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supervise-daemon-guide.md

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Using supervise-daemon

Beginning with OpenRC-0.21 we have our own daemon supervisor, supervise-daemon., which can start a daemon and restart it if it terminates unexpectedly.

The following is a brief guide on using this capability.

  • Use Default start, stop and status functions If you write your own start, stop and status functions in your service script, none of this will work. You must allow OpenRC to use the default functions.

  • Daemons must not fork Any deamon that you would like to have monitored by supervise-daemon must not fork. Instead, it must stay in the foreground. If the daemon forks, the supervisor will be unable to monitor it.

    If the daemon can be configured to not fork, this should be done in the daemon's configuration file, or by adding a command line option that instructs it not to fork to the command_args_foreground variable shown below.

Health Checks

Health checks are a way to make sure a service monitored by supervise-daemon stays healthy. To configure a health check for a service, you need to write a healthcheck() function, and optionally an unhealthy() function in the service script. Also, you will need to set the healthcheck_timer and optionally healthcheck_delay variables.

healthcheck() function

The healthcheck() function is run repeatedly based on the settings of the healthcheck_* variables. This function should return zero if the service is currently healthy or non-zero otherwise.

unhealthy() function

If the healthcheck() function returns non-zero, the unhealthy() function is run, then the service is restarted. Since the service will be restarted by the supervisor, the unhealthy function should not try to restart it; the purpose of the function is to allow any cleanup tasks other than restarting the service to be run.

Variable Settings

The most important setting is the supervisor variable. At the top of your service script, you should set this variable as follows:

supervisor=supervise-daemon

Several other variables affect the way services behave under supervise-daemon. They are documented on the openrc-run man page, but I will list them here for convenience:

command_args_foreground="arguments"

This should be used if the daemon you want to monitor forks and goes to the background by default. This should be set to the command line option that instructs the daemon to stay in the foreground.

healthcheck_delay=seconds

This is the delay, in seconds, before the first health check is run. If it is not set, we use the value of healthcheck_timer.

healthcheck_timer=seconds

This is the number of seconds between health checks. If it is not set, no health checks will be run.

respawn_delay

This is the number of seconds to delay before attempting to respawn a supervised process after it dies unexpectedly. The default is to respawn immediately.

respawn_max=x

This is the maximum number of times to respawn a supervised process during the given respawn period. The default is 10. 0 means unlimited.

respawn_period=seconds

This works in conjunction with respawn_max and respawn_delay above to decide if a process should not be respawned for some reason.

For example, if respawn period is 10 and respawn_max is 2, the process would need to die 3 times within 10 seconds to no longer be respawned. Note that respawn_delay will delay all of this, so in the above scenario a respawn_delay of greater than 5 will cause infinite respawns.

By default, this is unset and respawn_max applies to the entire lifetime of the service.