of the application, the internal details shouldn’t matter, but the external properties and how to achieve them (eg. CRDT style merging etc as a way to get strong consistency with less syncing, for certain domains).
I think I'm probably operating with definitions of client and commit that are different than yours.
Specifically, I don't really see how a client can "commit locally" and "commit globally" as separate things. I understand a client to be something that interacts with your metastore API, which provides a single "commit" operation, that AFAICT will return success/failure based on local commit state, not global state.
Is that not correct?
Later on in the design.md you say
> The Client will be a Rust library ...
which might be the missing link in this discussion. Is the system model here assuming that clients are always gonna be Rust code in the same compilation unit as the Graft crate/library/etc.?
Graft's definition sounds more like a Git "commit" than one found in a SQL standard. Perhaps that's the source of this confusion?
That may be the case, but the Git notion of a "commit" is a domain concept that is unrelated to, and incompatible with, the distsys notion of a "commit". And terminology like "strict serialization" and "snapshot isolation" and so on -- which this system and its docs reference -- are distsys concepts, not Git concepts. So "commit" needs to abide distsys rules, not Git rules, as you can't really mix-and-match definitions across domains.
Git is a DVCS. The D stands for distributed, meaning (in old people language) masterless. Git doesn't have a "global."