A simple extension service does not need a REST API. You can define a service-specific REST API entrypoint and one or more schema definition files.

An extension service that does not require request or response bodies other than those that the vCloud API defines, Task, for example, can simply define the URL patterns that constitute its API filters and the service links that implement its operations.

A service that defines its own request or response bodies must also specify a URL to which clients can direct requests. The service must specify locations of the files, such as XML schema definition (XSD) files, to which its clients require access.