> db.getSiblingDB('mydb').createCollection('foo')
> db.getSiblingDB('mydb').foo.drop()
> db.getSiblingDB('Mydb').createCollection('foo')
MongoServerError: db already exists with different case already have: [mydb] trying to create [Mydb]

^^ The above happens even in replica sets (i.e., unsharded clusters). Happens in the last 4.2 version as well as 7.0.5.

Per matt.kneiser@mongodb.com, it looks like the reason is:

It looks like the DatabaseHolderImpl/DBsIndex, requires an explicit dropDb() call to unregister the namespace. Otherwise, we keep state around in memory that a database with "mydb" exists.

(Slack thread)

Ideas:

• The last collection drop should probably clear out the DB state.
• Should that mean we get an oplog dropDatabase entry (and change event), too?

Additional context: Mongosync can’t do `dropDatabase` commands—even in response to events of that name—because there’s no UUID.