Data Migration · shaheershoaib bundle Use when moving or reshaping DATA rather than code - a legacy-system migration, a backfill, a bulk import, an ETL, a re-keying, or a one-off correction script over existing rows. Triggers on "migrate the data", "backfill X", "import from the old system", "reconcile the migration", "why is this row wrong since the migration". NOT for schema-only DDL with no data movement (that is an ordinary schema change, handled by expand/contract), and NOT for code-wide mechanical sweeps like a codemod or rename (that is a mechanical code sweep). Applies to any store - relational, document, key-value, warehouse - and to moves between kinds. The defining property: the correctness of the output cannot be observed from the input, so the whole discipline is about proving it on the destination. Also when only a schema dump or an extract has been handed over and the application code for either side has not.
