This is interesting! Simply stick a physical hardware knob directly on the display of your iPad!
I saw these ages ago when the Kickstarted launched. Looks like they actually made it to production (TUNA KNOBS - Pepperdecks | Mix your Music, Anywhere, Anytime). I found a review here (Tuna Knobs Touchscreen Accessory Review - Digital DJ Tips). I don’t see Algoriddim djay listed as a compatible app on their website though so I wonder if these would work?
I can’t imagine that this requires any special compatibility on the part of the software. Rather, it seems to be a kind of stylus that is merely in the form of a knob. In this respect, it should work with any app as long as the element to be controlled is a knob.
In general, I find the idea quite funny, but at least for me and my way of working, this would not be practical. Of course, it would be nice to be able to control the elements on the tablet with a real knob as well. But that only works if the position of the elements doesn’t change, so the view always stays the same. Exactly that is not the case with me, because when I work with DJay, the display changes regularly (waveform, effects, library…). I can imagine this with a tablet, which is used as an additional device, such as a virtual 303 or similar. But all in all it seems to be more a funny gadget than a really useful tool.