It's quite a nice arrangement, and because the JBD is being developed as a separate, generic entity, it could be used to add journaling capabilities to other filesystems in the future.

Data services - also known as Entity services - are reusable because of several characteristics - generic nature, platform interoperability, and support for multiple consumption patterns.

Because it is generic, it should deal with types rather than a concrete implementation of an entity. As a summary, it should adhere to the following interface definition.