Bogdan Kanivets

@mitake @ahrtr I've tried implementing reason codes, but it feels cumbersome to have both reasonCode and cancelReason string. Here are some concerns: 1. All other response objects have string error....

@mitake main problem with codes is that they add extra duplication to exposed `rpctypes`. For example there will be new `AuthOldRevision` code and existing ErrAuthOldRevision. Also errors from [mvcc/watcher.go](https://github.com/etcd-io/etcd/blob/main/server/storage/mvcc/watcher.go#L27) will...

Sounds good. I think this requires google doc to summarize everything and allow folks to comment. I'll put it together.

@chaochn47 I've created a proposal. I'd like to polish but didn't have time. https://docs.google.com/document/d/12uoUM6tyn5BX9LvYrewnEuR0U9MpiVUdv3agumrPlj0/edit#heading=h.kuoiyzt3m5ff Feel free to comment what you'd like to see there.

@mitake yes, I'll work on proposal this week or next. Agreed about finishing it before 3.6. Would appreciate your review.

@serathius main change since last review is that I had to add better defaults for fields to make sure they can be read on upgrade.

> Would like to also have a offline downgrade test. Basically start etcd v3.5, do some requests, use etcdutl do downgrade it, do couple of requests, upgrade it using etcdutl,...

related doc [Downgrade Support 3.5 to 3.4 ](https://docs.google.com/document/d/1siSo-tA_xLRo5li5wK8KqWsOiq34PIgtmuSDcyxxwsY/edit?usp=sharing)

Please assign this to me, we already have a minimal internal patch to address this. In current form - it's a 3.4 patch that allows 3.4 to be deployed within...

Quick update - trying to get POC to work. The idea is to run `etcdutl migrate --data-dir data-3.5 --target-version 3.4` and get a data dir that etcd 3.4 can be...