What does authSource means in mongo database url?

This is the name of the database that has the collection with the user credentials.

https://docs.mongodb.com/manual/reference/connection-string/#connections-connection-options

This is where the usernames and passwords are set up.

I spent a few hours today trying to solve this problem and unfortunately, this SO post didn't appear in my searches. That was too bad because it's the post with the best answer to the problem.

I hope to help the next person by adding the error message I got.

I'm setting up the connection inside a Docker container that is spun up via a docker-compose yaml file. The user name and password are passed into docker-compose from a .env file.

Connection URI that works

mongodb://aUser:aPassword@mevn-app-mongo:27017/myDb?authSource=admin

Error:

SASL SCRAM-SHA-1 authentication failed for aUser on myDb from client 172.25.0.3:37352 ; UserNotFound: Could not find user aUser@myDb