[erlang-questions] Order of monitor signal(s)

[Joe Armstrong]

On Thu, Nov 30, 2017 at 4:28 PM, Sverker Eriksson
<sverker.eriksson@REDACTED> wrote:
> On tor, 2017-11-30 at 16:05 +0100, Dmytro Lytovchenko wrote:
>> message delivery is instant the moment you execute send command (send
>> opcode).
>
> NO NO NO.
> Messages are asynchronous signals. The return of the "send command"
> does not guarantee ANYTHING.
>
>
>> So exit message well always arrive second, because ordering guarantee
>> - exit message was sent SECOND.
>>
> YES
> Erlang signals (messages, links, monitors, etc) guarantee ordered
> delivery between process pairs. THAT is what gives you the guarantee
> that all messages sent from process A are put in the message queue of
> process B before the monitor 'DOWN' message.

To start with these properties different depending upon the environment.
You might have a) pairs of processes in the same node
b) pairs of processes on different nodes on the same host and c) pairs of
processes on different nodes on different hosts.

The failure modes in the three cases all different.

I think you should say "in the absence of any failures and counter indications"
messages and exit signals between pairs of processes should arrive in
the order they were sent.

In the distributed case the messages and exits all go down the same
socket in order,
so they should arrive ordered. If the socket closes abnormally, or
ping watchguard timer
elapses you will be told.

The general idea is that ordering is preserved between pairs of
processes if nothing goes wrong,
and if something goes wrong you will be told.

In order to be told you need to be trapping exits and
node up/down messages - the 'something has gone wrong' message will
arrive *after* the
failure has occurred - it's up to you to figure out what to do.

Cheers

/Joe



>
> /Sverker
>
>
>> On 30 Nov 2017 4:02 pm, "Dmitry Kolesnikov" <dmkolesnikov@REDACTED>
>> wrote:
>> Hello,
>>
>> There is a statement that Erlang messages may be not delivered, but
>> message order is always preserved relative to two processes. This is
>> a statement regarding the ordinal message… Is this statement valid
>> when we are talking about monitor messages in relation to ordinal
>> message? I have strong feeling that it is not guaranteed.
>>
>> So, let’s assume we have a process A that sends async message to
>> process B and quits. The process B listens messages for process A and
>> monitors it with erlang:monitor(process, A). I am suspecting that
>> there are no guarantee that message will be delivered before exit
>> signal. Am I right?
>>
>> Thank you in advanced!
>>
>> Best Regards,
>> Dmitry
>>
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