Comment by lisper

Comment by lisper 10 months ago

> The guards have "processed" the message for this purpose, and the fact that they can deliver it to you over a perfectly reliable channel is moot.

No, it isn't, because the situation inside the fort is different than the situation outside. The odds of a courier being intercepted inside the fort are effectively zero. If your computational model includes a non-zero probability of failure "inside the fort" then you are no longer in the realm of distributed systems, but are now talking about fault-tolerant computing, which is a whole nuther kettle o worms.

pyrolistical 10 months ago

No, one of the key ideas in distributed systems is the network is unreliable. It’s not bad actors “inside the fort” it’s just that messages get lost sometimes.

Internal switches can fail. There are countless reasons why packets will get lost

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lisper 10 months ago

"Inside the fort" is a metaphor for a single server, not anything involving a network.

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mlyle 10 months ago

No. An element of a distributed system that wants to debit from bank accounts must acknowledge the receipt of a message after making its effects durable. That system may fail between the change being durable and the acknowledgment making it back to the original sender.

That element must tolerate that debit being delivered multiple times. And we can't even solve that problem by a higher-order system providing "reliable delivery" on that element, unless that thing persists atomically with the application performing the transaction (or the application itself tolerates multiple delivery).

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lisper 10 months ago

> An element of a distributed system that wants to debit from bank accounts must acknowledge the receipt of a message after making its effects durable.
I do not and never have disputed that. But I fail to see what that has to do with the matter at hand, namely, whether or not exactly-once delivery is possible.

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