[erlang-questions] Future of epmd

David Mercer dmercer@REDACTED
Wed Nov 7 18:59:45 CET 2012

This seems like an outstanding idea.  If I understand correctly, you could have the first node that starts up on a host also start the erlpmd application.  If the node running erlpmd goes down, one of the other nodes on the same host starts the erlpmd application.  Do I have this right?






From: erlang-questions-bounces@REDACTED [mailto:erlang-questions-bounces@REDACTED] On Behalf Of Dmitry Demeshchuk
Sent: Wednesday, November 07, 2012 01:03
To: erlang-questions
Subject: [erlang-questions] Future of epmd


Hello, list.


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: http://sidentdv.livejournal.com/769.html ).


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: https://github.com/lemenkov/erlpmd


When hacking it, I've noticed several things:


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:


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?


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).



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.



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?


Best regards,
Dmitry Demeshchuk

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