Docker vs Vagrant - Round 1

Having kicked around Vagrant (https://docs.vagrantup.com/v2/) for a while, a colleague finally persuaded me to try out Docker instead, just to see what the competition was like.
... so, I spent a while working out ways of setting up a virtual machine / container to run an instance of the mongrel2 webserver, just to see how they compared.
The results are now in and ... well, it's a bit of a mixed bag, to be honest.
The Dockerfile and associated project are up and available on github, at https://github.com/nihilogist/docker-mongrel2, for those interested.

Setup

How easy are the two different solutions to set up?

Vagrant 

It's really easy. You write a few scripts to download and install the software you need - in this case it's a little more complex as you need to grab ZeroMQ and then make mongrel, but it's not hard at all. You copy what you need over to the VM and start the server. Great.

Docker

It took a while, I have to say. I found it a good deal harder to get my head around the way that the container works. Especially coming from Vagrant, which is purely and simply a method of managing VMs, it took some getting used to the idea that the container only persists as long as the process on PID1 is running. So this means that there were a few extra steps needed to ensure that PID1 keeps running, but happily there are plenty of tools to help with this.

Running

How do the two containers run?

Vagrant

You have a full VM running. You can ssh into it. You can see it running in VirtualBox (if that's the provider you're using). You can use it in exactly the ways you'd use any other virtual machine. But it is pretty heavy - it takes a while to boot up, but seems pretty solid once it's up, as you'd expect.

Docker

It's just a container - there are lots of limits on what you can do with it and what you can't. Well - not so much limits as recommendations. Using Vagrant I think  you could be easily tempted to start up a whole load of extra processes on the VM, just because it's easy, and if you have the machine running, then why not? Docker, on the other hand, really wants you to dedicate each container to a single process. It's really lightweight. It starts in an instant.

Which do I prefer?

Well, it's little early in the day, but I think that Docker has won me over. I've still got a heck of a lot to explore, like getting data volumes to work properly, but I'm really enjoying using it.
I'm also keen to explore the way that mongrel2 wants you to use many small services, and I think that Docker is perfectly aligned with that - each container running a single service, but running it well.