[erlang-questions] order-preserving send?

Hans Svensson <>
Thu Aug 28 15:56:39 CEST 2008


Matthias Lang wrote:
> On Wednesday, August 27, atomly wrote:
>> [Bill McKeeman <>]
>>>    Simple newbie question.  Suppose I have a process id U, and send two
>>>    messages
>>>
>>>
>>>    U ! Message1.
>>>
>>>    U ! Message2.
>>>
>>>
>>>    Is there a guarantee that the messages will be received in the order
>>>    they were sent?
>> No.  There's not even a guarantee that both (or either) messages will be
>> received.
> 
> That misleading and, for practical purposes, wrong.
> 
>>From the FAQ:
> 
>  | 10.9 Is the order of message reception guaranteed?
>  | 
>  | Yes, but only within one process.
>  | 
>  | If there is a live process and you send it message A and then message
>  | B, it's guaranteed that if message B arrived, message A arrived before
>  | it.
>  | 

That is however not true (in the distributed case), it is possible to 
drop A and then later receive B. The paper 'Programming Distributed 
Erlang Applications: Pitfalls and Recipes' describes this situation as 
well as some other quirks:

http://www.cs.chalmers.se/~hanssv/doc/ew07-dist.pdf

>  | On the other hand, imagine processes P, Q and R. P sends message A to
>  | Q, and then message B to R. There is no guarantee that A arrives
>  | before B. (Distributed Erlang would have a pretty tough time if this
>  | was required!)
> 

That is actually guaranteed in the local (non-distributed) case, then A 
is actually *always* delivered before B. But as mentioned it doesn't 
hold in the distributed setting.

> The next question is also relevant:
> 
> http://www.erlang.org/faq/academic.html#10.10
> 


/Hans




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