It is possible to use the library resteasy-cache-core to support the cache feature (ETag, etc) ; This library use the jboss tree cache internally to support the caching of response.

When a REST application is deployed in the Google apps engine, it is better to use the Memcached features of Google Apps Engine to do so.

As I see the implementation in resteasy-cache-core is already separated between a specification and a concrete implementation (for jboss tree cache), it should be interesting to have a second implementation (using memcached) and be able to choose which implementation to be used (parameter in web.xml for instance)