andygrove commented on PR #2514:
URL: 
https://github.com/apache/datafusion-ballista/pull/2514#issuecomment-5951038174

   I filed #2525 to design proper support for rolling upgrades.
   
   Thinking about it more, I don't think there's any harm in merging this PR 
now, because it doesn't take away a rolling upgrade that works today.
   
   - Within a major version it changes nothing. A 55.0.0 client and a 55.1.0 
scheduler accept each other, and the only effect is a warning.
   - Across a major version there's no rolling upgrade to break. Schedulers and 
executors already have to move together when `BALLISTA_PROTOCOL_VERSION` 
changes, and replacing the scheduler fails any query in flight because its 
state is only in memory. Nothing guarantees an old client works with a new 
cluster either. Today a mismatched client is let through and can get wrong 
results or fail late (#2367, #2370, #2376). This PR turns that into an upfront 
error.
   
   If the design in #2525 ends up letting a scheduler accept clients from the 
previous major for a while, that would loosen this check, which won't break 
anyone. So I'm moving this back to ready for review.
   


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