[Info-vax] VMS process communication

[Fred. Zwarts]

Op 14.sep..2022 om 19:40 schreef Dave Froble:
> On 9/14/2022 1:34 PM, Simon Clubley wrote:
>> On 2022-09-13, Dave Froble <davef at tsoft-inc.com> wrote:
>>>
>>> Not arguing any of that.  All I was implying was that both sides of a
>>> communication must insure a successful communication, or, it never 
>>> happened.
>>>
>>
>> Not that simple (for performance reasons).
>>
>> Protocols such as TCP, ZMODEM, and Kermit (with sliding windows enabled)
>> are all asynchronous protocols where the delivery of data to the end user
>> program is decoupled from the sending of ACKs back to the transmitter.
>>
>> The receiver end user program can easily have received a good amount of
>> data from the transmitter before the transmitter has received the ACKs
>> from the receiver to confirm receipt of that data.
>>
>> You need something that guarantees the receiver end user program and
>> the sender end user program are in sync at all times ? You can do that
>> with a layer on top of the underlying protocol, but you will take a
>> serious performace hit for doing so.
>>
>> Simon.
>>
> 
> What type of hit might one experience if the customer billing is say 
> $1000 and the last 2 zeros are chopped off the message?
> 
> Not that that is anything real.  You cannot convince me that if the 
> communication was not verified, it is anything but "it never happened".
> 
> Accuracy is a great deal more important than performance.  Actually, 
> accuracy isn't everything, it's the ONLY thing.
> 
If accuracy is the only thing, it will be impossible to do a 
transaction. In theory it will need an endless series of acknowledging 
the acknowledgements. In practice we usually have enough trust in the 
underlying communication channels that a one or two level 
acknowledgement is sufficient.
There is a trade-of between the needed accuracy and the performance. Not 
all communications need the same accuracy. A damaged frame when viewing 
a video is a smaller problem than a missing 0 in a bank transfer. So, 
the latter will use a more accurate communication protocol. But even 
then we accept an error once in a million years, because otherwise 
transactions are not possible at all.