[metheus]

Disclosure: I work for MongoDB.

You're in the right ballpark, for sure, but the SSPL addresses the difference explicitly. I'll use your example of Tumblr to clarify.

Tumblr is built on some component technologies, like a database, an app framework, operating systems, backup systems, load balancing, etc. But Tumblr itself is not any of those things. Nor does it make any of its component technologies available to the public as a service. You cannot pay Tumblr to backup your servers, or to rent you VMs running an OS, or to do load balancing for your infrastructure. Even if every single one of those components were licensed under the SSPL, Tumblr would not have to release a single line of their code under the SSPL, because they provide something else -- a publishing platform.

[politician]

Technically, Tumblr provides persistence for ramblings and pictures and makes them available to the public. Tumblr is a brand wrapped around a database.

I could do the same thing with a worse UI by hosting MongoDB on an open port.

[PinkMilkshake]

And taps are just zero-length swipes!