heart (3erl)

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.

An Erlang runtime system to be monitored by a heart program, should be started with the command line flag -heart (see also erl(1)). The heart process is then started automatically:

% erl -heart ...

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. However, if the hardware watchdog is used, it will trigger a reboot HEART_BEAT_BOOT_DELAY seconds later nevertheless (default is 60).

To reboot on the WINDOWS platform HEART_COMMAND can be set to heart -shutdown (included in the Erlang delivery) or of course to any other suitable program which can activate a reboot.

The hardware watchdog will not be started under Solaris if the environment variable HW_WD_DISABLE is set.

The HEART_BEAT_TIMEOUT and HEART_BEAT_BOOT_DELAY environment variables can be used to configure the heart timeouts, they can be set in the operating system shell before Erlang is started or be specified at the command line:

% erl -heart -env HEART_BEAT_TIMEOUT 30 ...

The value (in seconds) must be in the range 10 < X <= 65535.

It should be noted that if the system clock is adjusted with more than HEART_BEAT_TIMEOUT seconds, heart will timeout and try to reboot the system. This can happen, for example, if the system clock is adjusted automatically by use of NTP (Network Time Protocol).

If a crash occurs, an erl_crash.dump will not be written unless the environment variable ERL_CRASH_DUMP_SECONDS is set.

% erl -heart -env ERL_CRASH_DUMP_SECONDS 10 ...

Furthermore, ERL_CRASH_DUMP_SECONDS has the following behaviour on heart:

ERL_CRASH_DUMP_SECONDS=0: Suppresses the writing a crash dump file entirely, thus rebooting the runtime system immediately. This is the same as not setting the environment variable.

ERL_CRASH_DUMP_SECONDS=-1: Setting the environment variable to a negative value will not reboot the runtime system until the crash dump file has been completly written.

ERL_CRASH_DUMP_SECONDS=S: Heart will wait for S seconds to let the crash dump file be written. After S seconds heart will reboot the runtime system regardless of the crash dump file has been written or not.

In the following descriptions, all function fails with reason badarg if heart is not started.

set_cmd(Cmd) -> ok | {error, {bad_cmd, Cmd}}

Types:

Cmd = string()

Sets a temporary reboot command. This command is used if a HEART_COMMAND other than the one specified with the environment variable should be used in order to reboot the system. The new Erlang runtime system will (if it misbehaves) use the environment variable HEART_COMMAND to reboot.

Limitations: The Cmd command string will be sent to the heart program as a ISO-latin-1 or UTF-8 encoded binary depending on the file name encoding mode of the emulator (see file:native_name_encoding/0). The size of the encoded binary must be less than 2047 bytes.

clear_cmd() -> ok

Clears the temporary boot command. If the system terminates, the normal HEART_COMMAND is used to reboot.

get_cmd() -> {ok, Cmd}

Types:

Cmd = string()

Get the temporary reboot command. If the command is cleared, the empty string will be returned.