Backfill and Data Reconciliation Playbook

Backfill work is often discussed in terms of throughput, but real incidents usually happen after loading finishes. Row counts match, yet values drift. Some ranges are duplicated. Others are silently missing. That is why backfill should always be treated as a reconciliation problem, not only a loading problem.

Row counts are necessary, but not enough

Teams should compare more than total counts.

• total records
• range-by-range counts
• status distribution
• important aggregates such as sums or extrema

Global and segmented checks must both exist.

Design validation units before execution

Backfills are often chunked by time or ID ranges. Validation should follow the same units.

• batch-level checksums
• range-level missing record counts
• duplicate key detection

That structure makes partial reprocessing much easier.

Keep a final cross-check before switching authority

Even when the initial backfill is complete, source-of-truth handoff should wait until the final delta window is reconciled.

Conclusion

A good migration is not the one that only finishes quickly. It is the one where the team can immediately identify what is wrong, where it is wrong, and how to rerun only that slice.

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