s6: an overview
Keywords: s6 overview supervision init process unix

s6
Software
skarnet.org

# An overview of s6

s6 is a collection of utilities revolving around process supervision and management, logging, and system initialization. This page is a high-level description of the different parts of s6.

## Process supervision

At its core, s6 is a process supervision suite, like its ancestor daemontools and its close cousin runit.

### Concept

The concept of process supervision comes from several observations:

- Unix systems, even minimalistic ones, need to run long-lived processes, aka daemons. That is one of the core design principles of Unix: one service → one daemon.
- Daemons can die unexpectedly. Maybe they are missing a vital resource and cannot handle a certain failure; maybe they tripped on a bug; maybe a misconfigured administration program killed them; maybe the kernel killed them. Processes are fragile, but daemons are vital to a Unix system: a fundamental discrepancy that needs to be solved.
- Automatically restarting daemons when they die is generally a good thing. In any case, sysadmin intervention is necessary, but at least the daemon is providing service, or trying to, until the sysadmin can log in and investigate the underlying problem.
- Ad-hoc shell scripts that restart daemons suck, for several reasons that would each justify their own page. The difficulty of keeping track of the PID, explained below, is one of those reasons.
- It is sometimes necessary to send signals to a daemon. To kill it, of course, but also to make it read its config file again, for instance; signalling a daemon is a natural and very common way of sending it simple commands.
- Generally, to send a signal to a daemon, you need to know its PID. Without a supervision suite, knowing the proper PID is hard. Most non-supervision systems use a hack known as .pid files, i.e. the script that starts the daemon stores its PID into a file, and other scripts read that file. This is a bad mechanism for several reasons, and the case against .pid files would also justify its own page; the most important drawback of .pid files is that they create race conditions and management scripts may kill the wrong process.
- Non-supervision systems provide scripts to start and stop daemons, but those scripts may fail at boot time even though they work when run manually, and vice versa. If a sysadmin logs in and runs the script to restart a daemon that has died, the result might not be the same as if the whole system had been rebooted, and the daemon may exhibit strange behaviours! This is because the boot-time environment and the restart-time environment are not the same when the script is run; and a non-supervision system just cannot ensure reproducibility of the environment. This is a core problem of non-supervision systems: countless bugs have been falsely reported because of simple environment differences or configuration errors, countless man-hours have been wasted to try and understand what was going on.

A process supervision system organizes the process hierarchy in a radically different way.

- A process supervision system starts an independent hierarchy of processes at boot time, called a supervision tree. This supervision tree never dies: when one of its components dies, it is restarted automatically. To ensure availability of the supervision tree at all times, it should be rooted in process 1, which cannot die.
- A daemon is never started, either manually or in a script, as a scion of the script that starts it. Instead, to start a daemon, you configure a specific directory which contains all the information about your daemon; then you send a command to the supervision tree. The supervision tree will start the daemon as a leaf. In a process supervision system, daemons are always spawned by the supervision tree, and never by an admin's shell.
- The parent of your daemon is a supervisor. Since your daemon is its direct child, the supervisor always knows the correct PID of your daemon.
- The supervisor watches your daemon and can restart it when it dies, automatically.
- The supervision tree always has the same environment, so starting conditions are reproducible. Your daemon will always be started with the same environment, whether it is at boot time via init scripts or for the 100th automatic - or manual - restart.
- To send signals to your daemon, you send a command to its supervisor, which will then send a signal to the daemon on your behalf. Your daemon is identified by the directory containing its information, which is stable, instead of by its PID, which is not stable; the supervisor maintains the correct association without a race condition or the other problems of .pid files.

### Implementation

s6 is a straightforward implementation of those concepts.

- The s6-svscan and s6-supervise programs are the components of the supervision tree. They are long-lived programs.
  - s6-supervise is a daemon's supervisor, its direct parent. For every long-lived process on a system, there is a corresponding s6-supervise process watching it. This is okay, because every instance of s6-supervise uses very few resources.
  - s6-svscan is, in a manner of speaking, a supervisor for the supervisors. It watches and maintains a collection of s6-supervise processes: it is the branch of the supervision tree that all supervisors are stemming from. It can be run and supervised by your regular init process, or it can run as process 1 itself. Running s6-svscan as process 1 requires some effort from the user, because of the inherent non-portability of init processes; the s6-linux-init package automates that effort and allows users to run s6 as an init replacement.
  - The configuration of a daemon to be supervised by s6-supervise is done via a service directory.
  - The place to gather all service directories to be watched by a s6-svscan instance is called a scan directory.
- The command that controls a single supervisor, and allows you to send signals to a daemon, is s6-svc. It is a short-lived program.
- The command that controls a set of supervisors, and allows you to start and stop supervision trees, is s6-svscanctl. It is a short-lived program.

These four programs, s6-svscan, s6-supervise, s6-svscanctl and s6-svc, are the very core of s6. Technically, once you have them, you have a functional s6 installation, and the other utilities are just a bonus.

### Practical usage

To use s6's supervision features, you need to perform the following steps:

- For every daemon you potentially want supervised, write a service directory. Make sure that your daemon does not background itself when started in the `./run` script! Auto-backgrounding is a historical hack that was implemented when supervision suites did not exist; since you're using a supervision suite, auto-backgrounding is unnecessary and in this case detrimental.
- Write a single scan directory for the set of daemons you want to actually run. This set can be modified at run time.
- At some point in your initialization scripts, run s6-svscan on the scan directory. This will start the supervision tree, including your set of daemons. The exact way of running s6-svscan depends on your system: it is not quite the same when you want to run it as process 1 on a real machine, or under another init on a real machine, or as process 1 in a Docker container, or in another context entirely.
- Alternatively, you can start s6-svscan on an empty scan directory, then populate it step by step and send an update command to s6-svscan via s6-svscanctl whenever the supervision tree should pick up the differences and start the services you added.
- That's it, your services are running. To control them manually, you can use the s6-svc command.
- At the end of the system's lifetime, you can use s6-svscanctl to bring down the supervision tree.

## Service-specific logging

s6-svscan can monitor a supervision tree, but it can also do one more thing. It can ensure that a daemon's log, i.e. what the daemon outputs to its stdout (or stderr if you redirect it), gets processed by another, supervised, long-lived process, called a logger; and it can make sure that the logs are never lost between the daemon and the logger - even if the daemon dies, even if the logger dies.

If your daemon is outputting messages, you have a decision to make about where to send them.

- You can do as non-supervision systems do, and send the messages to syslog. It's entirely possible with a supervision system too. However, like auto-backgrounding, syslog is a historical mechanism that predates supervision suites, and is technically inferior; it is recommended that you do not use it whenever you can avoid it.
- You can send them to the daemon's stdout/stderr and do nothing special about it. The logs will then be sent to s6-svscan's stdout/stderr; what mechanism will read them depends on how you started s6-svscan.
- You can use s6-svscan's service-specific logging mechanism and dedicate a logger process to your daemon's messages.

s6 provides you with a long-lived process to use as a logger: s6-log. It will store your logs in one (or more) specific directory of your choice, and rotate them automatically.

## Helpers for run scripts

Creating a working service directory, and especially a good run script, is the most important part of the work when adapting a daemon to a supervision framework.

If you can find your daemon's invocation script on a non-supervision system, for instance a System V-style init script, you can see the exact options that the daemon is being run with: environment variables, uid and gid, open descriptors, etc. This is what you need to replicate in your run script.

(Do not replicate the auto-backgrounding, or things like start-stop-daemon invocation: start-stop-daemon and its friends are hideous and kludgy attempts to work around the lack of proper supervision mechanisms. Now that you have s6, you should remove them from your system, throw them into a bonfire, and dance and laugh while they burn. Generally speaking, as a system administrator you want daemons that have been designed following the principles described here, or at least you want to use the command-line options that make them behave in such a way.)

The vast majority of the tools provided by s6 are meant to be used in run scripts: they help you control the process state and environment in your script before it executes into your daemon. Or, sometimes, they are daemons themselves, designed to be supervised.

s6, like other skarnet.org software, makes heavy use of chain loading, also known as "Bernstein chaining": a lot of s6 tools will perform some action that changes the process state, then execute into the rest of their command line. This allows the user to change the process state in a very flexible way, by combining the right components in the right order. Very often, a run script can be reduced to a single command line - likely a long one, but still a single one. (That is the main reason why using the execline language to write run scripts is recommended: execline makes it natural to handle long command lines made of massive amounts of chain loading. This is by no means mandatory, though: a run script can be any executable file you want, provided that running it eventually results in a long-lived process with the same PID.)

Some examples of s6 programs meant to be used in run scripts:

- The s6-log program is a long-lived process. It is meant to be executed into by a `./log/run` script: it will be supervised, and will process what it reads on its stdin (i.e. the output of the `./run` daemon).
- The s6-envdir program is a short-lived process that will update its current environment according to what it reads in a given directory, then execute into the rest of its command line. It is meant to be used in a run script to adjust the environment with which the final daemon will be executed into.
- Similarly, the s6-softlimit program adjusts its resource limits, then executes into the rest of its command line: it is meant to set the resources the final daemon will have access to.
- The s6-applyuidgid program, part of the `s6-*uidgid` family, drops root privileges before executing into the rest of its command line: it is meant to be used in run scripts that need root privileges when starting but do not need it for the execution of the long-lived process.
- s6-ipcserverd is a daemon that listens to a Unix socket and spawns a program for every connection. It is meant to be supervised, so it should be used in a run script, and it's also meant to be a flexible super-server that you can use for different applications: so it is a building block that may appear in several of your run scripts defining local services.

## Readiness notification and dependency management

Now that you have a supervision tree, and long-lived processes running supervised, you may want to introduce dependencies between them: do not perform an action (e.g. start (with s6-svc -u) the Web server connecting to a database) before a given daemon is up and running (e.g. the database server). s6 provides tools to do that:

- The s6-svwait, s6-svlisten1 and s6-svlisten programs will wait until a set of daemons is up, ready, down (as soon as the `./run` process dies) or really down (when the `./finish` process has also died).
- Unfortunately, a daemon being up does not mean that it is ready: this page goes into the details. s6 supports a simple mechanism: when a daemon wants to signal that it is ready, it simply writes a newline to a file descriptor of its choice, and s6-supervise will pick that notification up and broadcast the information to processes waiting for it.
- s6 also has a legacy mechanism for daemons that do not notify their own readiness but provide a way for an external program to check whether they're ready or not: s6-notifyoncheck. This is polling, which is bad, but unfortunately necessary for many daemons as of 2019.

s6 does not provide a complete dependency management framework, i.e. a program to automatically start (or stop) a set of services in a specific order - that order being automatically computed from a graph of dependencies between services. That functionality belongs to a service manager, and is implemented for instance in the s6-rc package.

## Fine-grained control over services

s6 provides you with a few more tools to control and monitor your services. For instance:

- s6-svstat gives you access to the detailed state of a service
- s6-svperms allows you to configure what users can read that state, what users can send control commands to your service, and what users can be notified of service start/stop events
- s6-svdt allows you to see what caused the latest deaths of a supervised process

These tools make s6 the most powerful and flexible of the existing process supervision suites.

## Additional utilities

The other programs in the s6 package are various utilities that may be useful in designing servers, and more generally multi-process software. They can be used with or without a supervision environment, although it is of course recommended to have one; but they are not part of the core s6 functionality, and you may safely ignore them for now if you are just getting into the supervision world.

### Generic inter-process notification

The `s6-ftrig*` family of programs allows notifications between unrelated processes: a set of processes can subscribe to a certain channel - identified by a directory in the filesystem - and ask to be notified of certain events on that channel; another set of processes can send events to the channel.

The underlying mechanism is the same as the one used by the supervision tree for readiness notification, but the `s6-ftrig*` tools provide a more generic access to that mechanism.

### Helpers for designing local services

Local services, i.e. daemons listening to a Unix domain socket, are a powerful and flexible mechanism, especially with modern Unix systems that allow client authentication. s6 includes tools to take advantage of that mechanism.

- The `s6-ipc*` family of programs is about designing clients or servers that communicate over Unix domain sockets.
- The `s6-*access*` and s6-connlimit family of programs is about client access control.
- The `s6-sudo*` family of programs is about using a local service in order to give selected clients the ability to run a command line with the privileges of the server, without using suid programs.

### Keeping file descriptors open

Sometimes you want to keep a file descriptor open, even if the program normally using it dies - so the program can restart and use the same file descriptor without losing any data. To do that, you need to hold the descriptor in another process, i.e. that process should have it open but do nothing with it.

s6-svscan, for instance, holds the pipe existing between a supervised daemon and its logger, so even if the daemon or the logger dies while there are logs in the pipe, the pipe remains open and the logs are not lost.

s6 provides a mechanism to store and retrieve open file descriptors in a totally generic way: the `s6-fdholder*` family of programs.

- The s6-fdholder-daemon program is a daemon (or, rather, executes into the s6-fdholderd daemon), meant to be supervised, that will hold file descriptors on its clients' behalf.
- Other programs in the family, such as s6-fdholder-store, are client programs that interact with this daemon to store and retrieve file descriptors.

Note that "socket activation", one of the main advertised benefits of the systemd init system, sounds similar to fd-holding. The reality is that socket activation is a mixture of several different mechanisms, one of which is fd-holding; s6 allows you to implement the healthy parts of socket activation.

### Other miscellaneous utilities

This page does not list or classify every s6 tool. Please explore the "Reference" section of the main s6 page for details on a specific program.