Comment by philipov
7 years ago
I think that in user space, it is not clear what is an application and what is a subsystem. Is a database a system or an application? I could issue queries to it directly as part of some ad-hoc research, or I could build a production process using it as a component. In one use-case, it's the application, and in the other, it's just a subsystem. And so I think the distinction between kernel-space and user-space is the only one that makes sense. Once you're in user-space, the distinction between systems and applications can no longer be applied to a tool, but only to particular use-cases for that tool.
a modern RDBMS is largely in the systems layer since it's the glue used by other application.
Having a user interface doesn't really matter IMO, otherwise none of the usermode systems programming applications would be systems programming at all.
Perhaps it would help to add that user software applications generally aren't intended to be used by other applications. Your browser is such an application since it's primary interface is being directly used by the user. The output and state of a browser is not intended to be consumed by other applications running on the system, though it is capable of running code to supplement the output and state of a single website for the purpose of further user interfacing.
An RDBMS doesn't have such a thing, the user interface is largely what you use in the application, it's primary use case is gluing together applications like browsers or websites.