Imagine you are integrating one app to another app and more app is there. The integration point is hetero gen one to another application. One application provide flat file, while the other use web service, another one provide DB read only (view) for the integration point (we called it end point).
So, to handle this kind of situation two approach can use
1. Develop application and do formatting as per integration point.
2. Use ESB to do handle this.
Mule is one of the open source ESB dan provide community Edition and Enterprise Edition.
Here is the sample that I made in Mule Designer to integrate .Net (REST API) endpoint with Java (cxf API).
Using Community Edition