I tend to lean towards Josh's perspective. Gossip was poorly tested and implemented. I dont think it's a good parallel or at least I hope it's not. Taken to the extreme we shouldn't touch the database at all otherwise, which isn't practical. That said, anything touching important subsystems needs more care, testing, and time to bake. I think we're mostly discussing "being careful" of which I am totally on board with. I don't think Benedict ever said "don't use TCM", in fact he's said the opposite, but emphasized the care that is required when we do, which is totally reasonable.
Back to capabilities, Riak built them on an eventually consistent subsystem and they worked fine. If you have a split brain you likely dont want to communicate agreement as is (or have already learned about agreement and its not an issue). That said, I don't think we have an EC layer in C* I would want to rely on outside of distributed tables. So in the context of what we have existing I think TCM is a better fit. I still need to dig a little more to be convinced and plan to do that as I draft the CEP. Jordan On Sat, Dec 21, 2024 at 5:51 AM Benedict <bened...@apache.org> wrote: > I’m not saying we need to tease out bugs from TCM. I’m saying every time > someone touches something this central to correctness we introduce a risk > of breaking it, and that we should exercise that risk judiciously. This has > zero to do with the amount of data we’re pushing through it, and 100% to do > with writing bad code. > > We treated gossip carefully in part because it was hard to work with, but > in part because getting it wrong was particularly bad. We should retain the > latter reason for caution. > > We also absolutely do not need TCM for consistency. We have consistent > database functionality for that. TCM is special because it cannot rely on > the database mechanisms, as it underpins them. That is the whole point of > why we should treat it carefully. > > On 21 Dec 2024, at 13:43, Josh McKenzie <jmcken...@apache.org> wrote: > > > To play the devil's advocate - the more we exercise TCM the more bugs we > suss out. To Jon's point, the volume of information we're talking about > here in terms of capabilities dissemination shouldn't stress TCM at all. > > I think a reasonable heuristic for relying on TCM for something is whether > there's a big difference in UX on something being eventually consistent vs. > strongly consistent. Exposing features to clients based on whether the > entire cluster supports them seems like the kind of thing that could cause > pain if we're in a split-brain, cluster-is-settling-on-agreement kind of > paradigm. > > On Fri, Dec 20, 2024, at 3:17 PM, Benedict wrote: > > > Mostly conceptual; the problem with a linearizable history is that if you > lose some of it (eg because some logic bug prevents you from processing > some epoch) you stop the world until an operator can step in to perform > surgery about what the history should be. > > I do know of one recent bug to schema changes in cep-15 that broke TCM in > this way. That particular avenue will be hardened, but the fewer places we > risk this the better IMO. > > Of course, there are steps we could take to expose a limited API targeting > these use cases, as well as using a separate log for ancillary > functionality, that might better balance risk:reward. But equally I’m not > sure it makes sense to TCM all the things, and maybe dogfooding our own > database features and developing functionality that enables our own use > cases could be better where it isn’t necessary 🤷♀️ > > > On 20 Dec 2024, at 19:22, Jordan West <jorda...@gmail.com> wrote: > > > On Fri, Dec 20, 2024 at 11:06 AM Benedict <bened...@apache.org> wrote: > > > If TCM breaks we all have a really bad time, much worse than if any one of > these features individually has problems. If you break TCM in the right way > the cluster could become inoperable, or operations like topology changes > may be prevented. > > > Benedict, when you say this are you speaking hypothetically (in the sense > that by using TCM more we increase the probability of using it "wrong" and > hitting an unknown edge case) or are there known ways today that TCM > "breaks"? > > Jordan > > > This means that even a parallel log has some risk if we end up modifying > shared functionality. > > > > > On 20 Dec 2024, at 18:47, Štefan Miklošovič <smikloso...@apache.org> > wrote: > > > I stand corrected. C in TCM is "cluster" :D Anyway. Configuration is super > reasonable to be put there. > > On Fri, Dec 20, 2024 at 7:42 PM Štefan Miklošovič <smikloso...@apache.org> > wrote: > > I am super hesitant to base distributed guardrails or any configuration > for that matter on anything but TCM. Does not "C" in TCM stand for > "configuration" anyway? So rename it to TSM like "schema" then if it is > meant to be just for that. It seems to be quite ridiculous to code tables > with caches on top when we have way more effective tooling thanks to CEP-21 > to deal with that with clear advantages of getting rid of all of that old > mechanism we have in place. > > I have not seen any concrete examples of risks why using TCM should be > just for what it is currently for. Why not put the configuration meant to > be cluster-wide into that? > > What is it ... performance? What does even the term "additional > complexity" mean? Complex in what? Do you think that putting there 3 types > of transformations in case of guardrails which flip some booleans and > numbers would suddenly make TCM way more complex? Come on ... > > This has nothing to do with what Jordan is trying to introduce. I think we > all agree he knows what he is doing and if he evaluates that TCM is too > much for his use case (or it is not a good fit) that is perfectly fine. > > On Fri, Dec 20, 2024 at 7:22 PM Paulo Motta <pa...@apache.org> wrote: > > > It should be possible to use distributed system tables just fine for > capabilities, config and guardrails. > > I have been thinking about this recently and I agree we should be wary > about introducing new TCM states and create additional complexity that can > be serviced by existing data dissemination mechanisms (gossip/system > tables). I would prefer that we take a more phased and incremental approach > to introduce new TCM states. > > As a way to accomplish that, I have thought about introducing a new > generic TCM state "In Maintenance", where schema or membership changes are > "frozen/disallowed" while an external operation is taking place. This > "external operation" could mean many things: > - Upgrade > - Downgrade > - Migration > - Capability Enablement/Disablement > > These could be sub-states of the "Maintenance" TCM state, that could be > managed externally (via cache/gossip/system tables/sidecar). Once these > sub-states are validated thouroughly and mature enough, we could "promote" > them to top-level TCM states. > > In the end what really matters is that cluster and schema membership > changes do not happen while a miscellaneous operation is taking place. > > Would this make sense as an initial way to integrate TCM with capabilities > framework ? > > On Fri, Dec 20, 2024 at 4:53 AM Benedict <bened...@apache.org> wrote: > > > If you perform a read from a distributed table on startup you will find > the latest information. What catchup are you thinking of? I don’t think any > of the features we talked about need a log, only the latest information. > > We can (and should) probably introduce event listeners for distributed > tables, as this is also a really great feature, but I don’t think this > should be necessary here. > > Regarding disagreements: if you use LWTs then there are no consistency > issues to worry about. > > Again, I’m not opposed to using TCM, although I am a little worried TCM is > becoming our new hammer with everything a nail. It would be better IMO to > keep TCM scoped to essential functionality as it’s critical to correctness. > Perhaps we could extend its APIs to less critical services without > intertwining them with membership, schema and epoch handling. > > > On 20 Dec 2024, at 09:43, Štefan Miklošovič <smikloso...@apache.org> > wrote: > > > I find TCM way more comfortable to work with. The capability of log being > replayed on restart and catching up with everything else automatically is > god-sent. If we had that on "good old distributed tables", then is it not > true that we would need to take extra care of that, e.g. we would need to > repair it etc ... It might be the source of the discrepancies / > disagreements etc. TCM is just "maintenance-free" and _just works_. > > I think I was also investigating distributed tables but was just pulled > towards TCM naturally because of its goodies. > > On Fri, Dec 20, 2024 at 10:08 AM Benedict <bened...@apache.org> wrote: > > > TCM is a perfectly valid basis for this, but TCM is only really > *necessary* to solve meta config problems where we can’t rely on the rest > of the database working. Particularly placement issues, which is why schema > and membership need to live there. > > It should be possible to use distributed system tables just fine for > capabilities, config and guardrails. > > That said, it’s possible config might be better represented as part of the > schema (and we already store some relevant config there) in which case it > would live in TCM automatically. Migrating existing configs to a > distributed setup will be fun however we do it though. > > Capabilities also feel naturally related to other membership information, > so TCM might be the most suitable place, particularly for handling > downgrades after capabilities have been enabled (if we ever expect to > support turning off capabilities and then downgrading - which today we > mostly don’t). > > > On 20 Dec 2024, at 08:42, Štefan Miklošovič <smikloso...@apache.org> > wrote: > > > Jordan, > > I also think that having it on TCM would be ideal and we should explore > this path first before doing anything custom. > > Regarding my idea about the guardrails in TCM, when I prototyped that and > wanted to make it happen, there was a little bit of a pushback (1) (even > though super reasonable one) that TCM is just too young at the moment and > it would be desirable to go through some stabilisation period. > > Another idea was that we should not make just guardrails happen but the > whole config should be in TCM. From what I put together, Sam / Alex does > not seem to be opposed to this idea, rather the opposite, but having CEP > about that is way more involved than having just guardrails there. I > consider guardrails to be kind of special and I do not think that having > all configurations in TCM (which guardrails are part of) is the absolute > must in order to deliver that. I may start with guardrails CEP and you may > explore Capabilities CEP on TCM too, if that makes sense? > > I just wanted to raise the point about the time this would be delivered. > If Capabilities are built on TCM and I wanted to do Guardrails on TCM too > but was explained it is probably too soon, I guess you would experience > something similar. > > Sam's comment is from May and maybe a lot has changed since in then and > his comment is not applicable anymore. It would be great to know if we > could build on top of the current trunk already or we will wait until > 5.1/6.0 is delivered. > > (1) > https://issues.apache.org/jira/browse/CASSANDRA-19593?focusedCommentId=17844326&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-17844326 > > On Fri, Dec 20, 2024 at 2:17 AM Jordan West <jorda...@gmail.com> wrote: > > Firstly, glad to see the support and enthusiasm here and in the recent > Slack discussion. I think there is enough for me to start drafting a CEP. > > Stefan, global configuration and capabilities do have some overlap but not > full overlap. For example, you may want to set globally that a cluster > enables feature X or control the threshold for a guardrail but you still > need to know if all nodes support feature X or have that guardrail, the > latter is what capabilities targets. I do think capabilities are a step > towards supporting global configuration and the work you described is > another step (that we could do after capabilities or in parallel with them > in mind). I am also supportive of exploring global configuration for the > reasons you mentioned. > > In terms of how capabilities get propagated across the cluster, I hadn't > put much thought into it yet past likely TCM since this will be a new > feature that lands after TCM. In Riak, we had gossip (but more mature than > C*s -- this was an area I contributed to a lot so very familiar) to > disseminate less critical information such as capabilities and a separate > layer that did TCM. Since we don't have this in C* I don't think we would > want to build a separate distribution channel for capabilities metadata > when we already have TCM in place. But I plan to explore this more as I > draft the CEP. > > Jordan > > On Thu, Dec 19, 2024 at 1:48 PM Štefan Miklošovič <smikloso...@apache.org> > wrote: > > Hi Jordan, > > what would this look like from the implementation perspective? I was > experimenting with transactional guardrails where an operator would control > the content of a virtual table which would be backed by TCM so whatever > guardrail we would change, this would be automatically and transparently > propagated to every node in a cluster. The POC worked quite nicely. TCM is > just a vehicle to commit a change which would spread around and all these > settings would survive restarts. We would have the same configuration > everywhere which is not currently the case because guardrails are > configured per node and if not persisted to yaml, on restart their values > would be forgotten. > > Guardrails are just an example, what is quite obvious is to expand this > idea to the whole configuration in yaml. Of course, not all properties in > yaml make sense to be the same cluster-wise (ip addresses etc ...), but the > ones which do would be again set everywhere the same way. > > The approach I described above is that we make sure that the configuration > is same everywhere, hence there can be no misunderstanding what features > this or that node has, if we say that all nodes have to have a particular > feature because we said so in TCM log so on restart / replay, a node with > "catch up" with whatever features it is asked to turn on. > > Your approach seems to be that we distribute what all capabilities / > features a cluster supports and that each individual node configures itself > in some way or not to comply? > > Is there any intersection in these approaches? At first sight it seems > somehow related. How is one different from another from your point of view? > > Regards > > (1) https://issues.apache.org/jira/browse/CASSANDRA-19593 > > On Thu, Dec 19, 2024 at 12:00 AM Jordan West <jw...@apache.org> wrote: > > In a recent discussion on the pains of upgrading one topic that came up is > a feature that Riak had called Capabilities [1]. A major pain with upgrades > is that each node independently decides when to start using new or modified > functionality. Even when we put this behind a config (like storage > compatibility mode) each node immediately enables the feature when the > config is changed and the node is restarted. This causes various types of > upgrade pain such as failed streams and schema disagreement. A > recent example of this is CASSANRA-20118 [2]. In some cases operators can > prevent this from happening through careful coordination (e.g. ensuring > upgrade sstables only runs after the whole cluster is upgraded) but > typically requires custom code in whatever control plane the operator is > using. A capabilities framework would distribute the state of what features > each node has (and their status e.g. enabled or not) so that the cluster > can choose to opt in to new features once the whole cluster has them > available. From experience, having this in Riak made upgrades a > significantly less risky process and also paved a path towards repeatable > downgrades. I think Cassandra would benefit from it as well. > > Further, other tools like analytics could benefit from having this > information since currently it's up to the operator to manually determine > the state of the cluster in some cases. > > I am considering drafting a CEP proposal for this feature but wanted to > take the general temperature of the community and get some early thoughts > while working on the draft. > > Looking forward to hearing y'alls thoughts, > Jordan > > [1] > https://github.com/basho/riak_core/blob/25d9a6fa917eb8a2e95795d64eb88d7ad384ed88/src/riak_core_capability.erl#L23-L72 > > [2] https://issues.apache.org/jira/browse/CASSANDRA-20118 > > >
