leventov commented on issue #7458: New Coordinator segment balancing/loading 
algorithm
URL: 
https://github.com/apache/incubator-druid/issues/7458#issuecomment-482689404
 
 
   > There was discussion previously about what can/should be done in an 
extension vs what must be done in core druid.
   
   Could you please link to that discussion?
   
   > Can you highlight what is needed as far as changes to the core code vs 
what changes can be put in extensions?
   
   The whole algorithm described in this proposal needs to be in core. 
Something that may not need to be core is query metrics-based balancing (as 
opposed to the current cost-based balancing). (Out of scope of this proposal.)
   
   I will add a fallback option to the old behavior, but that's going to be off 
by default and removed timely because I don't see any point in supporting the 
old behavior. The old (current) implementation has many problems (see issues 
referenced in this proposal, but there are many more, at some point into the 
development of the new algorithm I stopped putting them into separate issues) 
and I don't see why would anybody want to use it instead of the new algorithm, 
other than for the sake of stability, when some rough edges will be found in 
the new algorithm. On the other hand, the new algorithm is not that different 
from the current implementation (as, for example, metrics-based vs. cost-based 
balancing, again) and it's expected to work at least as well as the current 
implementation in all cases.

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