[erlang-questions] net_kernel:start/1 and EPMD gotcha

[BOUDEVILLE Olivier]

Hi,

Shouldn't net_kernel:start/1 do the same as when a VM is run from the command-line with a short or long name, i.e. (among other things) also launch an EPMD instance if needed?

For the context (maybe it will help people encountering the same issue): we had an application which was running with long names until we had to include an optional information (an instance identifier) in the node names, an information that gets in some cases only available at runtime.

As renaming a node created with -sname or -name is apparently not possible (not_allowed), we resorted to changing the application so that its first node is launched initially in non-distributed mode and then, only once the identifier is available, turning this node in a distributed one, with net_kernel:start/1.

Unfortunately past launches had spawned an EPMD that was lingering in the background and gave us the impression that everything was working properly. However, while we were working on another feature, an update and a reboot had to occur, and the application became then unable to launch at all, with a somewhat cryptic message akin to:

{'EXIT',nodistribution}}},{child,undefined,net_sup_dynamic,{erl_distribution,start_link,[['my-foobar-node',longnames]]},permanent,1000,supervisor,[erl_distribution]}}...

It has been a long way to find IT updates, IPv4/IPv6 settings, host resolution, firewalls, cookies and all not guilty, and that the application was working iff another Erlang program had triggered the launch of an EPMD that had not been wiped yet.

At least, if EPMD was not launched transparently, as an improvement probably the error message could be clearer (like: "unable to connect to EPMD on port XXXX") and this sentence in http://erlang.org/doc/man/net_kernel.html#start-1 "Turns a non-distributed node into a distributed node by starting net_kernel and other necessary processes." could be rephrased in: "Turns a non-distributed node into a distributed node by starting net_kernel and other necessary (Erlang) processes, knowing that an EPMD instance is expected to be already running in the background."

Maybe it would be more user-friendly and consistent to have EPMD be launched automatically with net_kernel:start/1, though.

Thanks!

Best,

Olivier.

Olivier Boudeville
EDF R&D, Département SINETICS, Groupe ASICS, Bureau O2-E10
7, Boulevard Gaspard Monge, 91120 Palaiseau, France

Tél. : +33 (0)1 78 19 43 63
Tél. mobile : +33 (0)6 16 83 37 22





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