Handle external services as external resources of the application.
Examples:
- database
- log services
- ...
This ensure the application is loosely coupled with the services so it can easily switch provider or instance if needed
At this point, the only external service the application is using is MongoDB database. The loosely coupling is already done by the MONGO_URL used to pass the connection string.
If something wrong happens with our instance of MongoDB (assuming a single instance is used, which is generally a bad idea...), we can easily switch to a new instance, providing a new MONGO_URL environment variable and restart the application.