Dual-Write and Backfill

1. The Ordering Rule

Dual write and backfill are two halves of one operation, and the order between them is not a preference. Getting it wrong loses data silently.

Turn on dual writing first. Start the backfill second. Never the other way round.

The reason is a gap. Suppose you backfill first and enable dual writes afterwards. Between the moment the backfill reads a row and the moment dual writing goes live, the application may update that row. The old representation gets the update, the new one does not, and nothing in the system records that it happened