When we created the index, we specified the sort direction for the index, in this case both ascending. Sorting on this indexes would work only is they are both indexed ascending or descending. But if you ix the direction of either fields, mongo will skip indexing all together.
What does the server do in order to find this document? It does an innocuous for-each loop looking for the values, document by document ( assuming the documents were placed contiguously and there was no other logic involved in storing the document) until it finds the value (trying to make a point so I am over simplifying it).
Obviously what I have just stated is a very slow & inefficient way for searching the document. The solution is to create an Index ( as in all other modern databases). Each document has its own location on disk and an index logically holds mapping to those locations from field values.
Typically we have a primary server along with a backup server. If the primary fails and if you are well prepared ( and if everything goes well) we can restore the primary from the backup.
MongoDB uses the concept of replication which is a much better alternative to traditional backup solutions. Mongo uses an arrangement called the Relica Set. The members of the Relica Set are a Primary DB, Secondary DB(s) and an Arbiter DB (potentially). Each of these servers play a specific role in the Replica Set