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!
The Story
Years ago I discovered Daniel J. Bernstein‘s qmail, and with it came the rest of the “DJB-ware” software stack: djbdns, ucspi, daemontools… and their philosophy!)
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 (Ctrl-Z, wait, 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 had.
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 the given command to the input of the log-command.
- Spawn both the command and the log-command.
- 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 dmon will trigger some of the extra features provided:
- Passing -n makes it run in the foreground. This is very useful in conjunction with -1: with tha latter the processes will be only respawned if their exit status is non-zero.
- If you want to log messages from standard error, use -e and 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, use -L. After pausing execution (by means of the SIGSTOP signal), it will be resumed when the system load falls below the value given with -l (by sending 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 logging command: 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 daemontools, like multilog (part of it) or my own rotlog 
DMon use-cases & Examples
Running 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 syslog, and saving the PID of the process (the second line terminates dmon, vfand and dsyslog in a single shot):
dmon -e -p /var/run/vfand.pid vfand -- dsyslog vfand
kill $(cat /var/run/vfand.pid)
Starting the MediaTomb UPnP server as a user mediatomb (i.e. at bootup), saving auto-rotated logs with rotlog running as user log:
dmon -e -u mediatomb -g mediatomb -U log -G log
mediatomb --interface eth0 --home /mnt/mediafiles
-- rotlog -c /var/log/mediatomb/
Final Words
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!
Have a lot of fun… 
