You can find sample implementations of the Merchant Backend at this Github repository. Currently there are available the following:
Please refer to the sample code documentation for instructions for running a local development server, and/or deploying to selected cloud services. Do note that if you are running a local server over http, you will need to allow insecure communications in the mobile application; otherwise any payments will immediately fail due to a security error.
The samples provide a fully functioning API for the SDK, but the implementations are, of course, devoid of any business logic. They should, nevertheless, provide a good starting point toward integration with your business systems.