IBackendClient
Connecting your own backend to xpfw is done by implementing the IBackendClient interface.
The implementation of the NeDB and feathers client offer a great example on how to do on and offline backends!
For an offline plugin you only need to implement
getcreateremovefindpatch
For an online plugin you also need to implement
connectTodisconnectloginregisterlogout
Since xpfw is strongly typed your IDE should tell you what parameters to expect. In case you are not using typescript see an abstract from the API docs below.