Mongo Express authentication failed with docker compose - mongodb

Question

I am trying to run mongodb and mongo express in two containers using docker-compose. It seems like the mongodb container works fine but the mongo express container keeps exiting with code 0.
The error I get is: Could not connect to database using connectionString: mongodb://rootuser:rootpass#mongodb:27017/"
mongo-express | (node:8) UnhandledPromiseRejectionWarning: MongoNetworkError: failed to connect to server [mongodb:27017] on first connect [MongoError: Authentication failed.
My docker-compose.yml looks like this:
version: "3.8"
services:
mongodb:
image: mongo
container_name: mongodb
ports:
- 27017:27017
volumes:
- data:/data
environment:
- MONGO_INITDB_ROOT_USERNAME=rootuser
- MONGO_INITDB_ROOT_PASSWORD=rootpass
mongo-express:
image: mongo-express
container_name: mongo-express
restart: always
ports:
- 8081:8081
environment:
- ME_CONFIG_MONGODB_ADMINUSERNAME=rootuser
- ME_CONFIG_MONGODB_ADMINPASSWORD=rootpass
- ME_CONFIG_MONGODB_SERVER=mongodb
volumes:
data: {}
networks:
default:
name: mongodb_network

Answer 1

In MongoDB you can create user accounts in any database you wish. For example, I can create a user account called 'JohnDoe' in database 'MyDatabase1' and another account called 'JohnDoe' in database 'MyDatabase2'. These are two different accounts. You specify the authorizing database in your database connection string. Because of this feature it can become confusing. As such it has become the standard, and recommended approach to create database users in a single database called 'admin'. By adding users to a single database it is easier to manage.
In this post, you did not identify/specify which database is your authorization database for the user 'rootuser' and the connection info does not seem to contain such a concept. I suspect your user is defined in a database but when you attempt to connect it is looking at the temp database and not finding the user.