What does the term "single source of truth" mean?

"Single source of truth" is a principle where certain data is stored in only one place, and every part of the application reads from exactly that place.

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#Why it's needed:

• So there are no discrepancies between different copies of the same data.
• So that all updates are centralized and controlled.
• To make the application easier to debug and maintain.

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#Example:

If the user is stored in a component, in a service, and in local storage, sooner or later you will forget to update one of them. But if the entire user object is stored only in AuthService.user$, and every component reads from there alone, everything stays under control.

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#In Angular:

• A BehaviorSubject in a service is a common "single source of truth"
• The Store in NgRx is a global, formal single source of truth for the whole application

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Conclusion: A single source of truth is the one place where the real data lives, and everything else just looks there. That means fewer bugs and more predictability.