Jonathan, Your request to separate consensus from execution is about as sensical as asking for this separation in Paxos, or any other distributed consensus protocol. I have made these statements repeatedly, so let me break it down step by step.
1. Accord is an optimal leaderless distributed consensus protocol, offering multi-shard CAS semantics in one round-trip (or two under contention and clock skew). 2. By simple virtue of this property, it already achieves Calvin semantics with no other work. It remains a distributed consensus protocol, and the whitepaper compares to these as peers. 3. To build distributed transactions with more complex semantics, the remaining candidates are the CockroachDB or YugaByte approach. These must utilise a distributed consensus protocol. They do so using Raft today. Accord is as optimal as Raft, therefore, Accord may be used to implement this technique *without penalty*. Through its multi-shard consensus it has the added advantage of supporting stronger isolation (but not requiring it – a read/write intent design may choose weaker isolation). You continue to refuse to engage with these and other points. Please respond directly to ALL of the below, that I have been asking you to answer now for several weeks. 1. Since Accord supports all of your mooted transaction systems without penalty the conversation about which semantics to pursue may be conducted in parallel with its development. What about this claim do you not yet understand? If you understand, why should a vote on CEP-15 be delayed? 2. Which SPECIFIC transaction semantics do you want to achieve? You are all over the shop today, demanding Cockroach/YugaByte interactive semantics, but also LOCAL_SERIAL operation and proposing SLOG. These are conflicting demands. 3. Why do you think Accord cannot support your preferred semantics? 4. Will you accept a video call so we may discuss this with you in detail, so we may understand your difficulty understanding these points I keep repeating? After several weeks of back and forth you should already be able to answer these questions. If you cannot invest the time to answer them now, I perceive this as obstructive and I will escalate this to a PMC vote to break the deadlock. From: Jonathan Ellis <jbel...@gmail.com> Date: Wednesday, 13 October 2021 at 04:21 To: dev <dev@cassandra.apache.org> Subject: Re: Tradeoffs for Cassandra transaction management Blake (and Benedict), I’ll ask for your patience here. We don’t have a precedent of pushing through major initiatives in this project in a matter of weeks. We [members of the PMC that weren’t involved in creating Accord] need time to do thorough research and make sure both that we understand what is being proposed and that we have evaluated reasonable alternatives. One of the difficulties in evaluating Accord is that it combines a state-of-the-art consensus/ordering protocol with a fairly limited transaction manager. So it may be useful to decouple the consensus and transaction processing components, which would both allow non-Cassandra usage of the consensus piece, and also make explicit the boundaries with transaction processing with the consequence of making it easier to evolve independently. In the meantime, it’s very important to me to understand on which dimensions the transaction manager can be improved easily, and which dimensions resist such improvement. I get that Accord is your [plural] baby and it’s awkward for me to come along and start pointing at its limitations, but that’s part of creating a complete understanding of any system. If I keep coming back to the subject of SQL support and interactive transactions, that’s because it’s becoming table stakes in the distributed database space. People are using Cockroach or Yugabyte or Cloud Spanner for use cases where a couple years ago they would have used Cassandra. We can expect this trend to continue and strengthen. On Mon, Oct 11, 2021 at 11:39 PM Blake Eggleston <beggles...@apple.com.invalid> wrote: > Let’s get back on topic. > > Jonathan, in your opening email you stated that, in your view, the 2 main > areas of tradeoff were: > > > 1. Is it worth giving up local latencies to get full global consistency? > > Now we’ve established that we don’t need to give up local latencies with > Accord, which leaves: > > > 2. Is it worth giving up the possibility of SQL support, to get the > benefits of deterministic transaction design? > > I pointed out that this was a false dilemma and that, in the worst case, a > hypothetical SQL feature could have it’s own consensus system. I hope that > won’t be necessary, but as I later pointed out (and you did not address, > although maybe I should have phrased it as a question), if we’re going to > weigh accord against a hypothetical SQL feature that lacks design goals, or > any clear ideas about how it might be implemented, how can we rule that out? > > So Jonathan, how can we rule that out? How can we have a productive > discussion about a feature you yourself are unable to describe in any > meaningful detail? > > > On Oct 11, 2021, at 6:34 PM, Jonathan Ellis <jbel...@gmail.com> wrote: > > > > On Mon, Oct 11, 2021 at 5:11 PM bened...@apache.org <bened...@apache.org > > > > wrote: > > > >> If we want to fully unpack this particular point, as far as I can tell > >> claiming ANSI SQL would indeed require interactive transactions in which > >> arbitrary conditional work may be performed by a client within a > >> transaction in response to other actions within that transaction. > >> > >> However: > >> > >> 1. The ANSI SQL standard permits these transactions to fail and > >> rollback (e.g. in the event that your optimistic transaction fails). So > if > >> you want to be pedantic, you may modify my statement to “SQL does not > >> necessitate support for abort-free interactive transactions” and we can > >> leave it there. > >> > >> 2. I would personally consider “SQL support” to include the capability > >> of defining arbitrary SQL stored procedures that may be executed by > clients > >> in an interactive session > > > > > > I note your personal preference and I further note that this is not the > > common understanding of "SQL support" in the industry. If you tell 100 > > developers that your database supports SQL, then at least 99 of them are > > going to assume that you can work with APIs like JDBC that expose > > interactive transactions as a central feature, and hence that you will be > > reasonably compatible with the vast array of SQL-based applications out > > there. > > > > Historical side note: VoltDB tried to convince people that stored > > procedures were good enough. It didn't work, and VoltDB had to add > > interactive transactions as fast as they could. > > > > 3. Most importantly, as I pointed out in the previous email, Accord is > >> compatible with a YugaByte/Cockroach-like approach, and indeed makes > this > >> approach both easier to accomplish and enables stronger isolation than > the > >> equivalent Raft-based approach. These approaches are able to reduce the > >> number of conflicts, at a cost of significantly higher transaction > >> management burden. > >> > > > > If you're saying that you could use Accord instead of Raft or Paxos, and > > layer 2PC on top of that as in Spanner, then I agree, but I don't think > > that is a very good design, as you would no longer get any of the > benefits > > of the deterministic approach you started with. If you mean something > > else, then perhaps an example would help clarify. > > > > -- > > Jonathan Ellis > > co-founder, http://www.datastax.com > > @spyced > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: dev-unsubscr...@cassandra.apache.org > For additional commands, e-mail: dev-h...@cassandra.apache.org > > -- Jonathan Ellis co-founder, http://www.datastax.com @spyced
