Ask HN: Open source API connection engines?

I was wondering if there are any open source engines that would allow us to build on in order to connect different APIs together. There are a lot of very expensive SaaS platforms out there that do this kind of thing but we would like to use something open source and add multiple sources and targets. We want to be able to connect Google Analytics API calls for example as a source with a middle layer that runs some transformations and then targets a marketing tool. Any suggestions?