If a Product Has Two Sources of Truth, It Has None
Multiple state copies are normal. Multiple authorities are not. Name each state role and keep canonical ownership singular.
Cloud says A. Local storage says B. Which one wins?
If the answer is “it depends” and the dependency is not explicit, the system does not really have a source of truth.
Multiple representations are normal: cache, mirror, optimistic state and recovery journal can all exist. The problem begins when more than one of them behaves as canonical.
Name the roles. Canonical changes. Cache reflects. Optimistic waits for acceptance. Recovery preserves intent. Derived state is calculated.
Synchronization becomes much easier to reason about when authority is singular even if state copies are not.
Architecture in the Wild
Field architecture through live state, information, interaction and product-boundary decisions rather than abstract diagrams.