Hi Driss,
One of the common ways of integration between SAP ERP and Ariba is using web services (iDOC ->SOAP->WSDL). Is there a way you have already figured out to distinguish direct material in SAP technically? if not you will have to buid that identification first. Also, what is the intention to replicate material master in Ariba Sourcing? Is your sourcing instance new? with exiting master data or no master data exists? Generally Ariba Sourcing comes with pre-loaded UNSPSC commodity code you might have to create custom commodity code domain and provide mappings to UNSPSC.
Regards
Vilas