Should I Protect My Software Back-end with a Patent or Trade Secret?

This is your judgment call. If someone files a patent on your back-end before you do, you risk expensive legal fees down the road proving that you were the first to use the technology. A valuable patent covers more than an exact rendition of your back-end, it may cover general architecture and inter-component operation and functionality, without covering all the implementation details. Provisionals help build a fence around your technology.