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NAME
heart - 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 completely 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.