Single Slave with Multiple Masters. Does this pattern exist in MongoDB? Can we have a single mongoDB remotely (online) which can act as a slave to multiple master mongoDBs (which operate offline)?
For example, I have huge number of transactions which I like to perform on my local MongoDBs parallely on multiple systems (offline).
At the end of the day, I want to push minute part of my offline MongoDB records to my single slave MongoDB which is remote and online.
Does this pattern exist?
I don't think so Single Slave with Multipe Master Possible. As per MongoDB BOL Here Master-slave replication preceded replica sets and made it possible to have a large number of non-master (i.e. slave) nodes, as well as to restrict replicated operations to only a single database; however, master-slave replication provides less redundancy and does not automate failover.


  WARNING
  Deprecated since version 3.6: MongoDB 3.6 deprecates the use of
  master-slave replication.
  
  Master-slave replication has been deprecated for components of sharded
  clusters since MongoDB 3.2.


Replica sets replace master-slave replication for most use cases. If possible, use replica sets rather than master-slave replication for all new production deployments. This documentation remains to support legacy deployments and for archival purposes only.

For further your ref Master Slave Replication