Do I need a NVR if I have Survail?
You do not need a NVR if you have survail, as long as you have a PoE switch that is powering the cameras. That being said, there's no downside to having one and there's quite a few upsides.
Survail: works with any camera, NVR, or encoder
Survail works be receiving RTSP feeds, which is a common video output on nearly all security camera manufacturers and brands. This means that you can pull the RTSP feed from a NVR or from an IP Camera, assuming it is powered by a POE injector or PoE switch, or even from a video encoder. If you have an NVR, we can get the RTSP feed from there; if you just have a camera, we can get the RTSP feed from there.
Why you might still want a NVR with Survail
Survail was built with several assumptions. The first is that we wanted to dedicate 100% of all computing resources to filtering your video surveillance down to something manageable rather than wasting time watching video. By removing the need for survail to display anything on the video output, we can squeeze more computer vision out of the application. The second assumption was that we built it for multi-location clients who are often watching the video remotely anyways. The third assumption is that the video footage needed to be stored in a more secure location that where many people place a monitor and NVR. Survail hardware belongs in your surveillance closet.
Because of these design decisions, you may want to still have a NVR on-site in order to have local video output.
You may also want to have a NVR on-site to have yet another layer of storage redundancy. Security is all about having multiple layers of protection; the survail hardware will store a local copy and create a copy in the cloud, but if you put a hard drive in that NVR*, it will also store a copy of the footage that you can use in its user interface.
*If you are pulling RTSP feeds from the NVR, you do not need to have a hard drive in it for survail to work.