Description:

There are new server options to allow clients to override the maxConns setting and reserve threads for admin/internal connections.

Engineering Ticket Description:

MongoDB Cloud Services always need to be able to connect to database processes in order to perform their core admin operations–gather statistics, perform backups, change configuration, perform various maintenance operations, etc. 

In order to ensure that they can always connect and are not locked out by user sessions – using up all available slots under maxConns – we need a way to specify a CIDR block that is exempt from the maxConns limit. This can then be used with all mongod/mongos processes managed by MongoDB Cloud Services, ensuring that the Cloud Services can always perform their critical tasks.