[erlang-questions] Heart behavior

[Lukas Larsson]

Hello Bogdan,

See some answers inline:

On Mon, Jul 6, 2015 at 10:33 AM, Bogdan Andu <bog495@REDACTED> wrote:

> Hi,
>
> I was made some experiments with heart
> and I found something surpizing, athough it
> does the job.
>
> I start a erlang vm in daemon mode under user called
> _user0 with home in /var/app like this
> (from a shell script /var/app/appd run with sudo as a priv user):
>
> case $1 in
>   start)
>
> su - _user0 -c "$ERL -boot start_sasl -config $LOG +K true +A 4 -sname
> $NODE  -heart -detached -s app_ctl start $NODE"
>
> ;;
>
>   restart)
>      /usr/local/lib/erlang/lib/erl_interface-3.7.20/bin/erl_call -q -sname
> $NODE
>      sleep 2
>      $ERL -boot start_sasl -config $LOG +K true +A 4 -sname $NODE\
>                                  -heart -detached -s app_ctl start $NODE
>     ;;
>
> ....
>
> exit 0
>
> environment vars are(under user _user0):
>
> HEART_COMMAND=/bin/sh /var/app/appd restart
> ERL_CRASH_DUMP_SECONDS=10
>
> I have noticed 3 problems:
> 1)  Starting the daemon (as a priv user) with sudo sh /var/app/appd start
>      it starts the heart subsystem, but when I issue sudo kill -9
> <pid-of_erlang-
>     vm-monitored-byheart>, the erlang vm is killed but heart never
> restarts it;
>     Running as _user0 the command /bin/sh /var/app/appd restart manually
>     heart restarts the system monitored after was killed;
>

It sounds as if heart for some reason cannot execute the HEART_COMMAND. Why
that might be I don't know, maybe you could try to run it as a non-daemon,
or at least redirect the stderr printouts to some file. heart might print
things to stderr if it cannot execute HEART_COMMAND.


>
> 2) Everytime I kill a vm monitored by heart with kill -9 <pid-of-vm> the
> heart procces restarts it immediately, and after that the heart process
> dies itself,and if in restart is not mentioned -heart option, the heart
> process is not restarted for the newly restarted erlang vm.
>

When you supply -heart to the erlang vm command line you tell that VM to
monitor itself using the heart mechanism. So if, as you say, you don't pass
-heart on the HEART_COMMAND command, the new vm will not be restarted. This
is by design.


>
> 3) It seems the default timeout of 60 seconds is not respected because
> the vm is restarted immediately -heart option is specified in restart
> script;
>

Which timeout is it that you are referring to here? HEART_BEAT_TIMEOUT or
HEART_BEAT_BOOT_DELAY? HEART_BEAT_TIMEOUT is the maximum time it will take
for heart to detect that something is wrong with the VM, if it can detect
that something is wrong earlier then it will.


>
> So a heart process is tied up to an erlang vm that it monitors and it dies
> after it spawns another erlang vm?
>

yes


> The docs are not clear about these.
>
> Having said these what are the best practices to use heart and why
> heart behaves like above?
>
> It seems heart works with kill -KILL|SIGV <pid-of-vm>, but I  am not sure
> what happens if the erlang vm crashes when runs out of memory of file
> descriptors.
> Is the vm restarted by heart in these conditions?
>

It should be. The only reason for heart not to restart the VM (that I can
think of right now) is if you call init:stop(), or if the command line that
you gave to HEART_COMMAND does not work.


>
> System: OTP 17.5 64 bits
>
> Thanks,
> Bogdan
>
>
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