Audit my app for lost-update bugs: anywhere the code reads a value, modifies it in application code, and writes it back. Counters (likes, views, credits), balances, inventory, seats available, and 'increment usage' logic are the classic spots. For each one: replace the read-modify-write with an atomic operation the database performs (UPDATE ... SET count = count + 1, MongoDB's $inc, or the ORM's equivalent), so two concurrent requests can't both read 10 and both write 11. Where the logic genuinely needs the read (checking sufficient balance before a debit), do it inside a transaction with a conditional write (UPDATE ... WHERE balance >= amount) and verify the row actually updated, retrying or failing cleanly if it didn't. For records edited by users simultaneously (documents, settings), add optimistic locking with a version field: the write only succeeds if the version hasn't changed, and the loser gets told to reload. Don't serialize everything through locks; atomic operations and conditional writes handle the common cases without contention. Don't change the user-facing behavior of counters. Deliver: each race found, the fix applied, and a concurrency test: fire parallel increments or debits and confirm the final value is exactly right, not approximately right.