s6
Software
www.skarnet.org
Why another supervision suite ?
Supervision suites are becoming quite common. Today, we already have:
- Good (?) old System V init, which can be made to supervise services if you perform /etc/inittab voodoo.
BSD init can also be used the same way with the /etc/ttys file, but for some reason, nobody among BSD
developers is using /etc/ttys to this purpose, so I won't consider BSD init here.
- daemontools, the pioneer
- daemontools-encore, Bruce Guenter's upgrade to daemontools
- runit, Gerrit Pape's suite, well-integrated with Debian
- perp, Wayne Marshall's take on supervision
- and even Upstart, Ubuntu's init system, which performs real supervision.
Fedora's systemd and MacOSX's
launchd
are very similar in spirit to Upstart, so the same comments apply to them.
Why is s6 needed ? What does it do differently ? Here are the criteria I used.
Supervision suites should not wake up unless notified.
- System V init fails the test: it wakes up every 5 seconds, for the reason that
/dev/initctl might have changed.
m(
- daemontools fails the test: it wakes up every 5 seconds to check for new services.
- daemontools-encore does the same.
- the current version of runit fails the test: it wakes up every 14 seconds. But this is a workaround for a bug in some Linux kernels;
there is no design flaw in runit that prevents it from passing the test.
- perp works.
- Upstart works.
- s6 works. By default, s6-svscan wakes up every 5 seconds, to emulate
svscan behaviour; but it
can be told not to do so. (s6-svscan -t0)
Supervision suites should provide a program that can run as process 1.
- System V init and Upstart are process 1, so no problem here.
- daemontools was not designed to take over init, although
it can be made to work with
enough hacking skills. Same thing with daemontools-encore.
- runit provides an init functionality, but the mechanism is
separate from the supervision itself; the runit process, not the
runsvdir process, runs as process 1. This lengthens the supervision
chain.
- perp was not designed to run as process 1. It probably could be made to work too
without too much trouble.
- s6-svscan was designed from the start to be run as process 1, although it
does not have to.
Supervision suites should be bug-free, lightweight and easy to understand.
- daemontools, daemontools-encore, runit and perp all qualify. All of this is excellent quality
code, unsurprisingly.
- This is where System V init and Upstart fail, hard. SysVinit is too big
for what it (poorly) does. Upstart is clever, but it's waaaaaay too
complex. Come on people... using ptrace to watch your children fork()?
Linking process 1 against libdbus? This is
insanity. Process 1 should be absolutely stable, it should be guaranteed
to never crash, so the whole of its source code should be under control. At
Upstart's level of complexity, those goals are outright impossible to achieve,
so the Upstart approach is flawed by design.
- Of course, systemd and launchd suffer from the same problem. Guys, I'm
glad you eventually realized that supervision was a good thing, and that it
had to be rooted in process 1, but that does not mean that all the supervision
logic has to go into process 1.
No, really.
- s6, which has been designed with embedded environments in mind, tries
harder than anyone to pass this. It tries so hard that s6-svscan
and s6-supervise, the two long-running programs that make the
supervision chain, do not even allocate heap memory, and their main
program source files are less than 500 lines long.
Supervision suites should provide a basis for high-level service management.
- Neither System V init, daemontools, runit or perp
provides any hooks to wait for a service to go up or down. runit provides a
waiting mechanism, but it's based on polling, and the ./check script
has to be manually written for every service.
- daemontools-encore qualifies: the notify script can be used for
inter-service communication. But it's just a hook: all the real notification
work has to be done by the notify script itself, no notification framework is
provided.
- Upstart already is a service management tool. But, again,
it fails the test of simplicity: it does in process 1 what can and should be
done outside of process 1. Process supervision is not the same as service
management, and Upstart confuses the two. So do systemd and launchd.
- s6 comes with libftrig, an event notification
library, and command-line tools based on this library, thus providing a simple
API for future service management tools to build upon.
Artistic considerations
- s6-svscan and s6-supervise are entirely asynchronous.
Even during trouble (full process table, for instance), they'll remain reactive
and instantly respond to commands they may receive. s6-supervise has
even been implemented as a full deterministic finite automaton, to ensure it
always does the right thing under any circumstance. Other supervision suites
do not achieve that for now.
- daemontools' svscan
maintains an open pipe between a daemon and its logger, so even if the daemon,
the logger, and both
supervise processes
die, the pipe is still the same so no logs are lost, ever, unless
svscan itself dies.
- runit has only one supervisor, runsv,
for both a daemon and its logger. The pipe is maintained by runsv.
If the runsv process dies, the pipe disappears and logs are lost.
So, runit does not offer as strong a guarantee as daemontools.
- perp has only one process, perpd,
acting both as a "daemon and logger supervisor" (like runsv) and as a
"service directory scanner" (like runsvdir). It maintains the pipes
between the daemons and their respective loggers. If perpd dies, everything
is lost. Since perpd cannot be run as process 1, this is a possible SPOF for
a perp installation; however, perpd is well-written and has virtually no risk of
dying, especially compared to process 1 behemoths like Upstart, systemd and
launchd.
- Besides, the runsv model, which has to handle both a daemon
and its logger, is more complex than the supervise model (which
only has to handle a daemon). Consequently, the runsvdir model is
simpler than the svscan model, but there is only one svscan
instance when there are several runsvs and supervises.
The perpd model is obviously the most complex; while very understandable,
perpd is unarguably harder to maintain than the other two.
- So, to achieve maximum simplicity and code reuse, and minimal memory
footprint, s6's design is close to daemontools' one.
And when s6-svscan is run as process 1,
pipes between daemons and loggers are never lost.
Conclusion
All in all, I believe that s6 offers the best overall implementation of a
supervision suite as it should be designed. At worst, it's just another
take on daemontools with a reliable
base library and a few nifty features.