First off, let me apologize for my initial reply, it came off harsher than I had intended.
I know I didn't say it initially, but I like the idea of making it easier to replace a node. I think it's probably not obvious to folks that you can use rsync (with stunnel, or alternatively rclone), and for a lot of teams it's intimidating to do so. Whether it actually is easy or not to do with rsync is irrelevant. Having tooling that does it right is better than duct taping things together. So with that said, if you're looking to get feedback on how to make the CEP more generally useful, I have a couple thoughts. > Managing the Cassandra processes like bringing them up or down while migrating the instances. Maybe I missed this, but I thought we already had support for managing the C* lifecycle with the sidecar? Maybe I'm misremembering. It seems to me that adding the ability to make this entire workflow self managed would be the biggest win, because having a live migrate *feature* instead of what's essentially a runbook would be far more useful. > To verify whether the desired file set matches with source, only file path and size is considered at the moment. Strict binary level verification is deferred for later. Scott already mentioned this is a problem and I agree, we cannot simply rely on file path and size. TL;DR: I like the intention of the CEP. I think it would be better if it managed the entire lifecycle of the migration, but you might not have an appetite to implement all that. Jon On Thu, Apr 11, 2024 at 10:01 AM Venkata Hari Krishna Nukala < n.v.harikrishna.apa...@gmail.com> wrote: > Thanks Jon & Scott for taking time to go through this CEP and providing > inputs. > > I am completely with what Scott had mentioned earlier (I would have added > more details into the CEP). Adding a few more points to the same. > > Having a solution with Sidecar can make the migration easy without > depending on rsync. At least in the cases I have seen, rsync is not enabled > by default and most of them want to run OS/images with as minimal > requirements as possible. Installing rsync requires admin privileges and > syncing data is a manual operation. If an API is provided with Sidecar, > then tooling can be built around it reducing the scope for manual errors. > > From performance wise, at least in the cases I had seen, the File > Streaming API in Sidecar performs a lot better. To give an idea on the > performance, I would like to quote "up to 7 Gbps/instance writes (depending > on hardware)" from CEP-28 as this CEP proposes to leverage the same. > > For: > > >When enabled for LCS, single sstable uplevel will mutate only the level > of an SSTable in its stats metadata component, which wouldn't alter the > filename and may not alter the length of the stats metadata component. A > change to the level of an SSTable on the source via single sstable uplevel > may not be caught by a digest based only on filename and length. > > In this case file size may not change, but the timestamp of last modified > time would change, right? It is addressed in section MIGRATING ONE > INSTANCE, point 2.b.ii which says "If a file is present at the destination > but did not match (by size or timestamp) with the source file, then local > file is deleted and added to list of files to download.". And after > download by final data copy task, file should match with source. > > On Thu, Apr 11, 2024 at 7:30 AM C. Scott Andreas <sc...@paradoxica.net> > wrote: > >> Oh, one note on this item: >> >> > The operator can ensure that files in the destination matches with >> the source. In the first iteration of this feature, an API is introduced to >> calculate digest for the list of file names and their lengths to identify >> any mismatches. It does not validate the file contents at the binary level, >> but, such feature can be added at a later point of time. >> >> When enabled for LCS, single sstable uplevel will mutate only the level >> of an SSTable in its stats metadata component, which wouldn't alter the >> filename and may not alter the length of the stats metadata component. A >> change to the level of an SSTable on the source via single sstable uplevel >> may not be caught by a digest based only on filename and length. >> >> Including the file’s modification timestamp would address this without >> requiring a deep hash of the data. This would be good to include to ensure >> SSTables aren’t downleveled unexpectedly during migration. >> >> - Scott >> >> On Apr 8, 2024, at 2:15 PM, C. Scott Andreas <sc...@paradoxica.net> >> wrote: >> >> >> Hi Jon, >> >> Thanks for taking the time to read and reply to this proposal. Would >> encourage you to approach it from an attitude of seeking understanding on >> the part of the first-time CEP author, as this reply casts it off pretty >> quickly as NIH. >> >> The proposal isn't mine, but I'll offer a few notes on where I see this >> as valuable: >> >> – It's valuable for Cassandra to have an ecosystem-native mechanism of >> migrating data between physical/virtual instances outside the standard >> streaming path. As Hari mentions, the current ecosystem-native approach of >> executing repairs, decommissions, and bootstraps is time-consuming and >> cumbersome. >> >> – An ecosystem-native solution is safer than a bunch of bash and rsync. >> Defining a safe protocol to migrate data between instances via rsync >> without downtime is surprisingly difficult - and even moreso to do safely >> and repeatedly at scale. Enabling this process to be orchestrated by a >> control plane mechanizing offical endpoints of the database and sidecar – >> rather than trying to move data around behind its back – is much safer than >> hoping one's cobbled together the right set of scripts to move data in a >> way that won't violate strong / transactional consistency guarantees. This >> complexity is kind of exemplified by the "Migrating One Instance" section >> of the doc and state machine diagram, which illustrates an approach to >> solving that problem. >> >> – An ecosystem-native approach poses fewer security concerns than rsync. >> mTLS-authenticated endpoints in the sidecar for data movement eliminate the >> requirement for orchestration to occur via (typically) high-privilege SSH, >> which often allows for code execution of some form or complex efforts to >> scope SSH privileges of particular users; and eliminates the need to manage >> and secure rsyncd processes on each instance if not via SSH. >> >> – An ecosystem-native approach is more instrumentable and measurable than >> rsync. Support for data migration endpoints in the sidecar would allow for >> metrics reporting, stats collection, and alerting via mature and modern >> mechanisms rather than monitoring the output of a shell script. >> >> I'll yield to Hari to share more, though today is a public holiday in >> India. >> >> I do see this CEP as solving an important problem. >> >> Thanks, >> >> – Scott >> >> On Apr 8, 2024, at 10:23 AM, Jon Haddad <j...@jonhaddad.com> wrote: >> >> >> This seems like a lot of work to create an rsync alternative. I can't >> really say I see the point. I noticed your "rejected alternatives" >> mentions it with this note: >> >> >> - However, it might not be permitted by the administrator or >> available in various environments such as Kubernetes or virtual instances >> like EC2. Enabling data transfer through a sidecar facilitates smooth >> instance migration. >> >> This feels more like NIH than solving a real problem, as what you've >> listed is a hypothetical, and one that's easily addressed. >> >> Jon >> >> >> >> On Fri, Apr 5, 2024 at 3:47 AM Venkata Hari Krishna Nukala < >> n.v.harikrishna.apa...@gmail.com> wrote: >> >>> Hi all, >>> >>> I have filed CEP-40 [1] for live migrating Cassandra instances using the >>> Cassandra Sidecar. >>> >>> When someone needs to move all or a portion of the Cassandra nodes >>> belonging to a cluster to different hosts, the traditional approach of >>> Cassandra node replacement can be time-consuming due to repairs and the >>> bootstrapping of new nodes. Depending on the volume of the storage service >>> load, replacements (repair + bootstrap) may take anywhere from a few hours >>> to days. >>> >>> Proposing a Sidecar based solution to address these challenges. This >>> solution proposes transferring data from the old host (source) to the new >>> host (destination) and then bringing up the Cassandra process at the >>> destination, to enable fast instance migration. This approach would help to >>> minimise node downtime, as it is based on a Sidecar solution for data >>> transfer and avoids repairs and bootstrap. >>> >>> Looking forward to the discussions. >>> >>> [1] >>> https://cwiki.apache.org/confluence/display/CASSANDRA/CEP-40%3A+Data+Transfer+Using+Cassandra+Sidecar+for+Live+Migrating+Instances >>> >>> Thanks! >>> Hari >>> >> >>
