I didn't realize the CEP hadn't been updated. I think I was mixing up the convo on the ML and what was actually in the CEP. I had remembered it behaving much closer to clickhouse, where it keeps the data on disk and pushes the compacted files to S3. I don't think serving directly off S3 is going to be much use to anyone. The latency will be awful, and they issue a backoff warning fairly quickly. In my mind it only makes sense to treat S3 as the authority and cache SStables to local disk. I think the Astra folks may have done something along these lines, I'd be willing to bet there's a ton of knowledge there as they operationalized it successfully.
Anyways - the reason I asked my question, was to frame it in the context of a longer term project strategy. Asking more directly - If we're going to be pushing and pulling from S3 during compaction, and it's a major feature, would that go in Sidecar? It certainly could, but I'm concerned about having a critical component of C* living it's own independent lifecycle. Example of why I get concerned: https://the-asf.slack.com/archives/C07V0FK0M2P/p1788964591823929 Imo, having 2 projects that are tightly coupled, but developed independently, is going to be fairly high risk. I really don't like having responsibility spread this way, and I probably wouldn't want to run it in prod. Jon On Wed, Oct 7, 2026 at 7:15 AM Štefan Miklošovič <[email protected]> wrote: > I have a draft of CEP-36 here (1). S3 deps + proxy impl is meant to be > implemented in a different project (2) which once built is added to > Cassandra via custom ClassLoader so it is all encapsulated and we can > not have any dependency clashes. > > There is the impl of s3 proxy with e2e tests. It shows how Cassandra > starts to serve reads from s3. When it comes to writes, these SSTables > will be created locally as they are not and then "offloaded" to S3 for > reads. Once SSTable is created it is immutable so we can upload it and > then switch SSTableReader to read via S3 proxy. > > At least that is the idea. Heavy demo. Dragons everywhere. AI-driven a > lot. It more _does not work_ than does :D But it genuinely starts to > read from S3 via proxy which is quite ... promising, but I think that > if we want to seriously consider CEP-36 and implement it fully that is > a serious piece of work for two quarters :D > > Give me some time here please, like a week or two, I would like to > explore this in more detail. Maybe it will be better to modify > SnapshotManager to work with ChannelProxy instead of File and when > taking snapshots we would use S3 ChannelProxy plus we would tweak the > hardlinking bit. > > You can of course do this all in Sidecar, I just want to see where > this gets us. You would have two ways to choose from (and CEP-36 > building blocks would be in place to extend on further). > > (1) https://github.com/apache/cassandra/pull/5238/changes > (2) https://github.com/smiklosovic/cassandra-s3-channel-proxy > > On Wed, Oct 7, 2026 at 3:30 PM Isaac Reath <[email protected]> wrote: > > > > In fairness, CEP-36 does explicitly call out that anything that uses a > hard link will no longer work once data is written in an alternative > location: > > > > > Once triggered any data written to alternative locations will not be > included in the snapshotting, incremental backups or other processes that > use hard file links. > > > > In a world where we have CEP-36 and an S3-backed implementation of > ChannelProxy, I would imagine that a call to the > ColumnFamilyStore.takeSnapshot function would either no-op (sticking to > what is defined in the CEP), wrap a call to take an on-demand backup via > AWS Backup, or do something similar to running aws s3 sync. In this world, > if you wanted to take a backup across a whole cluster, you could still use > Sidecar to manage the coordination. Or you could just use continuous > backups on your S3 buckets and call it a day. Depends on the semantics and > use-cases of what we're looking for out of a backup solution. > > > > CEP-36 offers details around how to get the ChannelProxy interface in > place, but leaves the details of how to implement an object store-backed > version of the storage engine as an exercise for the reader. I think if we > were to get CEP-36 voted on and implemented, we'd need a follow-up CEP that > dives into the S3 implementation, or otherwise we'd need to expand the > current scope of CEP-36 to cover it. > > > > On Wed, Oct 7, 2026 at 6:31 AM Štefan Miklošovič <[email protected]> > wrote: > >> > >> BTW I am still not in favor of putting this to Cassandra, just have > >> snapshots in Sidecar with its own endpoints, it is way easier that > >> way. I am pretty tired at this point to go through these discussions > >> all over again yielding no actual resolution of what we are going to > >> do. "cloud-aware Cassandra" was so long on a roadmap I am losing hope > >> we will ever see that. The amount of work needed is enormous, we would > >> need to go case by case and decide what is still to be written on a > >> local disk and what goes to the cloud. Some functionality is not even > >> there: > >> > >> 1) How do you atomically rename a file? S3 does not have the concept > >> of rename. There is some "S3 Express One Zone" which seems to support > >> RenameObject but that seems quite niche > >> 2) Hard links ... none, we would need to copy > >> 3) mmap, how is this done in S3? You can not memory map an object > >> store, no? We would need to do disk_access_mode: standard > >> 4) Latency ... > >> > >> Commit logs, hints, caches ... Accord stuff. I don't know what else, > >> all the local disk interaction which is illogical to move to cloud > >> would need to stay local. (anti)-compaction, repair ... all of that > >> stuff would be cloud-native too? The performance of that would be ... > >> tragic? > >> > >> I know it is something to aspire for and I do not want to be too > >> negative about this but I think it makes sense to stop for a while and > >> actually see what this would mean and how much work it is. > >> > >> For that reason I would be doing backup / restores in Sidecar and be > >> done with that. It is a very well understood domain, s3 deps already > >> there, endpoints endpointing, a snapshot is a JMX call ... why to > >> complicate this more really. > >> > >> On Tue, Oct 6, 2026 at 9:11 PM Štefan Miklošovič < > [email protected]> wrote: > >> > > >> > Well ... that's quite a topic. > >> > > >> > The OP was asking specifically about backup / restore so I was > >> > replying in that spirit. > >> > > >> > One note for backup / restores, we talk about "snapshots", right? So > >> > if you back them up, then one would expect that Sidecar would also > >> > offer a way to list them, then to clear them ... It would not be "just > >> > about backup and restore". There would be a family of endpoints around > >> > snapshot management, very likely. That means we kind of duplicate what > >> > is in Cassandra already, if we do not put this into Cassandra > >> > directly, then Cassandra would not "see" the snapshots, except those > >> > taken locally. > >> > > >> > If we want to entertain CEP-36 specifically, that would change the > >> > picture quite significantly. In either case, snapshots would need to > >> > be copied over (SSTables, that is), because we can not do a hard-link > >> > from local storage to a S3 bucket. So we would need to copy that, > >> > right? If that is the case then we would need to probably throttle how > >> > fast it is uploaded to not interfere with other operations and > >> > similar. > >> > > >> > I was completely over-hauling the snapshot management in > >> > CASSANDRA-18111, together with Paulo Motta who did a great job > >> > reviewing it, everything is centralized in SnapshotManager(MBean). The > >> > only tasks touching the disk are TakeSnapshotTask, ClearSnapshotTask > >> > and SnapshotLoader which loads them. Their representation / metadata > >> > are in memory. So when you list it is not going to S3. That isolates > >> > what would need to be accommodated to talk to a cloud. So if you > >> > wanted to start to use S3 as a storage, taking CEP-36 into account, we > >> > would need to: > >> > > >> > 1) create "cloude-aware" implementation of > >> > org.apache.cassandra.io.util.File / java.nio.file.Path > >> > 2) create "S3 TakeSnapshotTaks" which would need to start to copy > >> > instead of hardlinking etc. > >> > 3) create another version of SnapshotLoader or modify it heavily as > >> > currently it is pretty-much wired to talk to local file system > >> > 4) Find some way how to "switch" between local / cloud via > >> > configuration in a complex manner > >> > > >> > I still think it holds that we would need to introduce some > >> > "class-path modularisation" like you would say, via configuration, > >> > what cloud storage provider you want to use and the related libraries > >> > would be auto-magically put into the class path. If we had this in > >> > place then CEP-36 might be potentially closer to the implementation as > >> > these building blocks would be there already. > >> > > >> > On Tue, Oct 6, 2026 at 6:32 PM Jon Haddad <[email protected]> > wrote: > >> > > > >> > > For the folks who want to put this in sidecar, what are your > thoughts on how that would work with a potential future improvement using > s3 as part of its normal operations? I’m thinking about CEP-36 > specifically. > >> > > > >> > > Thanks! > >> > > Jon > >> > > > >> > > > >> > > On Tue, Oct 6, 2026 at 7:37 AM Isaac Reath <[email protected]> > wrote: > >> > >> > >> > >> > If I am not mistaken, Sidecar already exposes a set of > >> > >> > endpoints related to the restoration of data from S3 when > running some > >> > >> > Spark jobs, right? > >> > >> > >> > >> You are correct Stefan. > >> > >> > >> > >> I'll echo Stefan's concerns around the weight of adding a cloud > provider dependency into Cassandra. Especially now, with the increasing > volume of CVE discovery across all sorts of open-source projects, keeping > the dependency scope of the core Cassandra process small is a distinct > advantage. As an operator, it's much easier for me to push out patches > across thousands of Sidecars than it is to push them across thousands of > Cassandra nodes. > >> > >> > >> > >> I also think that from a resource isolation perspective, running > these types of maintenance operations out-of-process can create much more > predictable performance, especially when paired with OS-level resource > governance tools like cgroups. What a dream it might be if we had > out-of-process compaction or out-of-process Merkle tree calculation for > repair. > >> > >> > >> > >> I understand your point that adding another Sidecar just for > backups can increase operational complexity, but I'd argue that > cassandra-sidecar is shaping up to offer far more than just backups. Today > we have Sidecar acting as the entrypoint for bulk analytics, CEP-44 > bringing CDC publishing to Kafka, CEP-40 adding zero-copy online data > migration between nodes and with CEP-53 and CEP-62 we are getting rolling > restarts, configuration management and a framework for executing other > distributed jobs across a Cassandra cluster. Framed this way it's becoming > a one-stop-shop for these types of operations. I also think that building > on top of the operational job framework being added with CEP-53 could go a > long way towards making the implementation easier. > >> > >> > >> > >> > >> > >> On Tue, Oct 6, 2026 at 8:56 AM Victor Araujo < > [email protected]> wrote: > >> > >>> > >> > >>> Hi Bernardo and Stefan, > >> > >>> > >> > >>> Thanks for the feedback. I agree that there are several benefits > to having backup and restore in Sidecar, especially regarding isolation, > dependencies, and being able to evolve the feature independently from > Cassandra itself. I would be happy to explore and contribute to that > direction, potentially in my free time, and I may also be able to bring > this discussion to my company later. > >> > >>> > >> > >>> My main concern is more about the entry point for users. In my > current environment, we are actually trying to reduce our use of Sidecars > and other additional processes to reduce the overall footprint. I also > think there may be users who would prefer having a basic built-in solution > in Cassandra rather than having to deploy another component just to get > backup and restore. > >> > >>> > >> > >>> Maybe there is a middle ground here. We could have an opt-in, > generic backup/restore capability in Cassandra core, similar in spirit to > the unified repair solution, without bringing any third-party cloud > dependencies into Cassandra. For example, the core could provide the > backup/restore abstractions and lifecycle, with a pluggable storage > implementation provided by the user or another component. This could serve > as a simple entry-level solution, while a more complete implementation with > cloud-specific integrations could live in Sidecar. > >> > >>> > >> > >>> I don't know yet whether this split makes sense from an > architectural perspective, but I think it could be worth exploring. I would > be interested in hearing what others think about this approach. > >> > >>> > >> > >>> Thanks, > >> > >>> Victor > >> > >>> > >> > >>> > >> > >>> Em ter., 6 de out. de 2026 às 07:49, Štefan Miklošovič < > [email protected]> escreveu: > >> > >>>> > >> > >>>> I also think that we should investigate how to integrate backup / > >> > >>>> restores in Sidecar rather than doing it in Cassandra. > >> > >>>> Dependency-wise, I think Sidecar already depends on s3 so that > is for > >> > >>>> free already. If I am not mistaken, Sidecar already exposes a > set of > >> > >>>> endpoints related to the restoration of data from S3 when > running some > >> > >>>> Spark jobs, right? So this would be just a natural extension of > that - > >> > >>>> a set of endpoints backuping to and restoring from an S3 bucket. > >> > >>>> > >> > >>>> However, I think that we should make it generic enough in such a > way > >> > >>>> that if somebody wants to do this against Azure or GCP they > could. We > >> > >>>> should not "wire" S3 only. I am not sure how to do this though. > These > >> > >>>> deps are quite "heavy", depending on their own dependency chain > ... I > >> > >>>> do not have good experience with mixing various cloud libraries. > >> > >>>> > >> > >>>> What we do in our backup / restore solution is that we > modularize it > >> > >>>> in such a way that a user can choose what cloud they go to deal > with > >> > >>>> and they have only that cloud-specific libraries on the class > path. > >> > >>>> > >> > >>>> On Tue, Oct 6, 2026 at 12:14 PM Bernardo Botella > >> > >>>> <[email protected]> wrote: > >> > >>>> > > >> > >>>> > Hi Victor! > >> > >>>> > > >> > >>>> > Thanks for bringing up this discussion 😊 > >> > >>>> > > >> > >>>> > I completely agree that a first citizen backup and restore > mechanism in Cassandra would be great. > >> > >>>> > > >> > >>>> > I’d like to explore a little bit more the solution involving > Sidecar. While it is true that it would “force” you to have yet another > component running, it definitely comes with a lot of benefits besides the > current other features it provides (control plane, analytics library, CDC > over Kafka, to name some). > >> > >>>> > > >> > >>>> > It allows for a more modular approach to a Cassandra > deployment. In my humble opinion, having smaller, more specialized > processes is the direction we should be following, as opposed to keep > making the core of the project bigger and more complex. > >> > >>>> > It allows for a faster iteration in terms of software. When it > comes to version upgrades, it is definitely way easier to update just the > sidecar than the entire Cassandra core. It is also less risky. > >> > >>>> > Blast radius is also smaller. It is better to have the Sidecar > affected if a backup process stalls/breaks/misbehaves than the whole > Cassandra core. > >> > >>>> > > >> > >>>> > > >> > >>>> > That’s my two cents! > >> > >>>> > Bernardo > >> > >>>> > > >> > >>>> > From: Victor Araujo <[email protected]> > >> > >>>> > Date: Monday, 5 October 2026 at 13:34 > >> > >>>> > To: [email protected] <[email protected]> > >> > >>>> > Subject: [DISCUSS] Backup and restore in Cassandra > >> > >>>> > > >> > >>>> > Hi everyone, > >> > >>>> > > >> > >>>> > I would like to discuss the possibility of having a built-in > backup and restore mechanism in Cassandra, including incremental backups > and support for external object storage such as S3. The main goal would be > to provide a supported solution while keeping the implementation > provider-agnostic and avoiding unnecessary operational overhead. > >> > >>>> > > >> > >>>> > I initially discussed this on Slack and was pointed to CEP-36 > and the previous discussions around external storage and snapshots. We also > discussed whether S3 would be too provider-specific, and whether Cassandra > Sidecar could be part of the solution. My concern with Sidecar is that > introducing another component would increase the operational footprint, > which goes against one of the goals of this work. I also have a small PoC > on top of trunk exploring some of these ideas. > >> > >>>> > > >> > >>>> > I'd be interested in hearing other opinions and any previous > work I should look at. If there is interest in pursuing this, would a draft > CEP be a useful next step so we can start working on the design in parallel? > >> > >>>> > > >> > >>>> > Link of the first slack discussion about this topic: > https://the-asf.slack.com/archives/CK23JSY2K/p1789021792614639 > >> > >>>> > > >> > >>>> > Thanks, > >> > >>>> > Victor Cavichioli >
