gortiz opened a new pull request, #17649:
URL: https://github.com/apache/pinot/pull/17649

   In https://github.com/apache/pinot/pull/17576 we detected an issue that 
proves we cannot downgrade from 1.4 or 1.5 to 1.3 without downtime, as MSE 
queries may be broken due to an incompatibility in the stage stats. It is 
important to know the issue is not introduced by 
https://github.com/apache/pinot/pull/17576 but detected by it.
   
   The main issue is that in 1.4 we introduced a cluster listener that detects 
whether the cluster is homogeneous or not, and in case it is not, MSE doesn't 
send stage stats. We did that because in 1.3 we detected a bug and the only way 
to fix it was to change the stats in 1.4 in a way that 1.3 fails when they are 
received. This worked fine when upgrading the system, but the listener is based 
on a version Helix property we started to set in 1.4. When rolling back to 1.3 
that property is not changed (as it is unknown by 1.3), so the other nodes see 
the old value, which means they think the cluster is in fact homogeneous.
   
   Fixing this issue may be complex and given we don't expect people to 
downgrade from 1.5 to 1.3, we think it is just better to remove that check 
here. As always, it is recommended to upgrade one version at the time, so in 
case you need to downgrade from 1.5, you should go to 1.4, which is still 
tested.


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