[erlang-questions] Future of epmd

Kenneth Lundin <>
Thu Nov 8 17:45:41 CET 2012


We have discussed having epmd implemented in Erlang several times and think
it
would be a good idea for several reasons especially if there is only one
Erlang node per host. But it could work even if there are more nodes. It
could also be an alternative to run a separate E-node just for the epmd
service. We have unfortunately not been able to give this high enough
priority yet, so this initiative
is interesting (have not looked at the code and other details)

The benefits with having epmd implemented in Erlang would be:

- Easier to maintain
- Easier to extend
- Easy to prototype other solutions, for example heterogenous distribution,
secure epmd communication via TLS , etc

The client part is already written in Erlang, see the erl_epmd module.

/Kenneth , Erlang/OTP Ericsson
Den 7 nov 2012 08:03 skrev "Dmitry Demeshchuk" <>:

> 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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