Modular like in 'really modular'. That is, OSGi.
skysail is a collection of OSGi bundles, depending on other bundles, and defining an API for domains like authentication, security, persistence and so on. If some API implementation does not fulfill your needs, it is easy to replace it with your own implementation.
DDD is "in". .
skysail lets you concentrate on your business (or domain) logic. Http Server, database, configuration setup and the like are already there. Just use them.
Backend as a Service
skysail concentrates on the backend and tries to stick to RESTful principles.
Right now, skysail is a prototype itself. There are ideas, plans, sketches of documentation and all kinds of well-meant efforts. But it is not yet ready for any end-user to try. If you are a developer and like the ideas meantioned before, check out the sources and give it a spin.