DELIVERY GUARANTEES
The ack got lost.
He paid twice.
BOTH BROKERS SENT IT TWICE
A lost ack looks like a lost job.
So the broker sends it again.
AT-LEAST-ONCE
REDELIVERED
It already ran, so it runs again and the card is charged twice.
IDEMPOTENCY KEY
DEDUPED
The duplicate still arrives, but the gate has seen this key.
AT-LEAST-ONCE — the duplicate runs.
Your side effect happens twice.
IDEMPOTENCY KEY — the duplicate
arrives and does nothing.
The ack got lost.
He paid twice.
Check this before
you take money.
