[erlang-questions] Message send guarantees

Per Hedeland <>
Thu Jan 26 18:25:33 CET 2017


On 2017-01-26 18:05, Per Hedeland wrote:
> On 2017-01-26 14:09, Fred Hebert wrote:
>> On 01/25, Dmitry Kakurin wrote:
>>>
>>> The reason I'm asking is because I need a guarantee of proper message ordering in the following scenario when all processes involved are local to a node:
>>> 1. A --msg1--> C (process A sends msg1 to process C)
>>> 2. A --msg2--> B
>>> 3. B --msg2--> C (process B simply proxies msg2 from A to C)
>>>
>>> I need a guarantee that msg2 will always appear in C's queue after msg1.
>>>
>>> [...]
>>>
>>> Unfortunately I have a limitation. In my case A talks to C thru proxy B and would like to short-circuit the communication by getting rid of B and start talking to C directly. Process A needs to do
>>> that while messages thru B could be in flight. The limitation is that C is not aware of this proxy business, and cannot be changed (it's a regular GenServer that is not written by me).
>>> With lack of cooperation from C I cannot think of a communication protocol to accomplish what I want.

Sorry, I missed that last part in my comment below - indeed it prevents
the "ack solution" if C won't cooperate.

--Per

>>> Is there a way for processes A and B to observe C's queue in any way and derive some information about it to help me with my problem?
>>>
>>
>> As others have mentioned, this is not really possible. You *could* observe the queue, but it is not practical.
>>
>> The interesting thing with Erlang there is that a lot of these design decisions work the same whether the processes are local or remote. The problem you see here of message ordering is very similar to
>> the one you get of packet ordering when delivering data over a network.
>>
>> Generally, the way to solve the problem is to label every message you send with a sequence number that increments. On every message received, the final receiver can then look at the sequence numbers
>> and see if any message is missing. It can then block, ask for a retransmission, or disregard duplicated messages.
>>
>> Because Erlang's guarantees for message passing only involve any given pair of processes (A -> B, or A -> C) but not more, we get a case where the following race conditions are very possible:
>>
>>  A            B           C
>>  | --- 1 ---> |           |
>>  | --- 2 ---------------> |
>>  |            | --- 1 --> |
>>
>> The stop at B means that through scheduling, the message can be delayed for unkown periods of time and the delays can mess up the ordering.
>>
>> The only surefire way of getting the messages to C in order is to either
>> a) send them directly from a single sender every time, or b) to let C know if the messages are not in order so it can do the reordering itself.
> 
> Well, there is also (already hinted at I think) c) have A receive an ack
> from C for message 1 before it sends message 2. Doing this for every
> message may be an unacceptable throughput limitation in some cases, in
> some other cases it is what happens "anyway" due to the nature of the
> communication. In this particular case, "A talks to C thru proxy B and
> would like to short-circuit the communication by getting rid of B and
> start talking to C directly", it would be sufficient that A got an ack
> for only the *last* via-B-message (could be a special "let's skip the
> proxy" message) before it started sending directly to C. Seems like a
> clean solution to me.
>
> --Per
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