

You cannot have at-least-once broadcast - rumcajz
http://250bpm.com/blog:61

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nemothekid
While I understand the intent, I feel that is much less rigorous than the 2
problems mentioned ("You Cannot Have Exactly-Once Delivery", and CAP), both
deal with the fact the network is unreliable and _only_ that - even if you had
unlimited RAM and disk, the two proofs would still hold.

Here a possible solution is proposed " _Send the message to Bob even if Carol
is offline and store the message to Carol for later delivery._ ", and is
really just handwaved away by saying no one has unlimited disk space (or a
24/7 ops team) (also doesn't Amazon sort of solve this problem?). I'm sure
that is a practical limitation, but for the sake of building reliable systems
I'm unsure its strong one.

If thats the case, then one could also argue Raft also isn't linearizable,
because the replicated log file could get really large and no one has
unlimited disk space.

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StavrosK
It's just silly. You can't have any delivery to anyone, because if they're
offline your message buffer might get too large for your disk.

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PeterisP
The argument is a bit ridiculous - simple logging of all messages sent also
requires the same storage, and that is commonly done whenever each individual
message and it's delivery is considered important.

As long as the storage requirements are bounded by number of messages times
number of recipients times some reasonable constant, it _is_ perfectly
feasible to assume that you will have sufficient storage; the storage
requirements are finite and predictable, and it's only a matter of your choice
and priorities.

Acknowledging that doing X will require Y petabytes of storage and cost Z
dollars may mean that you won't do it, but it's very, very different from
stating that it's impossible.

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brongondwana
Also the earth could be demolished to make way for a hyperspace bypass, so you
can't even have at-least-once delivery to a single recipient.

It's amazing how much you can get done without perfect guarantees though.

