Run a contest without paying the prize twice
Snapshot the standings, pay from the snapshot, mark the contest closed — all in one transaction, with the entry rules decided before it opens.
Tagged “transactions”
6 entries
Idempotency keys on every operation that moves money
A caller-supplied key with a unique index turns every retry, refresh and double-click into a no-op. The single most valuable column in a payments schema.
Credit maths for a traffic exchange
Send N, receive M, and keep the difference as the house margin. The one transaction that moves credits between two members without leaking any.
A withdrawal that cannot be paid twice
Debit on request, not on approval. The state machine that makes double payouts and phantom balances impossible.
Move balance between two accounts without losing any
Lock both rows in a fixed order, check inside the transaction, and write a ledger entry for each side. Skipping any of the three is how balances drift.
A claim cooldown the client cannot argue with
Reward the claim and start the next cooldown in one transaction, so a double-submitted form pays once instead of twice.