Single Source of Truth

It’s important to have a single source of truth for the data in your system.

That is to say, a single source of truth. Copies of the data itself can spend some time closer to other parts of the system that need it. Eventual consistency can suffice for sufficiently slow requests.

Can you ask a user to wait a few seconds for systems to sync up? That could spare them the much longer delays - minutes or even hours - that come with confusing single sources of truth with single sources of data. No one likes waiting for an overburdened, unintentional dependency to catch up with the rest of the system.