Why s6-rc ?
The limits of supervision suites
Supervision suites such as
define a service as a long-lived process, a.k.a
daemon. They provide tools to run the daemon in a reproducible
way in a controlled environment and keep it alive if it dies;
they also provide daemon management tools to, among others,
send signals to the daemon without knowing its PID. They can
control individual long-lived processes perfectly well, and
s6 also provides
tools to manage a whole supervision tree. To any system administrator
concerned about reliability, supervision suites are a good thing.
However, a supervision suite is not a service manager.
Relying on a supervision suite to handle all the service
management is doable on simple systems, where there aren't
many dependencies, and where most of the one-time initialization
can take place in stage 1, before any daemons are launched.
On some embedded systems, for instance, this is perfectly reasonable.
On other systems, though, it is more problematic. Here are a few
- With a pure supervision tree, all daemons are launched in
parallel; if their dependencies are not met, daemons just die, and
are restarted by the supervisors, and so on; so eventually the
dependency tree is correctly built and everything
is brought up. This is okay with lightweight daemons that do not take
up too many resources when starting; but with heavyweight daemons,
bringing them up at the wrong time can expend CPU or cause heavy
disk access, and
increase the total booting time significantly.
- The runit model of
separating one-time initialization (stage 1) and daemon management
(stage 2) does not always work: some one-time initialization may
depend on a daemon being up. Example: udevd on Linux. Such daemons
then need to be run in stage 1, unsupervised - which defeats the
purpose of having a supervision suite.
- More generally, supervision suites do not perform
dependency management. Their job is to maintain daemons
alive and ease their administration; dependency across those
daemons is not their concern, and one-time initialization scripts
are entirely foreign to them. So a situation like udevd
where it is necessary to interleave daemons and one-time scripts
is not handled properly by them.
To manage complex systems, pure supervision suites are insufficient,
and real service managers, starting and stopping services
in the proper order and handling both oneshots (one-time
initialization scripts) and longruns (daemons), are needed.
Unix distributions usually come with their own init systems and
service managers; all of those have flaws one way or another. No
widely spread init system gets things right, which is the main
reason for the recent "init wars" - no matter what init system you
talk about, there are strong, valid reasons to like it and support it,
and there are also strong, valid reasons to dislike it.
Non-supervision init systems usually fall in one of two categories,
both with pros and cons.
Traditional, sequential starters
Those are either the historical Unix init systems, or newer systems
that still favor simplicity. Among them, for instance:
the historical GNU/Linux init system, and its companion set of
/etc/rc.d init scripts that some distributions like to
call sysv-rc. Note that sysvinit does have
supervision capabilities, but nobody ever bothered to use them
for anything else than gettys, and all the machine
initialization, including longruns, is done by the sysv-rc
scripts, in a less than elegant way.
init, which is very similar to sysvinit - including the
supervision abilities that are only ever used for gettys.
The /etc/rc script takes care of all the initialization.
an alternative, dependency-based rc system.
All these systems run sequentially: they will start services, either
oneshots or longruns, one by one, even when the dependency graph says
that some services could be started in parallel. Also, the daemons
they start are always unsupervised, even when the underlying init
system provides supervision features. There usually is no
notification support on daemons either, daemons are fire-and-forget
(but that's more on the
scripts themselves than on the frameworks). Another common criticism
of those systems is that the amount of shell scripting is so huge
that it has a significant performance impact.
(Note that OpenRC
has an option to start services in parallel, but at the time of this
writing, it uses polling on a lock file to check whether a service
has completed all its dependencies; this is heavily prone to race
conditions, and is not the correct mechanism to ensure proper service
ordering, so this option cannot be considered reliable.)
Another, less obvious, but important drawback is that service-launching
scripts run as scions of the shell that invoked the command, and so
they may exhibit different behaviours when they're run automatically at
boot time and when they're run manually by an admin, because the
environment is different. Scripts usually try to run in a clean
environment, but it's hard to think of everything (open file
descriptors!) and every script must protect itself with a gigantic
boilerplate, which adds to the inefficiency problem.
Monolithic init behemoths
The other category of service managers is made of attempts to cover
the flaws of traditional service starters, and provide supervision,
dependency management and sometimes readiness notification, while
reducing the amount of scripting needed.
Unfortunately, the results are tightly integrated, monolithic init
systems straying far away from Unix core principles, with
design flaws that make the historical inits' design flaws look like
- Upstart was the first
one. On the front page, in the "feature highlights" section:
"Tasks and Services are started and stopped
by events. Events are generated as tasks and services are started
and stopped." Do you understand what that means? I don't. Also,
Upstart was the first system that used ptrace on the
processes it spawned in order to keep track of their forks. If
you don't know what that means: it was pure insanity.
Darwin's init and service manager. The wikipedia page (linked here
because Apple doesn't see fit to provide a documentation page for
launchd) is very clear: it replaces init, rc, init.d/rc.d,
SystemStarter, inetd, crontd, atd and watchdogd. It does all of this
in process 1. And it uses XML for daemon configuration, so launchctl
has to link in a XML parsing library, and it communicates with process 1
via a Mach-specific IPC mechanism. Is this the sleek, elegant
design that Apple is usually known for? Stick to selling iPhones,
the main protagonist (or antagonist) in the "init wars". It has the same
problems as launchd, up by an order of magnitude;
here is why.
systemd avowedly aims to replace the whole low-level user-space of
Linux systems, but its design is horrendous. It doesn't even
get readiness notification right.
The problem of integrated init systems is that:
- They have been developed by companies and associations, not
individuals, and despite the licensing, they are for all intents and
purposes closer to proprietary software than free software; they also
suffer from many of the technical flaws of enterprise software
- As a result, and despite being backed by tremendous manpower,
they have been very poorly thought out. The manpower goes into the
coding of features, not into architecture conception; and the
architects were obviously not Unix experts, which is a shame when
it's about creating a process 1 for Unix. This is apparent because:
- They have been designed like application software, not
system software, which requires a fairly different set of
skills, and careful attention to details
that are not as important in application software,
such as minimal software dependencies and shortness of code paths.
Pages and pages could be - and have been - written about the shortcomings
init systems, but one fact remains: they are not a satisfying solution
to the problem of service management under Unix.
The best of both worlds
s6-rc aims to be such a solution: it is small and modular, but offers
full functionality. Parallel service startup and shutdown with
correct dependency management (none of the systemd nonsense where
services are started before their dependencies are met), correct
readiness notification support, reproducible script execution, and
short code paths.
- s6-rc is a service manager, i.e. the equivalent of
sysv-rc or OpenRC. It is not an init system.
You can run s6-rc with any init system of your choosing.
Of course, s6-rc requires a
s6 supervision tree to be running on
the system, since it delegates the management of longrun services
to that supervision tree, but it does not require that s6 be the
init system itself. s6-rc will work
runs as process 1 (on Linux, such a setup can be easily achieved
via the help of the
package), and it will also work
s6-svscan runs under another init process.
- The service manager runs on top of a supervision
suite. It does not try to make it perform boot/shutdown operations or
dependency management itself; and it does not substitute itself to it.
s6-rc uses the functionality provided by s6, but it is still possible
to run s6 without s6-rc for systems that do not need a service manager.
It would also be theoretically possible to run s6-rc on top of another
supervision suite, if said supervision suite provided the hooks that
- A significantly time-consuming part of a service manager is
the analysis of a set of services and computation of a dependency graph
for that set. At the time of writing this document, s6-rc is the only
service manager that performs that work offline, eliminating
the dependency analysis overhead from boot time, shutdown time, or
any other time where the machine state changes.
- The source format for the
s6-rc-compile tool is purposefully
simple, in order to allow external tools to automatically write
service definitions for s6-rc - for instance for conversions between
service manager formats.
- Like every
skarnet.org tool, s6-rc
is made of very little code, that does its job and nothing else.
The binaries are small, it is very light in memory usage, and the
code paths are extremely short.
The combination of s6 and s6-rc makes a complete, full-featured and
performant init system and service manager, with probably the lowest
total memory footprint of any service manager out there, and all the
reliability and ease of administration that a supervision suite can
provide. It is a real, viable alternative to integrated init
behemoths, providing equivalent functionality while being much
smaller and much more maintainable.