Kernel

Reference Manual

Version 6.0

Table of Contents

heart

Module

heart

Module Summary

Heartbeat monitoring of an Erlang runtime system.

Description

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 (defaults to 60 seconds), the system can be rebooted.

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

% erl -heart ...

If the system is to be rebooted because of missing heartbeats, or a terminated Erlang runtime system, environment variable HEART_COMMAND must be set before the system is started. If this variable is not set, a warning text is printed but the system does not reboot.

To reboot on Windows, HEART_COMMAND can be set to heart -shutdown (included in the Erlang delivery) or to any other suitable program that can activate a reboot.

The environment variable HEART_BEAT_TIMEOUT can be used to configure the heart time-outs; it 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.

When running on OSs lacking support for monotonic time, heart is susceptible to system clock adjustments of more than HEART_BEAT_TIMEOUT seconds. When this happens, heart times out and tries to reboot the system. This can occur, for example, if the system clock is adjusted automatically by use of the Network Time Protocol (NTP).

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

% erl -heart -env ERL_CRASH_DUMP_SECONDS 10 ...

If a regular core dump is wanted, let heart know by setting the kill signal to abort using environment variable HEART_KILL_SIGNAL=SIGABRT. If unset, or not set to SIGABRT, the default behavior is a kill signal using SIGKILL:

% erl -heart -env HEART_KILL_SIGNAL SIGABRT ...

If heart should not kill the Erlang runtime system, this can be indicated using the environment variable HEART_NO_KILL=TRUE. This can be useful if the command executed by heart takes care of this, for example as part of a specific cleanup sequence. If unset, or not set to TRUE, the default behaviour will be to kill as described above. 

% erl -heart -env HEART_NO_KILL 1 ...

Furthermore, ERL_CRASH_DUMP_SECONDS has the following behavior on heart:

ERL_CRASH_DUMP_SECONDS=0

Suppresses the writing of 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 does not reboot the runtime system until the crash dump file is completly written.

ERL_CRASH_DUMP_SECONDS=S

heart waits for S seconds to let the crash dump file be written. After S seconds, heart reboots the runtime system, whether the crash dump file is written or not.

In the following descriptions, all functions fail with reason badarg if heart is not started.

Data Types

heart_option() = check_schedulers

Exports

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 is to be used to reboot the system. The new Erlang runtime system uses (if it misbehaves) environment variable HEART_COMMAND to reboot.

Limitations: Command string Cmd is sent to the heart program as an ISO Latin-1 or UTF-8 encoded binary, depending on the filename 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()

Gets the temporary reboot command. If the command is cleared, the empty string is returned.

set_callback(Module, Function) ->
                ok | {error, {bad_callback, {Module, Function}}}

Types

Module = Function = atom()

This validation callback will be executed before any heartbeat is sent to the port program. For the validation to succeed it needs to return with the value ok.

An exception within the callback will be treated as a validation failure.

The callback will be removed if the system reboots.

clear_callback() -> ok

Removes the validation callback call before heartbeats.

get_callback() -> {ok, {Module, Function}} | none

Types

Module = Function = atom()

Get the validation callback. If the callback is cleared, none will be returned.

set_options(Options) -> ok | {error, {bad_options, Options}}

Types

Options = [heart_option()]

Valid options set_options are:

check_schedulers

If enabled, a signal will be sent to each scheduler to check its responsiveness. The system check occurs before any heartbeat sent to the port program. If any scheduler is not responsive enough the heart program will not receive its heartbeat and thus eventually terminate the node.

Returns with the value ok if the options are valid.

get_options() -> {ok, Options} | none

Types

Options = [atom()]

Returns {ok, Options} where Options is a list of current options enabled for heart. If the callback is cleared, none will be returned.

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