Sharding currentVersion after downgrade from 6.0.3 to 5.0.14

[AmitG (Amit Gupta)]

Background:
Hi, we have upgraded our sharded cluster for years without any problems. Recently we updated from 4.4 to 5.0 and it seemed to work fine (though we only let it burn in for a day or 2). Last night we tried to do the same exact steps to upgrade from 5.0.14 to 6.0.3. It looked like everything was smooth, but we ran into some issues (I’m going to create a separate post about that). We successfully downgraded our db cluster back to 5.0.14. We never set the FCV to 6.0 so we didn’t need to undo that.

Question:
When I run sh.status(), I see the following:

  sharding version: {
  	"_id" : 1,
  	"minCompatibleVersion" : 5,
  	"currentVersion" : 6,
  	"clusterId" : ObjectId("57cd1d5d6303e86ab8f6a764")
  }

Is the currentVersion supposed to be 6? Is there something I need to do to downgrade that to 5? I never increased the FCV to 6.0.

[Aasawari (Aasawari Sahasrabuddhe)]

Hi @AmitG and welcome to the MongoDB community forum!!

The two variables $minCompatibleVersion and $currentVersion are deprecated schema version versions for config server and they are different from Feature Compatibility Version.
With the introduction of of Feature Compatibility Version, these variables are no longer used and are scheduled to be removed at a later date.
The server ticket https://jira.mongodb.org/browse/SERVER-68889 mentions the removal of these variables in the near future.

Let us know if you have any further queries.

Best Regards
Aasawari