[erlang-questions] question re. message delivery

Miles Fidelman <>
Mon Sep 25 18:02:39 CEST 2017


On 9/24/17 11:53 PM, Raimo Niskanen wrote:

> On Sun, Sep 24, 2017 at 11:24:31PM -0700, Miles Fidelman wrote:
>> See below....
>>
>>
>> On 9/24/17 6:10 PM, zxq9 wrote:
>>> On 2017年09月24日 日曜日 16:50:45 Miles Fidelman wrote:
>>>> Folks,
>>>>
>>>> I've just been re-reading Joe Armstrong's thesis, and I'm reminded of a
>>>> question that's been nagging me.
>>>>
>>>> As I understand it, message delivery is not guaranteed, but message
>>>> order IS. So how, exactly does that work?  What's the underlying
>>>> mechanism that imposes sequencing, but allows messages to get lost?
>>>> (Particularly across a network.)  What are the various scenarios at play?
>>> This is sort of backwards.
>>>
>>> Message delivery is guaranteed, assuming the process you are sending a
>>> message to exists and is available, BUT from the perspective of the
>>> sender there is no way to tell whether the receiver actually got it,
>>> has crashed, disappeared, fell into a network blackhole, or whatever.
>>> Monitoring can tell you whether the process you are trying to reach
>>> is available right at that moment, but that's it.
>>>
>>> The point is, though, that whether the receiver is unreachable, has
>>> crashed, got the message and did its work but was unable to report
>>> back about it, or whatever -- its all the same reality from the
>>> perspective of the sender. "Unavailable" means "unavailable", not matter
>>> what the cause -- because the cause cannot be determined from the
>>> perspective of the sender. You can only know this with an out of
>>> context check of some sort, and that is basically the role the runtime
>>> plays for you with regard to monitors and links.
>>>
>>> The OTP synchronous "call" mechanism is actually a complex procedure
>>> built from asynchronous messages, unique reference tags, and monitors.
>>>
>> Note that I didn't ask about the synchronous calls, I asked about raw
>> interprocess messages.
>>
>>> What IS guaranteed is the ordering of messages *relative to two processes*
>>>
>>> If A sends B the messages 1, 2 and 3 in that order, they will certainly
>>> arrive in that order (assuming they arrive at all -- meaning that B is
>>> available from the perspective of A).
>> But that's the question.  Particularly when sent via network, 1, 2, 3
>> may be sent in that order, but, at the protocol level, they may not
>> arrive in that order.
> What protocol level?
>
> Erlang distribution has to use or implement a reliable protocol.  Today
> TCP, but anything is possible.  Note that this protocol is between two
> nodes, both containing many processes.  But the emulator relies on the
> transport protocol being reliable.

No.  It doesn't.  It could simply send UDP packets.  I'm asking about 
implementation details.  In Joe's thesis, he says that the behavior is a 
"design choice."  I'm asking about the implementation details.  How does 
BEAM actually handle message delivery - locally, via network?

>> With a reliable transport protocol - say TCP - if the message-containing
>> packets arrived as 1, 3, 2, the protocol engine would wait for 2 to
>> arrive and deliver 1,2,3 in that order.  If It received 1 & 3, but 2 got
>> lost, it would request a re-transmit, wait for it to arrive, and again,
>> deliver in that order.
>>
>> But the implication of Erlang's stated rules is that an unreliable
>> transport protocol is being used, if you send 1, 2, 3, and what arrives
> What?  What is stated?
 From Joe Armstrong's Thesis:

"Message passing is assumed to be unreliable with no guarantee of 
delivery."

"Since we made no assumptions about reliable message passing, and must 
write our application so that it works in the presence of unreliable 
message passing it should indeed work in the presence of message passing 
errors. The initial ecort involved will reward us when we try to scale 
up our systems."

"2. Message passing between a pair of processes is assumed to be ordered 
meaning that if a sequence of messages is sent and received between any 
pair of processes then the messages will be received in the same order 
they were sent."

"Note that point two is a design decision, and does not reflect any 
under- lying semantics in the network used to transmit messages. The 
underlying network might reorder the messages, but between any pair of 
processes these messages can be buffered, and re-assembled into the 
correct order before delivery. This assumption makes programming message 
passing applications much easier than if we had to always allow for out 
of order messages."

---
I read this as saying, messages will be delivered in order, but some may 
be missing.

I'm really interested in this design decision, and how it's 
implemented.  (I'm also interested in the logic of why it's easier to 
program around missing messages than out-of-order messages.)

Miles

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

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