Gradual Migration Between Derived Views (the dual-gauge railway) Diagram

The Third Rail

The old schema and the new schema are maintained side by side as two independently derived views onto the same underlying data
Reprocessing the accumulated history builds the new view while the old view keeps serving every user
Like a dual-gauge railway: add a third rail, run trains of both gauges on it, and remove the extra rail only once the conversion is done

Shifting Users Gradually

A small number of users are shifted to the new view first, to find bugs and measure its performance
Most users keep being routed to the old view while the proportion on the new view is increased gradually
Once all users run on the new view, the old view is dropped — the nonstandard rail is removed

Every Stage Is Reversible

If something goes wrong at any stage, the system reverts to the old view, which is still running correctly
There is always a working system to go back to, so a mistake cannot cause irreversible damage
Removing the risk of permanent damage makes it easier to be confident and to move faster in improving the system