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<div><font face="verdana,sans-serif" size="2" color="#3333FF">Thanks Lukas, please see a couple of questions/comments below.</font></div>
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<div>- Dmitry</div>
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<div id="divRplyFwdMsg"><font face="Calibri,sans-serif" size="2"><span style="font-size: 11pt;"><b>From:</b></span></font><font face="Calibri,sans-serif" size="2"><span style="font-size: 11pt;"> Lukas Larsson <lukas@erlang.org></span></font></div>
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<div id="divRplyFwdMsg"><font face="Calibri,sans-serif" size="2"><span style="font-size: 11pt;"><b>Sent:</b></span></font><font face="Calibri,sans-serif" size="2"><span style="font-size: 11pt;"> Wednesday, January 25, 2017 12:37 AM</span></font></div>
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<div><font face="Calibri,Arial,Helvetica,sans-serif">Hello,</font></div>
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<div><font face="Calibri,Arial,Helvetica,sans-serif">On Wed, Jan 25, 2017 at 12:38 AM, Dmitry Kakurin <<a href="mailto:dima_kakurin@hotmail.com" target="_blank">dima_kakurin@hotmail.com</a>> wrote:</font></div>
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<div id="gmail-m_-88027960317730910m_-6976247717624396004divtagdefaultwrapper"><font face="Calibri,Arial,Helvetica,sans-serif">When I execute "pid ! msg" and it returns, what are the guarantees if pid and self are on the same node?</font></div>
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<div id="gmail-m_-88027960317730910m_-6976247717624396004divtagdefaultwrapper"><font face="Calibri,Arial,Helvetica,sans-serif">Can I assume that by the time it returns, msg is in the pid's queue? And as a result all messages sent to pid afterwards will be queued
after msg?</font></div>
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<div><font face="Calibri,Arial,Helvetica,sans-serif">No you cannot assume that. The message is considered "in transit" until the receiving process inspects it somehow.</font></div>
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<font face="verdana,sans-serif" size="2" color="#3333FF">What aspect of implementation can cause such reordering? Could you give an example how this could happen? Maybe I'll be able to artificially create conditions where it would not happen. Or work around
it in some other way. Please see below why.</font><font face="Calibri,Arial,Helvetica,sans-serif" style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 16px;"><br>
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<div id="gmail-m_-88027960317730910m_-6976247717624396004divtagdefaultwrapper"><font face="Calibri,Arial,Helvetica,sans-serif">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:</font></div>
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<div id="gmail-m_-88027960317730910m_-6976247717624396004divtagdefaultwrapper"><font face="Calibri,Arial,Helvetica,sans-serif">1. A --msg1--> C (process A sends msg1 to process C)</font></div>
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<div id="gmail-m_-88027960317730910m_-6976247717624396004divtagdefaultwrapper"><font face="Calibri,Arial,Helvetica,sans-serif">2. A --msg2--> B</font></div>
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<div id="gmail-m_-88027960317730910m_-6976247717624396004divtagdefaultwrapper"><font face="Calibri,Arial,Helvetica,sans-serif">3. B --msg2--> C (process B simply proxies msg2 from A to C)</font></div>
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<div id="gmail-m_-88027960317730910m_-6976247717624396004divtagdefaultwrapper"><font face="Calibri,Arial,Helvetica,sans-serif">I need a guarantee that msg2 will always appear in C's queue after msg1.</font></div>
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<div>You have to do this using messages or some other sync mechanism.</div>
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<font face="verdana,sans-serif" size="2" color="#3333FF">You are right, <i>in general</i> it's easy to devise a simple scheme for that. </font></div>
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<font face="verdana,sans-serif" size="2" color="#3333FF">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).</font></div>
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<font face="verdana,sans-serif" size="2" color="#3333FF">With lack of cooperation from C I cannot think of a communication protocol to accomplish what I want.</font></div>
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<font face="verdana,sans-serif" size="2" color="#3333FF">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?</font></div>
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<div id="gmail-m_-88027960317730910m_-6976247717624396004divtagdefaultwrapper"><font face="Calibri,Arial,Helvetica,sans-serif">P.S. I've read the FAQ 10.8 and 10.9 :-)</font></div>
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<div><font face="Calibri,Arial,Helvetica,sans-serif">Please add to the FAQ if you think that you can make it clearer on this point. <a href="https://github.com/matthiasl/Erlang-FAQ" target="_blank">https://github.com/matthiasl/Erlang-FAQ</a></font></div>
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<div><font face="Calibri,Arial,Helvetica,sans-serif">Lukas</font></div>
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