While investigating HELP-55723, we found that when the client closes the connection abruptly, somehow the mongod fails to drop the virtual collection. And the number of virtual collections was ever increasing for the lifetime of the compute mode mongod.

And there were 5 ~ 7 steady active readers hanging around. Given that the ADF team only sends aggregate requests to the compute mode mongod, these active readers are highly likely to be readers for virtual collections. If virtual collections are not cleaned up, it may mean those active readers are still hanging around.

This is not a confirmed theory, but the proposed counters will be helpful for future similar investigations.