Problem:
There are events that may cause an administrator to want to restore specific databases in a running mongod process. For example, in a multi-tennat situation where each DB is associated with a tenant. You may want to restore one database from a backup but not take the mongod process for all the other tenants.

You can call closeAllDatabases which will essentially forces the mongod process to reload the data files.

Solution:
a) ensure that this use case is tested and support across all platforms
b) provide a mechanism to "lock" the database during the restore, to prevent a client connect trying to create the database and/or collections, indexes etc. while the restore is in progress.