Although Modulus.io offers MongoDB hosting, Modulus does not grant access to Meteor's oplog tailing feature and does not have any support for multiple replica sets. Both these features are must-haves for production and this is why.
A MongoDB Replica Set is an exact copy of your database. When you first create a database in Compose, you are automatically given a primary Replica Set with a secondary Replica Set to support it. Since failures happen, the secondary Replica Set exists to replace the primary replica set immediately when the primary fails. In the database world, this is known as data redundancy.
This layer of security adds an interesting problem; how do the secondary databases know about the changes that are happening to the primary? The operations log, or oplog for short, is a special collection that Mongo uses to record all the changes that are being made to the database. These changes are read by the Replica Sets to reflect all necessary changes.
Great, now let...