S6-SUPERVISE(8)

NAME
s6-supervise - monitor a long-lived process (or service), making sure it stays alive, sending notifications to registered processes when it dies, and providing an interface to control its state

SYNOPSIS
s6-supervise servicedir

DESCRIPTION
s6-supervise is designed to be the last non-leaf branch of a supervision tree, the supervised process being a leaf. s6-supervise' s behaviour is approximately the following:

•   s6-supervise changes its current directory to servicedir.

•   It exits 100 if another s6-supervise process is already monitoring this service.

•   It forks and executes the ./run file in the service directory.

•   ./run should be a long-lived process: it can chain load (i.e. exec into other binaries), but should not die. It's the daemon that s6-supervise monitors and manages.

•   When ./run dies, s6-supervise spawns ./finish, if it exists. This script should be short-lived: it's meant to clean up application state, if necessary, that has not been cleaned up by ./run itself before dying.

•   When ./finish dies, s6-supervise spawns ./run again.

•   s6-supervise operation can be controlled by the s6-svc(8) program. It can be sent commands like “restart the service”, “bring the service down”, etc.

•   s6-supervise normally runs forever. If told to exit by s6-svc(8), it waits for the service to go down one last time, then exits 0.

For a precise description of s6-supervise' s behaviour, check the Detailed operation section below, as well as the s6-service-directory(7) page: s6-supervise operation can be extensively configured by the presence of certain files in the service directory.

  Readiness notification support
If the s6-service-directory(7) contains a valid notification-fd file when the service is started, or restarted, s6-supervise creates and listens to an additional pipe from the service for readiness notification; refer to s6-notifywhenup(7). When the notification occurs, s6-supervise updates the ./supervise/status file accordingly, then sends a ‘U’ event to ./event.

If the service is logged, i.e. if the service directory has a log subdirectory that is also a service directory, and the s6-supervise process has been launched by that is also s6-svscan(8), then by default the service's stdout goes into the logging pipe. If you set notification-fd to 1, the logging pipe will be overwritten by the notification pipe, which is probably not what you want. Instead, if your daemon writes a notification message to its stdout, you should set notification-fd to (for instance) 3, and redirect outputs in your run script. For instance, to redirect stderr to the logger and stdout to a notification-fd set to 3, you would start your daemon as ‘fdmove -c 2 1 fdmove 1 3 prog...’ (in execline), or ‘exec 2>&1 1>&3 3<&- prog...’ (in shell).

  Signals
s6-supervise reacts to the following signals:

SIGTERM
    Bring down the service and exit, as if a ‘s6-svc -xd’ command had been received.

SIGHUP
    Close its own stdin and stdout, and exit as soon as the service stops, as if an ‘s6-svc -x’ command had been received.

SIGQUIT
    Exit immediately without touching the service in any way.

SIGINT
    Send a SIGINT to the process group of the service, then exit immediately. (The point here is to correctly forward SIGINT in the case where s6-supervise is running in a terminal and the user sent ‘^C’ to interrupt it.)

  Detailed operation

•   s6-supervise switches to the servicedir s6-service-directory(7).

•   It creates a supervise/ subdirectory (if it doesn't exist yet) to store its internal data.

•   It exits 100 if another s6-supervise process is already monitoring this service.

•   If the ./event s6-fifodir(7) does not exist, s6-supervise creates it and allows subscriptions to it from processes having the same effective group id as the s6-supervise process. If it already exists, it uses it as is, without modifying the subscription rights.

•   It sends an ‘s’ event to ./event. ( Refer to s6-ftrigw(7))

•   If the default service state is up (i.e. there is no ./down file), s6-supervise spawns ./run. One argument is given to the ./run program: servicedir, the name of the directory s6-supervise is being run on. It is given exactly as given to s6-supervise, without recanonicalization. In particular, if s6-supervise is being managed by s6-svscan(8), servicedir is always of the form foo or foo/log, and foo contains no slashes.

•   s6-supervise sends a ‘u’ event to ./event whenever it successfully spawns ./run.

•   If there is a ./notification-fd file in the service directory and, at some point after the service has been spawned, s6-supervise is told that the service is ready, it sends a ‘U’ event to ./event. There are several ways to tell s6-supervise that the service is ready:

    •   the daemon may do so itself; refer to s6-notifywhenup(7).

    •   the run script may have forked a s6-notifyoncheck(8) process that polls the service for readiness.

•   When ./run dies, s6-supervise sends a ‘d’ event to ./event. It then spawns ./finish if it exists. ./finish will have ./run' s exit code as first argument, or 256 if ./run was signaled; it will have the number of the signal that killed ./run as second argument, or an undefined number if ./run was not signaled; and it will have servicedir as third argument.

•   By default, ./finish must exit in less than 5 seconds. If it takes more than that, s6-supervise kills it with a SIGKILL. This can be configured via the ./timeout-finish file, see the description in s6-service-directory(7).

•   When ./finish dies (or is killed), s6-supervise sends a ‘D’ event to ./event. Then it restarts ./run unless it has been told not to.

•   If ./finish exits 125, then s6-supervise sends a ‘O’ event to ./event before the ‘D’, and it does not restart the service, as if ‘s6-svc -O’ had been called. This can be used to signify permanent failure to start the service.

•   There is a minimum 1-second delay between two ./run spawns, to avoid busylooping if ./run exits too quickly. If the service has been ready for more than one second, it will restart immediately, but if it is not ready when it dies, s6-supervise will always pause for 1 second before spawning it again.

•   When killed or asked to exit, it waits for the service to go down one last time, then sends a ‘x’ event to ./event before exiting 0.

Make sure to also check the s6-service-directory(7) documentation page, for the full list of files that can be present in a service directory and impact s6-supervise' s behaviour in any way.

s6-supervise is a long-lived process. It normally runs forever, from the system's boot scripts, until shutdown time; it should not be killed or told to exit. If you have no use for a service, just turn it off; the s6-supervise process does not hurt.

Even in boot scripts, s6-supervise should normally not be run directly. It's better to have a collection of s6-service-directory(7)s in a single s6-scan-directory(7), and just run s6-svscan(8) on that scan directory. s6-svscan(8) will spawn the necessary s6-supervise processes, and will also take care of logged services.

s6-supervise always spawns its child in a new session, as a session leader. The goal is to protect the supervision tree from misbehaved services that would send signals to their whole process group. Nevertheless, s6-supervise' s handling of SIGINT ensures that its service is killed if you happen to run it in a terminal and send it a ‘^C’.

You can use s6-svc(8) to send commands to the s6-supervise process; mostly to change the service state and send signals to the monitored process.

You can use s6-svok(8) to check whether s6-supervise is successfully running.

You can use s6-svstat(8) to check the status of a service.

s6-supervise maintains internal information inside the ./supervise subdirectory of servicedir. servicedir itself can be read-only, but both servicedir /supervise and servicedir /event need to be read-write.

If servicedir isn't writable by s6-supervise, for any reason, then the s6-svc(8) -D and -U commands will not work properly since s6-supervise will be unable to create or delete a servicedir/down file; in this case s6-supervise will print a warning on stderr, and perform the equivalent of -d or -u instead — it will just be unable to change the permanent service configuration.

OPTIONS
s6-supervise does not support options, because it is normally not run manually via a command line; it is usually launched by s6-svscan(8) as part of a supervision tree.

The way to tune s6-supervise' s behaviour is via files in the s6-service-directory(7). There are a lot of configuration knobs there; if you want to get the most out of your supervisor, reading this page is strongly advised.

IMPLEMENTATION NOTES
s6-supervise tries its best to stay alive and running despite possible system call failures. It will write to its standard error everytime it encounters a problem. However, unlike s6-svscan(8), it will not go out of its way to stay alive; if it encounters an unsolvable situation, it will just die.

Unlike other “supervise” implementations, s6-supervise is a fully asynchronous state machine. That means that it can read and process commands at any time, even when the machine is in trouble (full process table, for instance).

s6-supervise does not use malloc(3). That means it will never leak memory. However, s6-supervise uses opendir(3), and most opendir(3) implementations internally use heap memory - so unfortunately, it's impossible to guarantee that s6-supervise does not use heap memory at all.

s6-supervise has been carefully designed so every instance maintains as little data as possible, so it uses a very small amount of non-sharable memory. It is not a problem to have several dozens of s6-supervise processes, even on constrained systems: resource consumption will be negligible.

SEE ALSO
s6-notifyoncheck(8), s6-permafailon(8), s6-svc(8), s6-svdt(8), s6-svdt-clear(8), s6-svlisten(8), s6-svlisten1(8), s6-svok(8), s6-svscan(8), s6-svscanctl(8), s6-svstat(8), s6-svwait(8)

This man page is ported from the authoritative documentation at: https://skarnet.org/software/s6/s6-supervise.html

AUTHORS
Laurent Bercot Alexis <flexibeast@gmail.com> (man page port)