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<p class="MsoNormal"><font size="2" color="#1f497d" face="Calibri"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hi,<o:p></o:p></span></font></p>
<p class="MsoNormal"><font size="2" color="#1f497d" face="Calibri"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font size="2" color="#1f497d" face="Calibri"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">BTW what about using SCTP for distribution protocol?
<o:p></o:p></span></font></p>
<p class="MsoNormal"><font size="2" color="#1f497d" face="Calibri"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Would not bring it some advatages in high availability area? E.g. bacause of multihoming or posibility of setting
timeout and retransmission parameters?<o:p></o:p></span></font></p>
<p class="MsoNormal"><font size="2" color="#1f497d" face="Calibri"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font size="2" color="#1f497d" face="Calibri"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Best regards,<o:p></o:p></span></font></p>
<p class="MsoNormal"><font size="2" color="#1f497d" face="Calibri"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> Tomas<o:p></o:p></span></font></p>
<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
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<p class="MsoNormal"><b><font size="2" face="Tahoma"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";font-weight:bold">From:</span></font></b><font size="2" face="Tahoma"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">
erlang-questions-bounces@erlang.org [mailto:erlang-questions-bounces@erlang.org]
<b><span style="font-weight:bold">On Behalf Of </span></b>Dmitry Demeshchuk<br>
<b><span style="font-weight:bold">Sent:</span></b> Wednesday, November 07, 2012 8:03 AM<br>
<b><span style="font-weight:bold">To:</span></b> erlang-questions<br>
<b><span style="font-weight:bold">Subject:</span></b> [erlang-questions] Future of epmd<o:p></o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt">Hello, list.<o:p></o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt">As you may know, epmd may sometimes misbehave. Loses nodes and doesn't add them back, for example (unless you do some magic, like this:
<a href="http://sidentdv.livejournal.com/769.html">http://sidentdv.livejournal.com/769.html</a> ).<o:p></o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt">A while ago, Peter Lemenkov got a wonderful idea that epmd may be actually written in Erlang instead. EPMD protocol is very simple, and it's much easier to implement all
the failover scenarios in Erlang than in C. So far, here's a prototype of his: <a href="https://github.com/lemenkov/erlpmd">https://github.com/lemenkov/erlpmd</a><o:p></o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt">When hacking it, I've noticed several things:<o:p></o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt">1. When we send ALIVE2_REQ and reply with ALIVE2_RESP, we establish a TCP connection. Closing of which is a signal of node disconnection. This approach does have a point,
since we can use keep-alive and periodically check that the node is still here on the TCP level. But next, some weird thing follows:<o:p></o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt">2. When we send other control messages from a node connected to epmd, we establish a new TCP connection, each time. Could use the main connection instead. Was it a design
decision or it's just a legacy thing?<o:p></o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt">3. The client (node) part of epmd seems to be all implemented in C and sealed inside ERTS. However, it seems like this code could be implemented inside the net_kernel
module instead (or something similar).<o:p></o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt">Why bother and switch to Erlang when everything is already written and working? First of all, sometimes it doesn't really work in big clusters (see my first link). And,
secondly, using Erlang we can easily extend the protocol. For example, add auto-discovery feature, which has been discussed on the list a lot. Add an ability for a node to reconnect if its TCP session has been terminated for some reason. Add lookups of nodes
by prefix (like, "give me all nodes that match mynode@*"). The list can be probably extended further.<o:p></o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt">Do you think such a thing (with full backwards compatibility, of course) could go upstream? Also, a question for epmd maintainers: is it going to change at all, or the
protocol is considered to be full enough for its purposes?<o:p></o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt"><o:p> </o:p></span></font></p>
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<p class="MsoNormal"><font size="3" face="Times New Roman"><span style="font-size:12.0pt">--
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Best regards,<br>
Dmitry Demeshchuk<o:p></o:p></span></font></p>
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