Have you ever wanted to run a lengthy process making sure that it will be restarted on failure? Did you need at some point to daemonize a “normal” program? Well, maybe you already knew about daemontools, runit, freedt, Supervisor, upstart or —recently— systemd. They already do a good job respawning processes, but there are three single things that neither of them are capable of doing:
- Running a single command and exiting when it exits successfully.
- Running commands interactively, without daemonizing nor detaching from the terminal.
- Temporarily suspending execution when the system load goes over a configurable value, and resuming execution automatically as soon as the load drops below another configurable value.
If you ever needed at least one of those features, then DMon is probably what you want. If unsure, read the examples below — you might find some inspiration for use-cases!
Some weeks ago I coded vfand, a small “non-daemon” to control the speed of the fan in my Vaio laptop because it was overheating. I am lazy, so I deliberately left out daemonization and suggested launching it from init(8) — because I knew that DJB’s tools leave daemonization and logging to other tools which just do one thing well.
Days back I had to make a huge data copy in a mission-critical mail
server, and used the mighty rsync tool because I wanted the copy to
be interruptible so I could stop it when the system load was getting
high and then resuming the data copy. I did that manually (
fg, repeat), and I do not like performing automatable tasks.
Fortunately I seldom do this kind of tasks.
Do you recognize the pattern? DMon is a subproduct of what I have
been doing lately, applying the knowledge about
daemontools I already
How does it work? — Modus operandi
In short it works àla
daemontools without control sockets and without
using script files for launching processes. All options are specified in
the command line, as long as the commands to be run. Like this:
dmon [options] command [command-options] [-- log-command [log-command-options]
As an example, consider the following command line:
dmon cron -f -- dlog /var/log/cron.log
This is what DMon wil do:
- Daemonize itself.
- Create a pipe(2), which will be used to connect the output of
commandto the input of the
- Spawn both the
- Silently wait. If some child is terminated, it will be respawned.
That is pretty close to what the
supervise program included in
daemontools does, so it have already all the advantages of it, plus
without needing stuff in the file system. Passing options to
trigger some of the extra features provided:
-nmakes it run in the foreground. This is very useful in conjunction with
-1: with the latter the processes will be only respawned if their exit status is non-zero.
- If you want to log messages from standard error, use
-eand both standard output and standard error will be piped to the logging command.
- For faulty programs which could get somewhat “locked” and sometimes
take too much time to run, you may pass a maximum running time with
-t. When the timeout is reached the program will be forcibly killed and then started again.
- Finally, for pausing the program over a given value of system load,
-L. After pausing execution (by means of the
SIGSTOPsignal), it will be resumed when the system load falls below the value given with
SIGCONT). The signals used are the standard ones used for this duty e.g. by the shell, so almost every well-behaved program will work without modifications.
The DMon package already includes a couple tools ready to be used as
dlog will append lines to a log file, optionally
adding a timestamp to them, and
dsyslog will send lines to the system
log. You can use any logging tool which works with
multilog (part of it) or my own rotlog ;-)
DMon use-cases & Examples
rsync in a terminal (without detaching), pausing the copy when
the system load is above
4.0, retrying until the copy succeeds:
dmon -n -1 -L 4.0 rsync -az /path/to/srcdir /path/to/destdir
Launching vfand as a daemon, logging errors to the local
and saving the PID of the process (the second line terminates
dsyslog in a single shot):
dmon -e -p /var/run/vfand.pid vfand -- dsyslog vfand kill $(cat /var/run/vfand.pid)
dmon -e -u mediatomb -g mediatomb -U log -G log mediatomb --interface eth0 --home /mnt/mediafiles -- rotlog -c /var/log/mediatomb/
It was fun for me to hack in DMon because C is a language I learnt to love, and using it from time to time is nice to not lose contact with it. Also, I had a clear idea of what I wanted to do for solving a particular problem, which is great for keeping focus.
Albeit DMon is already in its third release (namely version 0.3) and I have been using it since its first inceptions, it may contain bugs as any other piece of software. Do not hesitate to drop me a line with your complaints and suggestions — or even better: get yourself a clone of the Git repository and use its send-email awesomeness!