We have never found a solution to the above question and is now in a predicament. Some error that we cannot locate is taking down the main site but not the backoffice tools. We are looking for a solution where we can have an isolated development environment from which we can upgrade/update a production environment. 2 databases. 2 host applications. How might we set that up? And how to promote changes and updates to the production environment? Both data and files. Files could also be updated.

Are you on Azure. If so you can implement deployment slots . https://learn.microsoft.com/en-us/azure/app-service/deploy-staging-slots?tabs=portal

No. We are selfhosting our sites.

+1 for Azure web apps and deployment slots

We self host on an Windows Server running IIS. We run 2 websites in IIS for every website we build...the first website is a subdomain qa.domain.com and the 2nd site is www.domain.com We use qa as our staging site. Get everything hunky dory there, then we copy from qa to www. So every site we have ends up having 3 versions, one local, one qa, and one www. @Holm76 feel free to DM me if you have any specific questions about how we're setup. uSync is a sweet package that's great for keeping all 3 versions in sync. That said, I typically manage synchronization by hand.

If you set up two different sites "as usual" it is impossible for the development site to interfere with the production site. The only tricky thing when hosting on the same server is that * Umbraco load balancing will not work out of the box (i.e., the the production site has a separate backoffice and frontend). * Make sure the ASPNETCORE_ENVIRONMENT is set correctly per-site. There is >1 way of doing this in IIS I think. The Umbraco log at startup will tell you the runtime environment I think.