usage of erlang heart

Question

How should heart be used to keep an application alive?

Lets say I have an application X, will it be monitored if I just call something like:

erl -boot X -heart -env HEART_BEAT_TIMEOUT 30 -detached

?

Answer 1

Yes, this will automatically start a heart process that monitors your node. See the heart documentation.

Update: Yes, Asymptote is correct. You also need a HEART_COMMAND environment variable to instruct heart of what to do when restarting the node.

Answer 2

http://www.erlang.org/doc/man/heart.html

This modules contains the interface to the heart process. heart
sends periodic heartbeats to an external port program, which is
also named `heart`. The purpose of the heart port program is to
check that the Erlang runtime system it is supervising is still
running. If the port program has not received any heartbeats within
`HEART_BEAT_TIMEOUT` seconds (default is 60 seconds), the system
can be rebooted. Also, if the system is equipped with a hardware
watchdog timer and is running Solaris, the watchdog can be used to
supervise the entire system.

<snip>

If the system should be rebooted because of missing heart-beats, or
a terminated Erlang runtime system, the environment variable
HEART_COMMAND has to be set before the system is started. If this
variable is not set, a warning text will be printed but the system
will not reboot.

Right now I have a Makefile with a statement in it that will run erl -heart ... for me. When I execute it here's the process list:

ubuntu    3814  3579  3814  3579  0 22:03 pts/0    00:00:00           make webstart
ubuntu    3829  3814  3814  3579 25 22:03 pts/0    00:00:01             /usr/local/lib/erlang/erts-5.8.3/bin/beam.smp -K true -A 5
ubuntu    3848  3829  3848  3848  0 22:03 ?        00:00:00               heart -pid 3829

When I kill PID 3829, the following output appears in the Erlang shell:

heart: Wed May 18 22:04:09 2011: Erlang has closed.
heart: Wed May 18 22:04:09 2011: Would reboot. Terminating.
make: *** [webstart] Terminated

So clearly I need to set HEART_COMMAND to some sort of reboot statement, and then heart will perform as required. AFAIK, given the description in the docs, heart is not intended to simply restart the Erlang VM on crashing; that sounds like something an Erlang supervisor should be doing for you, but I may be wrong.

(Of course, you could get just HEART_COMMAND to restart your Erlang program).