Getting into docker: the simple case

So last time I'd been left in the situation of moving from Vagrant over to Docker. And I found myself really appreciating what Docker was doing, and beginning to get my head around what it's capable of. There's still a long way to go in order to use it properly, but I think I'm beginning to get the basics.

I ended up in a situation where I just about had the Mongrel2 webserver being loaded up onto a dockerfile, and starting up.

I've managed to take that a little further in the right direction now.

Step 1: Getting Mongrel to start properly — and keep running
I touched on it last time, but the thing you really have to nail to get Docker to work is the ability to start a single process in a container and keep it running. This really isn't as easy as it could be, given a few of the limitations of the way Docker runs things.

A docker container will only keep running as long as the process started on id 1 keeps running. There are a few hacky ways of doing this, like piping together a series of shell scripts, and finishing by tailing a log file, but that's best avoided. The best way that I've found so far is to use the supervisor daemon, which starts on process 1 and then spools up the processes that you deem necessary. It actually turned out to be easier – with Mongrel – to fire up a second supervisor process called procer in order to manage the startup of the Mongrel server. There might be a better way of doing it, but this seems to work, and (in theory) gives a layer of resiliency to the Mongrel process by granting automatic restarts in case the process dies on us.

Step 2: Getting some static content in there
The next step was to get some static content onto the site. That was pretty easy, and we ended up with an infrastructure that looked a bit like this.

The Dockerfile controlling the Mongrel2 server pulled all the necessary files to install Mongrel2 and the dependencies, copied in a set of static files, and finally started the server.

Next steps:
This is all very well, but it seems like a lot of effort to go to in order to get some static content served up by a webserver – and it is. Next up is the first handler, the independent programs that make Mongrel2 interesting, and how I believe they are perfectly suited to running on a containerised platform.