SYSE.34:4.3 - Construct the write and backfill mechanism

Choose one authoritative representation during coexistence when that can satisfy the use, and derive the other views through a specified transactional or otherwise qualified mechanism. If independent writers are required, obtain an explicit conflict/order resolution Method; the instruction “write both” does not supply one.

Select a backfill rule that accounts for concurrent updates. Use the database’s qualified locking, current-row or version-check behavior so that an old observation cannot silently replace a later legitimate write. Bound work and resource effects, including locks, storage growth and effects on serving traffic.

Make retry behavior explicit. Determine which transaction effects and progress observations are committed together, which may be replayed, and which external effects cannot be repeated. After a lost acknowledgement, first recover the actual state; a migration log entry alone is not proof that every dependent effect occurred.