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 >>>> >>>
