nndiary is a back end very similar to
nnml (see Mail Spool). Actually, it could appear as a mix of
nndraft. If you know
nnml, you're already familiar with
the message storing scheme of
nndiary: one file per message, one
directory per group.
Before anything, there is one requirement to be able to run
nndiary properly: you must use the group timestamp feature
of Gnus. This adds a timestamp to each group's parameters. Group Timestamp to see how it's done.