Normally folks think of filesystems as directories and files. But these days, you have to consider the Extended Attribute (EA) interface that filesystems offer as well. This makes a filesystem much closer to a large XML repository than just a collection of files accessible through a tree namespace (the directories). In libferris, each EA key-value pair can also tell the developer/user what schema that value has. This closes the gap between what a filesystem is and what a plasma data engine is just a little bit more. In fact, one might think of a plasma data engine as a virtual filesystem with a touch of extra stuff to allow a plasmoid to poll the data engine easily. This is not to detract from plasma at all, saying its "just a filesystem" means it is like postgresql, xml, or emacs to me ;)
For example, to see some current weather using my ABOMiNation plasma data engine and libferris (dev trunk), I can see the wind and also what type of value that EA or data engine key-value is:
$ fls -l \
nnn1 22 15 26 schema://xsd/attributes/decimal/integer/long/int
One major upshot of looking at plasma in this way is the major upshot of everything being a filesystem. I can "cat" values directly from a data engine and also use fls to inspect data engines and their values from the command line while developing. plasmaengineexplorer is very nice, but its a bit of a pain to use a GUI tool to test out if the data engine is working when you are in a compile, run test cycle. It is also really easy to pluck out data from plasma with libferris, for example, the above fls with a --xml on the command line will do what you imagine. And if you are a nepomuk fan, using fls --rdf will give you an RDF/XML file to enjoy.
I found a few of the data engines would crash if they are started with a QApplication that forces GUI to off. So I black list
s == "tasks" || s == "mouse" || s == "keystate"
in order to mount plasma at the moment. Also, I'm using the signal/slot callbacks to get at the source key-value hash because the immediate mode methods don't seem to want to work for me :/