I\'m deploying my Meteor app on Modulus.io and for my database using compose.io with oplog, I\'m setting up environment url\'s I wondering if I need both MONGO_OPLOG_URL and
MONGO_URL
is the URL your app uses to communicate with your mongo instance. It is required for all production applications. Oplog transactions are not communicated through this URL.
MONGO_OPLOG_URL
is the URL your app uses to listen for changes to the binary oplog for your database. It is not required, but highly recommended.
Summary: add both.
For more information see this article.