Questions about "erl -epmd_start false"

[Jérôme de Bretagne]

You're welcome, Shino.

Could you describe a case when the epmd daemon is running but you
wouldn't want the registration to happen? In your original question,
what is the actual issue with the node being registered when epmd is
running?

For anyone interested by the details, the code that triggers the
registration (via 'register_node') is found here:
   https://github.com/erlang/otp/blob/master/lib/kernel/src/erl_epmd.erl#L224-L245
and the 'erl_epmd_port' option is used only as a fallback when
'do_register_node' doesn't succeed in registering with the epmd
daemon.

With the default 'inet_tcp' distribution protocol, the call to
'register_node' is currently mandatory in 'gen_listen' here:
   https://github.com/erlang/otp/blob/master/lib/kernel/src/inet_tcp_dist.erl#L93-L97

With the 'gen_tcp_dist' alternative distribution protocol example, the
call to 'register_node' is also mandatory, this time in 'listen' here:
   https://github.com/erlang/otp/blob/master/lib/kernel/examples/gen_tcp_dist/src/gen_tcp_dist.erl#L86-L94
but it should be relatively easy to make a custom version that gives
the priority to the 'erl_epmd_port' option if preferred.

Cheers,
Jérôme


Le mar. 22 déc. 2020 à 04:03, Shunichi Shinohara
<shino.shun@REDACTED> a écrit :
>
> Maybe there are anyone interested, discussion continues in github PR:
>   Document the -no_epmd option by JeromeDeBretagne · Pull Request #2945 · erlang/otp
>   https://github.com/erlang/otp/pull/2945
>
> Thanks for the PR, Jérôme!
>
> -----
> Shino
>
> 2020年12月21日(月) 10:11 Shunichi Shinohara <shino.shun@REDACTED>:
>>
>> Hi Jérôme,
>>
>> Thanks a lot for the useful information, it works like a charm!
>>
>> Thanks again,
>> Shino
>>
>>
>> 2020年12月18日(金) 17:50 Jérôme de Bretagne <jerome.debretagne@REDACTED>:
>>>
>>> Hi Shino,
>>>
>>> This behavior is indeed expected.
>>>
>>> To avoir all registrations to epmd, you can pass the -no_epmd option instead.
>>>
>>> It is mentioned in the following documentation:
>>> https://erlang.org/doc/apps/erts/alt_dist.html
>>>
>>> Cheers,
>>> Jérôme
>>>
>>> Le ven. 18 déc. 2020 à 03:40, Shunichi Shinohara <shino.shun@REDACTED> a écrit :
>>>>
>>>> Hi List,
>>>>
>>>> I try "epmdlessless" configuration described in:
>>>>   Running Erlang Releases without EPMD on OTP 23.1+ · Erlware Blog
>>>>   https://blog.erlware.org/epmdlessless/
>>>>
>>>> It works as described, thanks to people who contributed to the feature!
>>>> However, it behaves in an unexpected way (to me) under epmd daemon running case.
>>>>
>>>> Expected behavior is the case without running epmd daemon. After launching a node as:
>>>>
>>>> % erl -sname foo -start_epmd false -erl_epmd_port 14693
>>>> Erlang/OTP 23 [erts-11.1.4] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1]
>>>>
>>>> epmd daemon does *NOT* start, as expected:
>>>>
>>>> % epmd -names
>>>> epmd: Cannot connect to local epmd
>>>>
>>>> On the other hand, after epmd daemon started as epmd -daemon,
>>>> by executing the same erl command above, the node is registered to epmd daemon:
>>>>
>>>> % epmd -names
>>>> epmd: up and running on port 4369 with data:
>>>> name foo at port 14693
>>>>
>>>> The questions are:
>>>> - Is this behavior expected (to other than me)?
>>>> - Is there any option(s) of erl command to avoid registration to epmd?
>>>>   # I guess it's possible by using https://github.com/tsloughter/epmdless
>>>>
>>>> Thanks,
>>>> Shino