Can systemd replace monit?

Let me start by saying that in my opinion restarting any service automatically is a bad idea.

Answering partly your question - it is possible to design a primitive monitoring system based on systemd. I have never used monit, but the number of available nagios plugins ( http://www.nagios.org/projects/nagiosplugins ) is something hard to beat. Therefore the example below uses a nagios plugin from within systemd, which in turn sends emails in case the check performed by the plugin fails.

Example below for checking for httpd:

1. install the nagios plugin (and httpd to be monitored)

   su -c "yum -y install httpd"
   su -c "yum -y install nagios-plugins-http"
   

2. configure systemd mailer:

   su -c "yum -y install sendmail-cf"
   

   Configure sendmail with e.g. SMART_HOST , see http://fedoraproject.org/wiki/Administration_Guide_Draft/Mail#Smart_Host

   All steps as root from now.

   The systemd script (idea taken from https://mailman.archlinux.org/pipermail/arch-general/2014-February/035037.html ) reads:

   cat <<'EOF' > /root/mail
   #!/usr/bin/sh
   (echo "Subject: Failed Service: ${1} on `hostname`"; /usr/bin/systemctl status "${1}.service") | /usr/sbin/sendmail email@domain.com
   EOF
   chmod go-rwx /root/mail
   

   /usr/bin/mail did not work for me. The above script is executed by the following systemd service (for usage of the systemd specifiers (@, %i, etc), see http://www.freedesktop.org/software/systemd/man/systemd.unit.html ):

   cat <<'EOF' > /usr/lib/systemd/system/mail@.service
   [Unit]
   Description="Mailer"
   [Service]
   Type=oneshot
   ExecStart=/usr/bin/sh /root/mail %i
   [Install]
   WantedBy=multi-user.target
   EOF
   

3. configure the systemd timer service - this will periodically call the systemd service performing the actual nagios check. The timer uses OnCalendar systemd option ( http://www.freedesktop.org/software/systemd/man/systemd.time.html ) and is based on the ideas from https://wiki.archlinux.org/index.php/Systemd/cron_functionality#Starting_events_according_to_the_calendar and http://solpeth.wordpress.com/2013/12/27/using-systemd-as-a-cron-replacement/ . It will run every minute (haven't figured out how to set more complex time patterns yet - see https://bugzilla.redhat.com/show_bug.cgi?id=1074951 ):

   cat <<'EOF' > /usr/lib/systemd/system/min@.timer
   [Unit]
   Description=Run %i every minute
   [Timer]
   OnCalendar=*:*:00
   Unit=%i.service
   [Install]
   WantedBy=timers.target
   EOF
   

4. configure the actual systemd service performing the nagios check_http check and notifying the min@.timer through OnFailure:

   cat <<'EOF' > /usr/lib/systemd/system/check_http.service
   [Unit]
   Description=check_http
   OnFailure="mail@check_http.service"
   [Service]
   Type=simple
   ExecStart=/usr/lib64/nagios/plugins/check_http -H localhost -p 80
   EOF
   

Now, how to use those? The main controller is the timer, so:

su -c "systemctl start min@check_http.timer"

You should start receiving emails now. Let's start apache:

su -c "systemctl start httpd.service"

The emails with errors still coming? This is due to the way nagios check_http behaves - one has to create some web contents:

su -c "touch /var/www/html/index.html"

In order to disable the check:

su -c "systemctl stop min@check_http.timer"

or to have it permanently after reboot:

su -c "systemctl enable min@check_http.timer"

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