Ask HN: API as a service

I'm working in a group that has a homegrown ecosystem of in house developed applications and vendor apps. The interconnects between this system have been developed over a decade with different teams, technologies and methodologies (some API, some report passing, some cross application direct DB access).

This obviously has become a tangled web of systems which are so tightly intertwined all of the systems are now an immovable block of mud which can't move. You can't upgrade one system without breaking some backwards compatibility between another.

The homegrown system is about to undergo a large rewrite but, instead of just recreating the same tangled web of interconnect methodologies I'd like to create an API (or ESB) layer abstracting the systems from each other to allow for the ecosystem to evolve in a more sensible fashion.

We could develop this internally but, writing a system just to handle mostly boiler plate: documentation, registration, discovery, monitoring, authentication, rate-limiting, etc seems like a misuse of development resources instead of creating larger business value.

Before starting a formal RFI/P process, does HN have any suggestions of companies which could provide that abstraction layer as a service (hosted internally only, sorry)