Watch Vagrant Changes

November 07, 2013

Vagrant is a great tool for a building consistent, reproducible development environments. I can include a Vagrantfile in my code base, type vagrant up and vagrant ssh in my project folder, install project dependences in the Vagrant VM, and then run my code in the convenient /vagrant project mount. Meanwhile, I can continue to write my code and view the results on my Mac. It's a nice setup, especially when I need to hop among projects with very different setups or need to trash a VM due to conflicting libs or other damage.

One gotcha I've found using Vagrant and VirtualBox occurs when I use tools that watch the filesystem like node-supervisor and wr. These utils are handy when I want my test suite to execute, or my static code to compile, or my web app server to restart each time I make changes to my code base. Unfortunately, they fail to monitor the VirtualBox mounted /vagrant folder effectively, undoubtedly because of some VirtualBox shared folder magic.

For instance, when I do the following in the VM:

vagrant@vm:~$ npm install -g wr
vagrant@vm:~$ mkdir /vagrant/tmp
vagrant@vm:~$ touch /vagrant/tmp/foo
vagrant@vm:~$ wr "echo CHANGED" /vagrant/tmp
01:43:10 wr: watching 1 files, running 'echo CHANGED'

and then edit the foo file on my Mac, nothing happens. Occasionally, I'll see "CHANGED" printed, but it's entirely unpredictable.

An idea struck me the other day about how to overcome this problem: watch the filesystem outside the VM and get the command to run inside using SSH. It sounds nasty but the cost is minimal: a one-time setup and a minor addition to the command I want to run.

First, I install my watch utility on my Mac and save a copy of my project Vagrant SSH config file.

parente@mac:~/myproject$ npm install -g wr
parente@mac:~/myproject$ vagrant ssh-config > .ssh-config

Then, I run my watch tool on my Mac with an ssh command to run on change. Typically I hide this in whatever the project is using for automation (e.g., bash script, Makefile).

parente@mac:~/myproject$ wr "ssh -F .ssh-config default ls -l /vagrant/tmp" .
21:09:36 wr: watching 3 files, running 'ssh -F .ssh-config default ls -l /vagrant/tmp'

Finally, when I make a code change on my Mac, I see wr ssh into my VM and execute my command immediately, as desired.

21:11:16 wr: file changed

21:11:16 wr: running 'ssh -F .ssh-config default ls -l /vagrant/tmp'

total 4
-rw-r--r-- 1 vagrant vagrant 7 Nov  8 02:11 foo
21:11:16 wr: 0.1s - command succeeded

The same trick works whether I want to run make build or mocha or fab deploy within my VM.

There can be a slight delay as SSH reconnects on each change. But even this minor annoyance can be overcome by enabling SSH connection sharing and keeping a second secure shell connected to the VM.

parente@mac:~/myproject$ echo "\nControlMaster auto\nControlPath /tmp/ssh_mux_%h_%p_%r" >> ~/.ssh/config
parente@mac:~/myproject$ vagrant ssh
vagrant@tottbox:~$

I've been successfully using this recipe for a little while now. If you have an alternative or better workaround, I'd love to hear from you.