Integartion of SAP and Non-SAP systems

Hello experts,

We have a scenario, where are going to use PI as the middle ware to connect SAP and Non-SAP systems.

Connection between SAP and non-SAP can be done with IDoc to File scenario.

But not sure how we will have an Non-SAP to Non-SAP system intergartion the underlying database is oracle for the non-sap system.

We are thinking of using XML or web services, but not sure how to proceed, any help would be appreciated.

regards,

Abhishek