That is an excellent question. It is trivial to back up a live SQL Server instance because it supports shadow-volume snapshots. The same isn't true for MongoDB.

Instead, we had to resort to running our MongoDB database on a 3-node cluster and this is our main strategy for resiliency. Additionally, one of the nodes is set to do a daily full-database dump, which is pretty much guaranteed to be in an inconsistent state but still provides us an extra degree of peace of mind.