One single source of truth

Data should be held in one location, duplicates of that data should be by reference only.

Why

• Changes to data are always propagated to the rest of the system.
• Mutations to the data need only happen in one place.
• Single source of truth means no data will be out of sync or fail to be updated.

How

Only allow data writes to happen in one location. Whether that be a call to a rest API, system call or other write actions.

Don't allow data to be stored anywhere but the single source of truth.

Exceptions

Highly distributed systems Some systems rely on data consistency to be reached eventually or may never need to have accurate data.

References

Further reading: https://en.wikipedia.org/wiki/Single_source_of_truth