The Rubyists

Runit for Ruby (And Everything Else)

The Runit system is a process supervisor and set of utilities surrounding supervising processes.

It can be used as a sysvinit replacement or run side-by-side with many init systems to maintain supervised process heirarchies.

We began using runit as a replacement for daemontools in November 2001, because runit was OSI licensed at the time (daemontools was not) and for the extensive documentation offerred for runit utilities (including manpages).

Fully implementing runit, as a replacement for init/PID 1 or as a standalone supervisor, is covered in general at the runit page above and in detail for Arch Linux.

Some things we especially like about this system are:

Service View

Runit offers a concise but complete view of process status, including the optional log service.

bougyman@jimmy:~$ sudo sv s /service/*
run: /service/callcenter: (pid 2870) 5266009s
run: /service/cron: (pid 3769) 7700115s
run: /service/fxc: (pid 3714) 7700118s
run: /service/getty-1: (pid 3713) 7700118s
run: /service/getty-2: (pid 3730) 7700117s
run: /service/getty-3: (pid 3716) 7700118s
run: /service/getty-4: (pid 3690) 7700118s
run: /service/getty-5: (pid 3691) 7700118s
run: /service/helpdesk: (pid 3715) 7700118s
run: /service/lighttpd: (pid 27321) 5208602s; run: log: (pid 3724) 7700117s
run: /service/openntpd: (pid 3747) 7700116s; run: log: (pid 3741) 7700117s
run: /service/postgres: (pid 3732) 7700117s
run: /service/redmine: (pid 3729) 7700117s
run: /service/socklog-klog: (pid 3746) 7700116s; run: log: (pid 3733) 7700117s
run: /service/socklog-unix: (pid 3758) 7700116s; run: log: (pid 3756) 7700116s
run: /service/ssh: (pid 3757) 7700116s; run: log: (pid 3731) 7700117s  

As you can see, all services live in one place, defined by $SVDIR or /service by default, which runsvdir manages.
The services in /service/* are symlinks to directories (usually in /etc/sv/) which must contain one executable file, named ‘run’.

The run executable should exec the process in the foreground with stderr redirected to stdout.

Runit avoids unecessary complexity by separating the roles into small, dietlibc-friendly utilities, written in C without library dependencies.
The above listing represents many individual services, each with one instance of the per-process supervisor runsv running.

Example Service (sshd)


exec /usr/sbin/sshd -De 2>&1

If the directory log/ exists, it will be treated as a log service.

runsv will create a pipe and redirect standard out of the service (and its optional finish script) to log/run.

Log Service (for sshd)


exec svlogd -t /var/log/sshd/

Svlogd Logging

Runit includes a superb lightweight logging system in svlogd.

When you supervise a process, the optional log service’s stdout is directed to svlogd (or any other logger which accepts stdin).

Svlogd offers granular control of how to log that output, including rotation on many metrics (without stopping the process it’s logging), post-processing of logs, networked logging (both standard syslog style and offers its own network option), notifications, filtering, and more.

Config file for a log service



The above would set the max size of a log file to 100000 bytes, keep 10 files maximum, 5 files minimum (if the disk were full it would delete 6-10 until it had room), rotate every 86400 seconds (even if the size limit were not exceeded), network log to, prepend each log message with SSHD_LOG, not log messages matching the pattern “bougyman”, and upon rotation run each logfile through sh -c 'logwatcher'.

Not very descriptive, but well documented (in man svlogd).

Process respawning and control

If a process stops, it will be started again immediately without intervention or outside process monitoring.
runsv avoids a failed or faulty process hammering resources by delaying 1 second before starting a process back up if it’s failing instantly.

When a process stops, if a file named ‘finish’ exists and is executable, finish will be run with two arguments, the exit code and exit status of run.

Example finish script


if [ $status -ne 0 ];then
  echo "SSHD FAILED on $hostname: $code, $status"|mail
  echo "SSHD Stopped on $hostname: $code, $status"|mail

sv usage examples

sv is used to get and change status of a particular service. This is your swiss army knife of process control.

Get status

$ sv s sshd
run: sshd: (pid 4040) 6435s; run: log: (pid 4028) 6435s

Send the TERM signal, which will restart a process. This works because runsv will always keep a process started unless you tell it to stay down.

$ sv t sshd

Generally there will be no output from such commands, use -v to get some output (examples from here on out will use -v)

$ sv -v t sshd
ok: run: sshd: (pid 26764) 0s

Stop a service and keep it down (until the next boot or you specifically tell it to come back up)

$ sv -v d sshd
ok: down: sshd: 1s, normally up

Send USR1 to a service

$ sv -v 1 sshd
ok: down: sshd: 56s, normally up

Remove a service (stop it and make it not start back up, even on boot)

$ rm /service/sshd

Of course there will be no sv output from the above.

Removing a symlink in /service (or your $SVDIR if not /service) will send the equivalent of an sv d(own) to runsv, stopping the process and its log service, then stopping runsv.

Flexible dependency system

The sv program (with the check subcommand) also allows for any dependency tree you can dream of (and script).

Basic dependency example


#!/bin/sh -e
sv -w7 check postgresql
exec 2>&1 \
  lighttpd -f /etc/lighttpd/lighttpd.conf -D

This would wait 7 seconds for the postgresql service to be running, exiting with an error if that timout is reached.
runsv will then run this script again. Lighttpd will never be executed unless sv check exits without an error (postgresql is up).

Dependency hierarchies (Per-User service trees)

Building on the above dependency checking, we can go one step further and give a user control over their own $SVDIR, /home/username/service here.
This gives non-root users the same ability to guarantee services are running, supervised, properly logged, and all the advantages of the main $SVDIR.

User-level supervision example (with dependency checking)


#!/bin/sh -e
sv check postgresql couchdb memcached
exec 2>&1 \
  sudo -H -u callcenter runsvdir -P /home/callcenter/service 'log:.........................................................................................................'

Now any service directory the callcenter user symlinks in ~/service/ will get its own runsv process, running as callcenter. None of the callcenter user’s processes will be started (because runsvdir will not run) until postgresql, couchdb, and memcached are up and running successfully.

What’s with all the dots?

The log:………… is a construct runsvdir uses to log to proctitle (seen in ps aux). Each dot represents 15 seconds of time, with any stderr/out being flushed to proctitle for each dot. Thus if you saw

root      4001  0.0  0.0    184    28 ?        Ss   08:00   0:00 runsvdir -P /service log: ed with loglevel notice?Could not load host key: /etc/ssh/ssh_host_ecdsa_key??...

You’d know sshd had a problem, but it hasn’t had that problem in 45 seconds.

Example Ruby Service for a User


#!/bin/sh -e
echo Starting from $root
cd $root
if [ -s $HOME/.rvm/scripts/rvm ]; then
  source $HOME/.rvm/scripts/rvm
  echo "Using RVM $(which rvm)"
  rvm use 1.9.2 2>&1
exec 2>&1 \
  chpst -e $envdir ./bin/fs2ws 2>&1

Finally, a hint of Ruby!
This script includes a check to see if rvm is installed and if so uses its 1.9.2 version.
The -e to chpst specifies an environment directory, which is a way to set environment variables, but chpst can do much more.

Standalone environments

chpst allows multiple controls around processes we run, including memory capping, user privileges, nice levels, lock files, open files, data segments, cores, stdin/out, and all environment variables.
It is the swiss army knife for manipulating what a running process can do.
This command encapsulates all of the functionality of the daemontools commands setuidgid, envuidgid, envdir (used above), softlimit, and setlock. If called as one of those commands, it will behave as that utility.
chpst adds the behavior of pgrphack and fghack, as well as some behavior not available in daemontools such as nice level.

That’s a lot of manipulation in one tool, but most of them are rarely used.
The -e/envdir option is the one we take advantage of most, essentially using it as a replacement for options/config files.
A directory of files where an enviroment variable will be created for each file, with the value set to the contents of that file, may appear cumbersome at first glance.
In practice, however, we find that changing one or two options is the most likely workflow.
With the envdir setup, this becomes

echo 'sofia/gateway/default/%s' > ~/service/fs2ws/env/TCC_ProxyServerFormatString

Not so cumbersome for that! It may indeed take longer to set these variables up the first time, but maintenance is not so bad.
One advantage of having this data in environment variables is the ability to query /proc/PID/env/ for the options your ruby got on startup.
The second (more important) one we found is that we make fewer options.
That’s right, it makes us think about each option we add, knowing it will be another file in env/. So we don’t have as many options. Good Thing.

Now our (ruby) process has access to all of these in ENV["TCC_*"], how to use them?.
See the Tiny Call Center default envdir and options.rb for one way to glue them., but rolling your own should be trivial (just look at ENV in a chpst -e ./env irb).

Parallel startup

Services all start concurrently, drasticly reducing time spent changing runlevels (including the initial boot).

How Fast (Benchmarks!)

The only fair benchmark available for runlevel switching, because of the differences in what arbitrary runlevels mean in various systems, was initial boot time.
The results of the initial boot time are of course influenced by POST time, which is listed for each of the below reference systems. Each of these systems is a default Arch Linux install with the Arch Linux default (sysvinit), runit-run-git, or initscripts-systemd installed, running getty’s, a syslogger, ssh, and cron (no X).
Because POST has varied on the systems, the initial boot times and reboot times are an average of 10 runs of each action. A system is considered ‘booted’ when a user is able to log in to a getty.

Acer 1830t Small Form Factor Notebook
680U i7 1.47Ghz processor
4G Ram
640G 7200RPM Sata HD
Intel Graphics on board
Post time 5-7 seconds.
System Initial Boot Time Reboot (init 6/shutdown -r now) Time Max Boot Time Min Boot Time
Default SysV Init 28.73 39.41 33.48 27.10
SystemD 19.21 26.77 22.41 18.07
Runit 17.25 24.47 18.22 16.33
Toshiba Qosmio Portable Workstation
740QM i7 processor
8G Ram
Corsair Performance III SSD
Nvidia GTX460m 1.5G Discreet Graphics
Post time 2-4 seconds.
System Initial Boot Time Reboot (init 6/shutdown -r now) Time Max Boot Time Min Boot Time
Default SysV Init 22.13 29.41 24.83 19.94
SystemD 15.11 21.53 16.96 14.62
Runit 11.94 17.67 12.45 11.28

The main time difference we saw in the SystemD startup was the 2-3 seconds dbus itself takes to start, giving runit its advantage. If you started dbus in runit stage 1 we expect these times would be nearly identical.

Infinite runlevels

You are not limited to 0-6, with only current and previous as reserved.

Runlevels become (unlimited amount of) directories of services (in /etc/runit/runsvdir) which can be switched to quickly and simply.

# Switch to the services described in `/etc/runit/runsvdir/primary`
$ runsvchdir primary

# Switch to the services described in `/etc/runit/runsvdir/failover`
$ runsvchdir failover

No mysterious backgrounding

Processes run in the foreground logging to stdout/stderr.
This requirement for well-behaved processes can be a blessing but some may see it as a curse.
Processes that are not well behaved can be supported through a once service, if you absolutely cannot do without them.
We use this as a bit of a litmus test for potential new services. When they can not run in a well-behaved manner that’s a high-priority Con, one which takes extraordinary Pros or absolutely necessary business-case to overcome.