There are other platforms besides Galaxy. Plus, you can always host it on servers you have direct control over. That’s what I myself am doing.
MiniMongo is just what its names says: A miniature version of MongoDB running on the client. Basically, it runs like this:
You define a Collection on both client and server - this Collection thus now exists both in MiniMongo and MongoDB. You then run a publish() function on the server which determines which documents can get pushed from MongoDB (server) to MiniMongo(client). The client then subscribes to the published data.
When you insert into a Collection on the client, the change gets pushed into MiniMongo first and then pushed down to the server (dependent on whether you were actually allowed to insert something. Same goes for updates and deletions).