> Is there a document describing the architecture of a running system?

There is an architecture page: https://flynn.io/docs/architecture

Please let me know if anything specific is missing so that we can add it.

> Can I choose to dedicate a host to run a db, and at the same time run serveral mini-processes without launching every one in a container?

You can dedicate specific hosts to specific apps and process types using node tagging, though it looks like we are missing documentation for that. I'll make sure it gets added soon.

You are free to fork multiple processes inside of containers you deploy, there doesn't need to be a 1:1 mapping of container to process.