I've recently found the HAppS and Happstack projects, and the latter notes that one of its missions is to "leverage use of MACID".
What exactly is MACID ?
Found some info about MACID, which says:
MACID is the Happstack storage mechanism that allows you to use whatever data structure you want to hold your permanent data, without worrying about getting it into and out of tabular form fit for storage in a traditional rdbms.
I also found someone saying that MACID data will be consistent and permanent (following the ACID principle) and so on, so that you could pull the power on your computer, start the system back up, and everything would be left in memory as-is.
Also found a description of the MACID acronym: Monadic ACID transaction service.