[erlang-questions] Future of epmd

Dmitry Demeshchuk <>
Wed Nov 7 08:03:05 CET 2012


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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