Adventures with Docker

For a few years now, the new hotness in the software world has been Docker. It’s essentially a very-stripped-down virtual machine, where instead of each virtual machine needing to run an entire operating system as well as whatever application you’re running inside it, you have just your application and its direct dependencies and the underlying operating system handles everything else. This means you can package up your application along with whatever other crazy setup or specific versions of software is required, and as long as they have Docker installed, anyone in the world can run it on pretty much anything.

The process of converting something to run in Docker is called “Dockerising”, and I’d tried probably two or so years ago to Dockerise my website (which was at the time still in its Perl incarnation), but without success. Most of it was not properly understanding Docker but also Docker’s terminology not being hugely clear and information on Dockering Perl applications being a bit thin on the ground at the time.

My new job involves quite a lot of Docker so I figured I should probably have another crack at it, so I sat down in June and managed to get my website running in a Docker container! The two-or-so-years between when I tried it last and now definitely helped, as did having had a little bit of experience with it in the new job.

I think the terminology was one of the bits that I struggled with most, so maybe this explanation will help someone… you have a Docker image, that’s basically a blueprint for a piece of software and all its associated dependencies. From that image (blueprint), you start up one or more containers which are the actual running form of the image. If one container dies (the application inside crashes or whatever), you don’t care and just start up another one and it’s identical each time. To build your own image, you start with a Dockerfile that tells Docker exactly how to construct your application and all the different parts that are required to support it (see my Lessn Archive’s Dockerfile for an example). There really wasn’t any substitute for actually going in and doing it; by struggling and failing I eventually got there in the end.

Since my initial success with my website, I’ve gone on to put both my old site archive and my URL shortener in Docker containers as well! Next stop is Kristina’s website, but that’s still using Perl and Mojolicious and my initial attempts have not been successful. 😛

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