Straight to commercial, domestic, and API that were shared can only recommend to pay attention to the registrars from Devlin (Line). They have on the website is the API specification, the beauty of their solution is that the registrars inherent functionality of the program (server part) like PC ie that maybe the PC + Line can and their registrars. Google see the description. The model number of the regulars they are not great, but universal (there are IP and analogue) I personally use XVR8 on small objects, though the API is not necessary to use.
In any case, you need to see large software systems as well as those who issue the registrars usually hones them to fit your camera and invites us to use the utility for administration and monitoring. For example, I have not met a single Chinese reg with open APIs, they can certainly give the info on request, but the communication with the Chinese one more quest :))