Migrating a live financial ledger is the kind of project where "move fast" and "break things" cannot coexist. Every transaction matters, and every second of downtime shows up as a support ticket.
We split the migration into phases: dual-writing to the old and new stores, backfilling history, then a staged cutover per account segment. Feature flags let us roll back any segment independently if reconciliation checks failed.
The biggest lesson: invest in reconciliation tooling before the migration, not during it. Being able to diff old vs. new state in minutes, not hours, is what let us move with confidence.
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