s6
Software
skarnet.org

Service directories

A service directory is a directory containing all the information related to a service, i.e. a long-running process maintained and supervised by s6-supervise.

(Strictly speaking, a service is not always equivalent to a long-running process. Things like Ethernet interfaces fit the definition of services one may want to supervise; however, s6 does not provide service supervision; it provides process supervision, and it is impractical to use the s6 architecture as is to supervise services that are not equivalent to one long-running process. However, we still use the terms service and service directory for historical and compatibility reasons.)

Contents

• An executable file named run. It can be any executable file (such as a binary file or a link to any other executable file), but most of the time it will be a script, called run script. This file is the most important one in your service directory: it contains the commands that will setup and run your foo service. It is forked and executed by s6-supervise every time the service must be started, i.e. normally when s6-supervise starts, and whenever the service goes down when it is supposed to be up. A run script should normally:
  • adjust redirections for stdin, stdout and stderr. For instance, if your service is logged, the run script should make sure that its stderr goes into the log pipe (which is on stdout by default), which is achieved by fdmove -c 2 1 in execline, and exec 2>&1 in shell. By default, in a normal supervision tree situation, a run script's stdin will be /dev/null, and its stdout and stderr will both be a pipe to a catch-all logging program.
  • adjust the environment for your foo daemon. Normally the run script inherits its environment from s6-supervise, which normally inherits its environment from s6-svscan, which normally inherits a minimal environment from the boot scripts. Service-specific environment variables should be set in the run script.
  • adjust other parameters for the foo daemon, such as its uid and gid. Normally the supervision tree, i.e. s6-svscan and the various s6-supervise processes, is run as root, so run scripts are also run as root; however, for security purposes, services should not run as root if they don't need to. You can use the s6-setuidgid utility in foo/run to lose privileges before executing into foo's long-lived process; or the s6-envuidgid utility if your long-lived process needs root privileges at start time but can drop them afterwards.
  • execute into the long-lived process that is to be supervised by s6-supervise, i.e. the real foo daemon. That process must not "background itself": being run by a supervision tree already makes it a "background" task.
• An optional executable file named finish. Like run, it can be any executable file. This finish script, if present, is executed everytime the run script dies. Generally, its main purpose is to clean up non-volatile data such as the filesystem after the supervised process has been killed. If the foo service is supposed to be up, foo/run is restarted after foo/finish dies. A finish script must do its work and exit in less than 3 seconds; if it takes more than that, it is killed. (The point is that the run script, not the finish script, should be running; the finish script should really be short-lived.)
• A directory named supervise. It is automatically created by s6-supervise if it does not exist. This is where s6-supervise stores its information. The directory must be writable.
• An optional, empty, regular file named down. If such a file exists, the default state of the service is considered down, not up: s6-supervise will not automatically start it until it receives a s6-svc -u command. If no down file exists, the default state of the service is up.
• An optional, empty, regular file named nosetsid. If such a file exists, s6-supervise will not make the service a process group and session leader; the service will be run in the same process group as s6-supervise. If no nosetsid file exists, the service has its own process group and is started as a session leader.
• A fifodir named event. It is automatically created by s6-supervise if it does not exist. foo/event is the rendez-vous point for listeners, where s6-supervise will send notifications when the service goes up or down.
• An optional service directory named log. If it exists and foo is in a scandir, and s6-svscan runs on that scandir, then two services are monitored: foo and foo/log. A pipe is open and maintained between foo and foo/log, i.e. everything that foo/run writes to its stdout will appear on foo/log/run's stdin. The foo service is said to be logged; the foo/log service is called foo's logger. A logger service cannot be logged: if foo/log/log exists, nothing special happens.

Service directories describe the way services are launched. Once they are designed, they have little reason to change on a given machine. They can theoretically reside on a read-only filesystem - for instance, the root filesystem, to avoid problems with mounting failures.

However, two subdirectories - namely supervise and event - of every service directory need to be writable. So it has to be a bit more complex. Here are a few possibilities.

• The laziest option: you're not using s6-svscan as process 1, you're only using it to start a collection of services, and your booting process is already handled by another init system. Then you can just store your service directories and your scan directory on some read-write filesystem such as /var; and you tell your init system to launch (and, if possible, maintain) s6-svscan on the scan directory after that filesystem is mounted.
• The almost-as-lazy option: just have the service directories on the root filesystem. Then your service directory collection is for instance in /etc/services and you have a /service scan directory containing symlinks to that collection. This is the easy setup, not requiring an external init system to mount your filesystems - however, it requires your root filesystem to be read-write, which is unacceptable if you are concerned with reliability - if you are, for instance, designing an embedded platform.
• Some people like to have their service directories in a read-only filesystem, with supervise symlinks pointing to various places in writable filesystems. This setup looks a bit complex to me: it requires careful handling of the writable filesystems, with not much room for error if the directory structure does not match the symlinks (which are then dangling). But it works.
• Service directories are usually small; most daemons store their information elsewhere. Even a complete set of service directories often amounts to less than a megabyte of data - sometimes much less. Knowing this, it makes sense to have an image of your service directories in the (possibly read-only) root filesystem, and copy it all to a scan directory located on a RAM filesystem that is mounted at boot time. This is the setup I recommend. It has several advantages:
  • Your service directories reside on the root filesystem and are not modified during the lifetime of the system. If your root filesystem is read-only and you have a working set of service directories, you have the guarantee that a reboot will set your system in a working state.
  • Every boot system requires an early writeable filesystem, and many create it in RAM. You can take advantage of this to copy your service directories early and run s6-svscan early.
  • No dangling symlinks or potential problems with unmounted filesystems: this setup is robust. A simple /bin/cp -a or tar -x is all it takes to get a working service infrastructure.
  • You can make temporary modifications to your service directories without affecting the main ones, safely stored on the disk. Conversely, every boot ensures clean service directories - including freshly created supervise and event subdirectories. No stale files can make your system unstable.