Very interesting approach of PiBakery for rapid prototype a Serverless API Infrastructure with OpenFaaS on Raspberry Pi
https://medium.com/@JockDaRock/serverless-at-the-edge-up-and-running-w-openfaas-docker-on-a-raspberry-pi-multi-node-cluster-e0957f4d8a49
After the first setup, your API Gateway could be design like this with an additional Object Storage Server Minio
https://medium.com/statuscode/integrating-openfaas-and-graphql-experimental-1870bd22f2a
https://blog.alexellis.io/openfaas-storage-for-your-functions/
https://www.thepolyglotdeveloper.com/2017/02/using-raspberry-pi-distributed-object-storage-minio/
https://blog.alexellis.io/hardened-raspberry-pi-nas/