Comment by zinodaur
2 years ago
My understanding of ReadVersion is that the only point of calling it is to be able to read your own writes - so staleness wouldn't be good. There was another sibling comment that says the ReadVersion requests are batched up before hitting the sequencer, I could definitely believe that would work.
Yeah, read versions are just there to make what's known as "external consistency" work. That is, if transaction B starts after transaction A commits, then B will see the effects of A.
The reason external consistency is nice is that if you change the database, as soon as you get a commit signal, you can tell any other client "hey, go check the database and see what I did" and they will see the changes. No worries about whether the changes have sync'ed yet or anything like that.
We've been exploring ways of getting external consistency for our data model - but our Sequencers are different enough that I don't think we can use the FoundationDB approach.
We are trying out a version of what CockroachDB does - run transaction A possibly on stale data, spy on the things it reads, call our flavor of ReadVersion for the "shards" the transaction interacted with, and re-run transaction A with the data versions we got from ReadVersion. It will usually just return after the the first run, rarely requiring a second run, and in some pathological cases require infinite runs.