[erlang-questions] question re. message delivery

Miles Fidelman <>
Tue Sep 26 20:10:15 CEST 2017


On 9/26/17 10:05 AM, *Matthias Lang* wrote:

> On 26. September 2017, Miles Fidelman wrote:
>
>
>>> Miles, do you have some concrete examples of situations where you're
>>> worried about messages disappearing? Here's one from me: process 1
>>> sends two messages to process 2. The messages are A and B,
>>> respectively. Process 2 sends an ACK for message B back to process
>>> 1. For single-node Erlang, if message A disappears then that is a
>>> bug. I'll let others reason about distributed Erlang.
>> Sure.  Bank transactions.  Edits to a document.  Dispatch commands to a
>> vehicle.
> Those are not what I consider concrete examples.

How concrete do you need.  There are lots of systems where one needs 
reliable, ordered message delivery.  One generally implements an 
acknowledgement, but those can be unordered.

Let's say that a transaction is a test-and-set on a value.  With 
acknowledgement sent after transaction is executed.

Send transactions 1,2,3,4,5

If message delivery is ordered & guaranteed, then an acknowledgement 
from 5 guarantees that transactions 1-4 were performed, in order, even 
if some of the acknowledgements were not received.

If, either, a message can get dropped, or it can get delivered out of 
order, then one needs a protocol on top of basic message delivery to 
wait for an acknowledgement of each and every message before sending the 
next.

There are multiple approaches to implementing a reliable messaging 
protocol.  What approach would work best somewhat depends on the kinds 
of failure modes one has to deal with (i.e., the services provided by 
the next layer down).

Miles

-- 
In theory, there is no difference between theory and practice.
In practice, there is.  .... Yogi Berra

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