I am referencing to the smart-home-nodejs git repository on Actions On Google. This template provides an end to end example of how the smart-home app can be use. However, instead of using firebase, can I use my own database that host my devices and I would want to just call the Homegraph api to sync. Would this be possible?
Thank you.
Yes, the implementation of your backend can be designed however you want. You also are not restricted to using Cloud Functions or App Engine. While the end-to-end sample provides one example of how to do it, your backend just has to implement a webhook and respond to intents plus have support to do things like report state.
The API reference contains all the information you needed to build a custom implementation.