Monitoring BitTorrent Sync with Monit on Ubuntu 14.04 LTS

January 20th, 2015

Over the past few days, I've been doing tons of DevOps work. Lots of setting up server monitoring, security, and automation. Most recently I wanted to give monit a try, as I've heard some good things about it. I was able to install it and get it up and running pretty hassle-free. I added in a few process checks for nginx, php-fpm, and postgres and it instantly was able to start tracking those processes for me.

If you've gone through some of my other posts, you might remember a guide that I wrote about settings up BitTorrent Sync on a remote server, well today I'm going to expand on that a bit and add BitTorrent sync into my monit checks!

There were multiple apparent walls that I was going to run into, as BitTorrent sync wasn't exactly written to be placed on a web server, so it lacks a few things like PID logging and the ability to start/stop the service. In order to overcome this, I figured the wisest approach would be to write a wrapper for the btsync program that would enable stop/start.

If you're unsure what a PID is and why it's important, read on; otherwise, skip the next section and get into the meat of this tutorial.

What is a PID?

A PID is a process id. Have you ever run top or ps aux on your system? If so, you've probably seen a list with rows like this

root      4038  8.9  4.8 144436 49100 ?        Ssl  15:22   0:17 /BTSync/btsync

The number that is in the PID column is obviously the PID. These are unique identifiers for a single process. This unique PID can be used to kill or restart a process. I find that a lot of linux newbies often end up calling killall btsync if they wish to kill the process called btsync, but this isn't the proper way to do it, as there could be multiple instances of the btsync process. You should only use killall if you with to kill multiple processes at once. Like killall nginx to stop nginx and all child nodes.

Killing based on PID is much more precise. If we wish to kill the above BitTorrent sync process, we would just use kill -KILL 4038. The problem here is that the PID is unique, so upon starting back up, it would have changed.

The way around this, is most programs log the PID inside a file. Typically the PID file would be something like /var/run/ and it would contain nothing but the PID.

You'd then be able to use the contents of that file to kill or restart your process. Something along the lines of

$ kill -HUP $(cat /var/run/

could be used to restart the nginx process.

As I said above, by default BitTorrent sync doesn't log the PID in a file, so it's up to us to fix that.

The Meat

I opened up a new file with vim (my personal favorite CLI text editor)

$ vim /usr/local/bin/btsync

Then I wrote the following script in it

if [[ $1 = "start" ]]; then
    /BTSync/btsync --config /BTSync/sync.conf 2>/dev/null & PID=$!
    echo $PID > /var/run/
elif [[ $1 = "stop" ]]; then
    kill -KILL $(cat /var/run/
elif [[ $1 = "restart" ]]; then
    kill -HUP $(cat /var/run/
    echo "Invalid option. Use either start, stop, or restart."

If the script feels a little hack-y, that's because it is. For some reason upon starting btsync and retrieving the PID, it was always 1 behind the actual PID. So if it started with a PID of 420 it would end up logging 419 to the /var/run/ file. Strange, but I have other things I need to get done today, so doing a little bash-fu I was able to log the correct PID to the file. If it wasn't already obvious, you're also going to have to replace /BTSync/btsync with the actual location of the binary. If you followed my previous guide for setting it up, then you can leave it as /BTSync/btsync.

The next step is making the wrapper an executable. This is easily done by running

$ chmod +x /usr/local/bin/btsync


So now we get to add in our check to the monitor file. It's been more than easy to add in checks so far, so I expected nothing less when adding in another check for btsync.

$ vim /etc/monit/monitrc

I then added in the check like so

check process btsync with PIDfile /var/run/
    group utilities
    start program = "/usr/local/bin/btsync start"
    stop program = "/usr/local/bin/btsync stop"
    restart program = "/usr/local/bin/btsync restart"

After that, I ran a monit reload and it started monitoring btsync immediately! I was also able to click on the process and use monit's GUI to stop/start/restart the process.

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