> I would kind of expect this work, if it pans out, to _replace_ the current > paxos implementation
That’s a good point. I think the clear direction of travel would be total replacement of Paxos, but I anticipate that this will be feature-flagged at least initially. So for some period of time we may maintain both options, with the advanced CQL functionality disabled if you opt for classic Paxos. I think this is a necessary corollary of a requirement to support live upgrades – something that is non-negotiable IMO, but that I have also neglected to discuss in the CEP. I will rectify this. An open question is if we want to support live downgrades back to Classic Paxos. I kind of expect that we will, though that will no doubt be informed by the difficulty of doing so. Either way, this means the deprecation cycle for Classic Paxos is probably a separate and future decision for the community. We could choose to maintain it indefinitely, but I would vote to retire it the following major version. A related open question is defaults – I would probably vote for new clusters to default to Accord, and existing clusters to need to run a migration command after fully upgrading the cluster. From: Sylvain Lebresne <lebre...@gmail.com> Date: Wednesday, 15 September 2021 at 14:13 To: dev@cassandra.apache.org <dev@cassandra.apache.org> Subject: Re: [DISCUSS] CEP-15: General Purpose Transactions Fwiw, it makes sense to me to talk about CQL syntax evolution separately. It's pretty clear to me that we _can_ extend CQL to make sure of a general purpose transaction mechanism, so I don't think deciding if we want a general purpose transaction mechanism has to depend on deciding on the syntax. Especially since the syntax question can get pretty far on its own and could be a serious upfront distraction. And as you said, there are even queries that can be expressed with the current syntax that we refuse now and would be able to accept with this, so those could be "ground zero" of what this work would allow. But outside of pure syntax questions, one thing that I don't see discussed in the CEP (or did I miss it) is what the relationship of this new mechanism with the existing paxos implementation would be? I would kind of expect this work, if it pans out, to _replace_ the current paxos implementation (because 1) why not and 2) the idea of having 2 serialization mechanisms that serialize separately sounds like a nightmare from the user POV) but it isn't stated clearly. If replacement is indeed the intent, then I think there needs to be a plan for the upgrade path. If that's not the intent, then what? -- Sylvain On Wed, Sep 15, 2021 at 12:09 PM bened...@apache.org <bened...@apache.org> wrote: > Ok, so the act of typing out an example was actually a really good > reminder of just how limited our functionality is today, even for single > partition operations. > > I don’t want to distract from any discussion around the underlying > protocol, but we could kick off a separate conversation about how to evolve > CQL sooner than later if there is the appetite. There are no concrete > proposals to discuss, it would be brainstorming. > > Do people also generally agree this work warrants a distinct CEP, or would > people prefer to see this developed under the same umbrella? > > > > From: bened...@apache.org <bened...@apache.org> > Date: Wednesday, 15 September 2021 at 09:19 > To: dev@cassandra.apache.org <dev@cassandra.apache.org> > Subject: Re: [DISCUSS] CEP-15: General Purpose Transactions > > perhaps we can prepare these as examples > > There are grammatically correct CQL queries today that cannot be executed, > that this work will naturally remove the restrictions on. I’m certainly > happy to specify one of these for the CEP if it will help the reader. > > I want to exclude “new CQL commands” or any other enhancement to the > grammar from the scope of the CEP, however. This work will enable a range > of improvements to the UX, but I think this work is a separate, long-term > project of evolution that deserves its own CEPs, and will likely involve > input from a wider range of contributors and users. If nobody else starts > such CEPs, I will do so in due course (much further down the line). > > Assuming there is not significant dissent on this point I will update the > CEP to reflect this non-goal. > > > > From: C. Scott Andreas <sc...@paradoxica.net> > Date: Wednesday, 15 September 2021 at 00:31 > To: dev@cassandra.apache.org <dev@cassandra.apache.org> > Cc: dev@cassandra.apache.org <dev@cassandra.apache.org> > Subject: Re: [DISCUSS] CEP-15: General Purpose Transactions > Adding a few notes from my perspective as well – > > Re: the UX question, thanks for asking this. > > I agree that offering a set of example queries and use cases may help make > the specific use cases more understandable; perhaps we can prepare these as > examples to be included in the CEP. > > I do think that all potential UX directions begin with the specification > of the protocol that will underly them, as what can be expressed by it may > be a superset of what's immediately exposed by CQL. But at minimum it's > great to have a sense of the queries one might be able to issue to focus a > reading of the whitepaper. > > Re: "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 think it would be valuable to the project for the work to be incubated > in a separate repository as part of the Apache Cassandra project itself, > much like the in-JVM dtest API and Harry. This pattern worked well for > those projects as they incubated as it allowed them to evolve outside the > primary codebase, but subject to the same project governance, set of PMC > members, committers, and so on. Like those libraries, it also makes sense > as the Cassandra project is the first (and at this time) only known > intended consumer of the library, though there may be more in the future. > > If the proposal is accepted, the time horizon envisioned for this work's > completion is ~9 months to a standard of production readiness. The > contributors see value in the work being donated to and governed by the > contribution practices of the Foundation. Doing so ensures that it is being > developed openly and with full opportunity for review and contribution of > others, while also solidifying contribution of the IP to the project. > > Spinning up a separate ASF incubation project is an interesting idea, but > I feel that doing so would introduce a far greater overhead in process and > governance, and that the most suitable governance and set of committers/PMC > members are those of the Apache Cassandra project itself. > > On Sep 14, 2021, at 3:53 PM, "bened...@apache.org" <bened...@apache.org> > wrote: > > > 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 >
