Although there are literally dozens of tools out there that aim to facilitate SaaS, if you look at the definition of SaaS and what makes SaaS, SaaS. Things like “on demand” activation, licensing models, and resource management are the key definition points, not so much the nuts and bolts libraries needed behind the scenes.
I wrote a funcional specification for a SaaS provider platform about a month ago (see https://abstractionlayer.jamesbiv.tech/Abstraction%20Layer%2...) its initial chapters deal with the definition of SaaS.