Re: Downgradability

Mon, 20 Feb 2023 12:31:21 -0800

FWIW I think 8110 is the right approach, even if it isn’t a panacea. We will have to eventually also tackle system schema changes (probably not hard), and may have to think a little carefully about other things, eg with TTLs the format change is only the contract about what values can be present, so we have to make sure the data validity checks are consistent with the format we write. It isn’t as simple as writing an earlier version in this case (unless we permit truncating the TTL, perhaps) 

On 20 Feb 2023, at 20:24, Benedict <bened...@apache.org> wrote:


In a self-organising community, things that aren’t self-policed naturally end up policed in an adhoc manner, and with difficulty. I’m not sure that’s the same as arbitrary enforcement. It seems to me the real issue is nobody noticed this was agreed and/or forgot and didn’t think about it much. 

But, even without any prior agreement, it’s perfectly reasonable to request that things do not break compatibility if they do not need to, as part of the normal patch integration process.

Issues with 3.1->4.0 aren’t particularly relevant as they predate any agreement to do this. But we can and should address the problem of new columns in schema tables, as this happens often between versions. I’m not sure it has in 4.1 though?

Regarding downgrade versions, surely this should simply be the same as upgrade versions we support?


On 20 Feb 2023, at 20:02, Jeff Jirsa <jji...@gmail.com> wrote:


I'm not even convinced even 8110 addresses this - just writing sstables in old versions won't help if we ever add things like new types or new types of collections without other control abilities. Claude's other email in another thread a few hours ago talks about some of these surprises - "Specifically during the 3.1 -> 4.0 changes a column broadcast_port was added to system/local.  This means that 3.1 system can not read the table as it has no definition for it.  I tried marking the column for deletion in the metadata and in the serialization header.  The later got past the column not found problem, but I suspect that it just means that data columns after broadcast_port shifted and so incorrectly read." - this is a harder problem to solve than just versioning sstables and network protocols. 

Stepping back a bit, we have downgrade ability listed as a goal, but it's not (as far as I can tell) universally enforced, nor is it clear at which point we will be able to concretely say "this release can be downgraded to X".   Until we actually define and agree that this is a real goal with a concrete version where downgrade-ability becomes real, it feels like things are somewhat arbitrarily enforced, which is probably very frustrating for people trying to commit work/tickets.

- Jeff



On Mon, Feb 20, 2023 at 11:48 AM Dinesh Joshi <djo...@apache.org> wrote:
I’m a big fan of maintaining backward compatibility. Downgradability implies that we could potentially roll back an upgrade at any time. While I don’t think we need to retain the ability to downgrade in perpetuity it would be a good objective to maintain strict backward compatibility and therefore downgradability until a certain point. This would imply versioning metadata and extending it in such a way that prior version(s) could continue functioning. This can certainly be expensive to implement and might bloat on-disk storage. However, we could always offer an option for the operator to optimize the on-disk structures for the current version then we can rewrite them in the latest version. This optimizes the storage and opens up new functionality. This means new features that can work with old on-disk structures will be available while others that strictly require new versions of the data structures will be unavailable until the operator migrates to the new version. This migration IMO should be irreversible. Beyond this point the operator will lose the ability to downgrade which is ok.

Dinesh

On Feb 20, 2023, at 10:40 AM, Jake Luciani <jak...@gmail.com> wrote:


There has been progress on 

Which is similar to what datastax does for DSE. Would this be an acceptable solution?

Jake 

On Mon, Feb 20, 2023 at 11:17 AM guo Maxwell <cclive1...@gmail.com> wrote:
It seems “An alternative solution is to implement/complete CASSANDRA-8110” can give us more options if it is finished😉

Branimir Lambov <blam...@apache.org>于2023年2月20日 周一下午11:03写道：
Hi everyone,

There has been a discussion lately about changes to the sstable format in the context of being able to abort a cluster upgrade, and the fact that changes to sstables can prevent downgraded nodes from reading any data written during their temporary operation with the new version.

Most of the discussion is in CASSANDRA-18134, and is spreading into CASSANDRA-14277 and CASSANDRA-17698, none of which is a good place to discuss the topic seriously.

Downgradability is a worthy goal and is listed in the current roadmap. I would like to open a discussion here on how it would be achieved.

My understanding of what has been suggested so far translates to:
- avoid changes to sstable formats;
- if there are changes, implement them in a way that is backwards-compatible, e.g. by duplicating data, so that a new version is presented in a component or portion of a component that legacy nodes will not try to read;
- if the latter is not feasible, make sure the changes are only applied if a feature flag has been enabled.

To me this approach introduces several risks:
- it bloats file and parsing complexity;
- it discourages improvement (e.g. CASSANDRA-17698 is no longer a LHF ticket once this requirement is in place);
- it needs care to avoid risky solutions to address technical issues with the format versioning (e.g. staying on n-versions for 5.0 and needing a bump for a 4.1 bugfix might require porting over support for new features);
- it requires separate and uncoordinated solutions to the problem and switching mechanisms for each individual change.

An alternative solution is to implement/complete CASSANDRA-8110, which provides a method of writing sstables for a target version. During upgrades, a node could be set to produce sstables corresponding to the older version, and there is a very straightforward way to implement modifications to formats like the tickets above to conform to its requirements. 

What do people think should be the way forward?

Regards,
Branimir


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