Revising this question following more attempts:
I'm deploying a Meteor app on Galaxy for the first time and cannot connect to MongoDB Atlas.
I can connect to MongoDb through Mongo Shell using:
mongo "mongodb://host1:27017,host2:27017,host3:27017/<db_name>?replicaSet=<replica_set_name>" --ssl --authenticationDatabase admin --username <username> --password <password>
I tried an equivalent for the Mongo URL in my settings.json:
mongodb://<username>:<password>@host1:27017,host2:27017,host3:27017/<db_name>?authSource=admin&replicaSet=<replace_set_name>
But when I deploy my app, I get:
Error: Could not locate any valid servers in initial seed list.
I've also tried using the connection string provided by Atlas:
mongodb+srv://<username>:<password>@<host>/<db_name>?retryWrites=true&w=majority
But this just returns:
Error: URL must be in the format mongodb://user:pass@host:port/dbname
I've tried every combination I can come up with but no luck.
RESOLVED, WITH HELP FROM MONGODB ATLAS SUPPORT: "MONGO_URL": "mongodb://:@host1:27017,host2:27017,host3:27017/?ssl=true&replicaSet=&authSource=admin&retryWrites=true&w=majority",