Banks have never heard of SYN/ACK | Hacker News Reader

Banks have never heard of SYN/ACK

jwz.org

7 pointsAndrewDucker3 comments

From the fine article:

  1. Customer places an order.
  2. SYN: Can I charge $30?
  3. SYN/ACK: Yes.
  4. ACK + SYN: Do it.
  5. SYN/ACK: I am gonna do it.
  6. ACK: I see that you're gonna do it.

"If that was their model, then at no point does a communication failure cause a charge to be in an ambiguous state. If I never get the message in #5, the customer is not charged. If I get the message in #5 and my response in #6 is not received, the customer is not charged."

Er.....that doesn't appear to solve anything, instead it just pushes the error state down a level; there's still an ambiguous state where #6 is sent and not received.

The client thinks the charge is going to take place, and so thinks the client will be charged, but the bank never gets #6 and so never makes the charge, aka distributed atomic operations are hard.

He doesn't say that the change makes things perfect - he says "There's only one possible failure mode and not two, and that failure mode is the safer one"

https://en.wikipedia.org/wiki/Two_Generals'_Problem

"Because acknowledgement of message receipt can be lost as easily as the original message, a potentially infinite series of messages are required to come to consensus."

What you want isn't really a handshake, it's a commit, and no finite amount of messages will ensure agreement over a lossy network.

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