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Taking the reference of already implemented email, shipment and shipment payment gateway integration in OFBiz, the database schematic for SMS gateway integration can be designed as follows:
Brief about the data model:
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At service level, we will be writing one common generic service to send SMS as like sendEmail in OFBiz. The user/developer will need to write the custom service, specific to the API provider/vendor to prepare vendor specific parameters or data which will be needed for calling SMS APIs.