well to level3 I can tell you, I just used grep embedded in a nodejs server application :D

Everything boiled down to this: "cd " + data_path + "; grep -rno " + key + " | cut -d: -f1,2 | sort | uniq"

I read the result from stdout and transformed the result into json that was enough to make it. I did not want at that time to spend more time on that level ;)