In this scenario the X Engine is installed alongside the Composable Architecture Platform Application Server and they work as an integrated whole, turning the X Engine into a high-performance HTTP proxy with the ability to provide SSL termination and on the fly transformation of requests and responses between the existing applications and the existing APIs.
Use cases for this approach include:
Vendor abstraction, which provides the ability to create generic APIs for things such as text messages, geo-location, two-factor authentication and use those APIs in the existing applications instead of vendor specific APIs
API Sunsetting, where the X Engine is capable of transforming the structure of an API call between different versions, such as to facilitate the removal of older version code from the API server source code
API Accounting, enabling chargeback of API calls that have a monetary cost to the respective users of that API