Hi Paulo, > First and foremost, I believe this proposal in its current form focuses on > the protocol details (HOW?) but lacks the bigger picture on how this is going > to be exposed to the user (WHAT)?
In my opinion this CEP embodies a coherent distinct and complex piece of work, that requires specialist expertise. You have after all just suggested a month to read only the existing proposal 😊 UX is a whole other kind of discussion, that can be quite opinionated, and requires different expertise. It is in my opinion helpful to break out work that is not tightly coupled, as well as work that requires different expertise. As you point out, multi-key UX features are largely independent of any underlying implementation, likely can be done in parallel, and even with different contributors. > Can we not start using it as an external dependency I would love to understand your rationale, as this is a surprising suggestion to me. This is just like any other subsystem, but we would be managing it as a separate library primarily for modularity reasons. The reality is that this option should anyway be considered unavailable. This is a proposed contribution to the Cassandra project, which we can either accept or reject. > Isn't this a good chance to make the serialization protocol pluggable with clearly defined integration points It has recently been demonstrated to be possible to build a system that can safely switch between different consensus protocols. However, this was very sophisticated work that would require its own CEP, one that we would be unable to resource. Even if we could this would be insufficient. This goal has never been achieved for a multi-shard transaction protocol to my knowledge, and multi-shard transaction protocols are much more divergent in implementation detail than consensus protocols. > so we could easily switch implementations with different guarantees… (ie. > Apache Ratis) As far as I know, there are no other strict serializable protocols available to plug in today. Apache Ratis appears to be a straightforward Raft implementation, and therefore it is a linearizable consensus protocol. It is not multi-shard transaction protocol at all, let alone strict serializable. It could be used in place of Paxos, but not Accord. From: Paulo Motta <pauloricard...@gmail.com> Date: Tuesday, 14 September 2021 at 22:55 To: Cassandra DEV <dev@cassandra.apache.org> Subject: Re: [DISCUSS] CEP-15: General Purpose Transactions I can start with some preliminary comments while I get more familiarized with the proposal: - First and foremost, I believe this proposal in its current form focuses on the protocol details (HOW?) but lacks the bigger picture on how this is going to be exposed to the user (WHAT)? Is exposing linearizable transactions to the user not a goal of this proposal? If not, I think the proposal is missing the UX (ie. what CQL commands are going to be added etc) on how these transactions are going to be exposed. - Why do we need to bring the library into the project umbrella? Can we not start using it as an external dependency, and later re-evaluate if it's necessary to bring it into the project or even incubate it as another Apache project? I feel we may be importing unnecessary management overhead into the project while only a small subset of contributors will be involved with the core protocol. - Isn't this a good chance to make the serialization protocol pluggable with clearly defined integration points, so we could easily switch implementations with different guarantees, trade-offs and performance considerations while leaving the UX intact? This would also allow us to easily benchmark the protocol against alternatives (ie. Apache Ratis) and validate the performance claims. I think the best way to do that would be to define what the feature will look like to the end user (UX), define the integration points necessary to support this feature, and use accord as the first implementation of these integration points. Em ter., 14 de set. de 2021 às 17:57, Paulo Motta <pauloricard...@gmail.com> escreveu: > Given the extensiveness and complexity of the proposal I'd suggest leaving > it a little longer (perhaps 4 weeks from the publish date?) for people to > get a bit more familiarized and have the chance to comment before casting a > vote. I glanced through the proposal - and it looks outstanding, very > promising work guys! - but would like a bit more time to take a deeper look > and digest it before potentially commenting on it. > > Em ter., 14 de set. de 2021 às 17:30, bened...@apache.org < > bened...@apache.org> escreveu: > >> Has anyone had a chance to read the drafts, and has any feedback or >> questions? Does anybody still anticipate doing so in the near future? Or >> shall we move to a vote? >> >> From: bened...@apache.org <bened...@apache.org> >> Date: Tuesday, 7 September 2021 at 21:27 >> To: dev@cassandra.apache.org <dev@cassandra.apache.org> >> Subject: Re: [DISCUSS] CEP-15: General Purpose Transactions >> Hi Jake, >> >> > What structural changes are planned to support an external dependency >> project like this >> >> To add to Blake’s answer, in case there’s some confusion over this, the >> proposal is to include this library within the Apache Cassandra project. So >> I wouldn’t think of it as an external dependency. This PMC and community >> will still have the usual oversight over direction and development, and >> APIs will be developed solely with the intention of their integration with >> Cassandra. >> >> > Will this effort eventually replace consistency levels in C*? >> >> I hope we’ll have some very related discussions around consistency levels >> in the coming months more generally, but I don’t think that is tightly >> coupled to this work. I agree with you both that we won’t want to >> perpetuate the problems you’ve highlighted though. >> >> Henrik: >> > I was referring to the property that Calvin transactions also need to >> be sent to the cluster in a single shot >> >> Ah, yes. In that case I agree, and I tried to point to this direction in >> an earlier email, where I discussed the use of scripting languages (i.e. >> transactionally modifying the database with some subset of arbitrary >> computation). I think the JVM is particularly suited to offering quite >> powerful distributed transactions in this vein, and it will be interesting >> to see what we might develop in this direction in future. >> >> >> From: Jake Luciani <jak...@gmail.com> >> Date: Tuesday, 7 September 2021 at 19:27 >> To: dev@cassandra.apache.org <dev@cassandra.apache.org> >> Subject: Re: [DISCUSS] CEP-15: General Purpose Transactions >> Great thanks for the information >> >> On Tue, Sep 7, 2021 at 12:44 PM Blake Eggleston >> <beggles...@apple.com.invalid> wrote: >> >> > Hi Jake, >> > >> > > 1. Will this effort eventually replace consistency levels in C*? I >> ask >> > > because one of the shortcomings of our paxos today is >> > > it can be easily mixed with non serialized consistencies and therefore >> > > users commonly break consistency by for example reading at CL.ONE >> while >> > > also >> > > using LWTs. >> > >> > This will likely require CLs to be specified at the schema level for >> > tables using multi partition transactions. I’d expect this to be >> available >> > for other tables, but not required. >> > >> > > 2. What structural changes are planned to support an external >> dependency >> > > project like this? Are there some high level interfaces you expect >> the >> > > project to adhere to? >> > >> > There will be some interfaces that need to be implemented in C* to >> support >> > the library. You can find the current interfaces in the accord.api >> package, >> > but these were written to support some initial testing, and not intended >> > for integration into C* as is. Things are pretty fluid right now and >> will >> > be rewritten / refactored multiple times over the next few months. >> > >> > Thanks, >> > >> > Blake >> > >> > >> > > On Sun, Sep 5, 2021 at 10:33 AM bened...@apache.org < >> bened...@apache.org >> > > >> > > wrote: >> > > >> > >> Wiki: >> > >> >> > >> https://cwiki.apache.org/confluence/display/CASSANDRA/CEP-15%3A+General+Purpose+Transactions >> > >> Whitepaper: >> > >> >> > >> https://cwiki.apache.org/confluence/download/attachments/188744725/Accord.pdf >> > >> < >> > >> >> > >> https://cwiki.apache.org/confluence/download/attachments/188744725/Accord.pdf?version=1&modificationDate=1630847736966&api=v2 >> > >>> >> > >> Prototype: https://github.com/belliottsmith/accord >> > >> >> > >> Hi everyone, I’d like to propose this CEP for adoption by the >> community. >> > >> >> > >> Cassandra has benefitted from LWTs for many years, but application >> > >> developers that want to ensure consistency for complex operations >> must >> > >> either accept the scalability bottleneck of serializing all related >> > state >> > >> through a single partition, or layer a complex state machine on top >> of >> > the >> > >> database. These are sophisticated and costly activities that our >> users >> > >> should not be expected to undertake. Since distributed databases are >> > >> beginning to offer distributed transactions with fewer caveats, it is >> > past >> > >> time for Cassandra to do so as well. >> > >> >> > >> This CEP proposes the use of several novel techniques that build upon >> > >> research (that followed EPaxos) to deliver (non-interactive) general >> > >> purpose distributed transactions. The approach is outlined in the >> > wikipage >> > >> and in more detail in the linked whitepaper. Importantly, by adopting >> > this >> > >> approach we will be the _only_ distributed database to offer global, >> > >> scalable, strict serializable transactions in one wide area >> round-trip. >> > >> This would represent a significant improvement in the state of the >> art, >> > >> both in the academic literature and in commercial or open source >> > offerings. >> > >> >> > >> This work has been partially realised in a prototype. This partial >> > >> prototype has been verified against Jepsen.io’s Maelstrom library and >> > >> dedicated in-tree strict serializability verification tools, but much >> > work >> > >> remains for the work to be production capable and integrated into >> > Cassandra. >> > >> >> > >> I propose including the prototype in the project as a new source >> > >> repository, to be developed as a standalone library for integration >> into >> > >> Cassandra. I hope the community sees the important value proposition >> of >> > >> this proposal, and will adopt the CEP after this discussion, so that >> the >> > >> library and its integration into Cassandra can be developed in >> parallel >> > and >> > >> with the involvement of the wider community. >> > >> >> > > >> > > >> > > -- >> > > http://twitter.com/tjake >> > >> > >> > --------------------------------------------------------------------- >> > To unsubscribe, e-mail: dev-unsubscr...@cassandra.apache.org >> > For additional commands, e-mail: dev-h...@cassandra.apache.org >> > >> > >> >> -- >> http://twitter.com/tjake >> >
