Exploring Dockerfiles

I’d like to continue the previous entry on Docker a little further. Last time we talked about the installation process & a little more, so this time we’re going to talk about the next part of getting started with Docker – writing a Dockerfile.

Here’s what we talked about last time, and with one odd little exception (why did I promise to talk about load testing…) we’re going to cover all these things!

So, next steps, make the container persistent – it isn’t yet, and play around with Dockerfiles, and just do a little more spying on the produced container itself & probably try to do some babby’s frist load testing things in there & spy on the container as a process without the box & all its processes within!

First let’s take a look at the Docker process we created last time. Just like at your native command line, docker commands all resemble low-level Linux commands, so just like you’d use ps to look at the processes running at any given time on your machine, you can use docker ps to see all the Docker processes it is managing at any given time. If you followed along last time you’ll see some that have been exited but which you don’t have access to – each time you run the docker run -it bash you get a new process. But the old ones are still there! The all flag will show us these Exited boxes with a docker ps -a.

rachel $ docker ps -a
CONTAINER ID        IMAGE                      COMMAND                  CREATED             STATUS                     PORTS               NAMES
b5de9583d7b3        fedora                     "bash"                   10 minutes ago      Exited (0) 3 seconds ago                       pedantic_morse
35192bfa05d4        images/cowsay-dockerfile   "/usr/games/cowsay *P"   2 hours ago         Exited (0) 2 hours ago                         gigantic_goldberg
a0e40d55125a        images/cowsayimage         "/usr/games/cowsay 'D"   3 hours ago         Exited (0) 3 hours ago                         jovial_mcnulty
d32381833772        debian                     "bash"                   3 hours ago         Exited (0) 3 hours ago                         cowsay

You’ll notice a few things, first that the names are a mix of adjective_noun, except one – the cowsay container example is from the excellent Using Docker where I’ve gained a lot of my recent Docker information. Their status is all Exited. Some of the container-specific commands are similar to the init.d service commands, like start, stop, and rm, so let’s start the desired container in that list up there. The container we’re going to start up is similar to the one we made before & is Fedora, though it is true that I only made it ~10m ago!

docker start pedantic_morse

So now the output of docker ps includes the container we just made. So how do we keep it? We commit it, just like with Git! Replace pedantic_morse with whatever name yours has been assigned beneath the NAMES column.

rachel $ docker commit pedantic_morse images/morse
sha256:b398fe28d7fd26a52e0947fc8eebb7614b8a8d6d19a5332359df167c9296c04f

So what we’ve done here is create an image from which we can create containers. images/morse is the image, pedantic_morse is the Docker process that we crafted it from. For every time we run the image images/morse, it creates a new Docker process, so at this point it’s still not persistent in ONE image, HOWEVER we can use this image to perform one-offs.

Clearly we’re not getting into the strength of Docker, yet. So now it’s time for a very basic Dockerfile. Just like Vagrantfile and Procfile & probably a few other similarly intended setup files, the D in Dockerfile is capitalized and there’s no extension to it, because remember – Linux doesn’t care about file extensions!

The main piece to know with Dockerfiles is that their syntax can be as minimal as you like, and personally I recommend making them non-complex – major structural pieces, and insert kickoff scripts or use some config management in the container itself for anything much more complicated. I reserve the right to change my mind on this later! And this is also more for next time to learn. But the way it looks, the RUN command will run any bash you put in it, but if you need anything more complex, the contents become a lot more murky, in my opinion. Simple is better than complex, but complex is better than complicated, so let’s do what we need to here.

For posterity and a simplistic example, here’s the first Dockerfile I ever wrote. (ed note: I trimmed this down because each line of a Dockerfile creates a new filesystem – try to truncate Dockerfile lines as much as possible)

FROM fedora:23
RUN /bin/bash
RUN echo "the dockerfile took!"

RUN dnf install -y wget tar man

MAINTAINER Rachel!

The output of this, which is a bit long to post, pulls down version 23 of Fedora, uses bash for the following commands, prints “the dockerfile took!” to stdout, and then installs those three packages. I’m unsure why some of those aren’t present in a base Fedora image, but it doesn’t appear to be related to what I’m working on in this blog post, so we’ll leave it be for now.

This is about ten times longer than I thought it would be, woohoo! I hope you learned something, please please let me know if I’ve missed the mark on anything, cheers!

Tune in next time and we’ll talk about a more complicated Dockerfile, and syncing it up to… something πŸ™‚ come back and you’ll find out what!

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“Composting” Docker

Right now I’m fiddling with Docker and combing through their terrific, extensive docs. As I have a history of doing lately with this blog, I’ll talk about my own setup and the installation process, all the way to the Docker image I’ve created and messed with, and what’s next for my own knowledge.

New laptop

First, I recently purchased a machine from Free Geek so I’m going through the delightful process (really :D) of setting it up. When I volunteered at Free Geek (which I HIGHLY recommend, please ping me for details!!), the computer I received after building desktops for them for a while was 40G HDD, 512MB DDR2 RAM, and was capable of a wired-only connection to the internet. After a USB wifi antenna, the machine could do all the browsing I needed, which was all I did at the time. At the time, that kind of machine cost anywhere between $80-$130 if my memory serves me correctly. I believe this was about 2011.

Two days ago, I wanted a laptop that would be able to run VMs without too much trouble, and I wanted to spend $200 or less as I am unemployed. Five years after my volunteer stint, I got a machine with 8G ram DDR3, 250G SATA HDD, i5 dual-core processor, in a pretty giant old Dell Latitude E63340, for exactly $200. AMAZING. I’m astounded at how cheap it is. And what else has changed? It’s no longer Ubuntu that they ship the computers they clean, reassemble to spec, and sell or give away, it’s Linux Mint, specifically Edition 17.1. Thoughts so far are really just that it feels very Windows-y with its focus on the (not) Start menu, and you’ll get no complaints from me on this front. It’s Ubuntu with a coat of paint on top, and I’m not toooootally sure this is what I want to be running, but it definitely works and there is probably broadest reach of packages available for the end-user (not enterprise which is of course another red-colored ball of wax) so it will do, for now, and at some point I am sure I’ll explore other distros.

Docker

On to Docker! I’ve previously read up on Docker, and find its resource allocation methodology super interesting. I’ve used a lot of Vagrant/VMWare on my OSX box at my previous job, and while the mechanism to spin up/destroy boxes was pretty good, the amount of cruft left behind became frustrating at a work level – I had to clear out vast swaths of space with a machete on an irritatingly regular basis! Basically, Docker runs without a hypervisor, which in the spirit of keeping things in layperson’s terms, is a virtual machine management machine which is pretty heavy. That they’ve figured out a way around the heaviness of a hypervisor is a Big Deal. And as I understand it, it uses the resources of the host machine very intelligently. With the previous VM paradigm, you gave both disk and memory permanently (insomuch as a VM that you create & tear down relatively easily is permanent) to the machine, and the VM held onto that inaccessibly while in use. No such selfish tactics are employed by Docker and use of memory in particular is supposedly much more elastic.

Installation was just a few shell commands which worked with no fuss, I used this official site. The instructions are for Ubuntu 14.04 which Mint 17 is based on, and as I’ve covered above, all Ubuntu packages can be used on Linux Mint with little to no adaptation required.

sudo apt-get update
sudo apt-get install apt-transport-https ca-certificates
sudo apt-get install linux-image-extra-$(uname -r)
sudo apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
echo 'deb https://apt.dockerproject.org/repo ubuntu-trusty main' >> /etc/apt/sources.list.d/docker.list
sudo apt-get install docker-engine
sudo apt-get update

Then, the Docker engine can be started! First start the service (this may be different if your linux distro uses systemctl, like a newer version of Fedora or RHEL/CentOS) & then make sure their dummy example works.

sudo service docker start
sudo docker run hello-world

Ok! Now we’ve got Docker installed. If you encounter anything funny here during installation I’d love to hear about it, please leave a comment! I can try to help.

There are two primary ways for the new end-user to continue forward. One of them is writing a Dockerfile. I leave that for its own blog post, but hold me to it! I want to write a few basic Dockerfiles for my own reference & I imagine they’ll be of some use to others! The second way I’ll mention here is via DockerHub, which you don’t even directly need to interact with to use! To me it’s reminiscent of pulling images down from VagrantCloud, but again, you don’t even have to go there to grab things. You can feed a few different common distros into the docker run command and it will pop you right into a container of that OS! It’s still rather magical at this point, so I’m still learning more about it so it becomes a bit less magical πŸ˜‰

Getting into the box I wanted was as simple as plugging in the distro I wanted, Fedora, into the docker run command, like so:

sudo docker run -it fedora /bin/bash

What this led to for me, which I need to learn SO much more about, is an EXTREMELY spare version of Fedora that has very few executables I’m used to. Because it’s the latest version of Fedora, its package manager is dnf & not yum, but it still knows what you mean & permits installation “via” yum – but really it’s just aliased to /usr/bin/dnf, haha, which is fine.

Quick fun fact: dnf is an abbreviation of “Dandified Yellowdog Updater, Modified” – you can see the yum in there as the commonly used RHEL and RHEL-flavored linuxes’ package manager. To me, whenever I type dnf install -- it looks as though I’m typing “do not f&$%ing install” πŸ˜€

So, next steps, make the container persistent – it isn’t yet, and play around with Dockerfiles, and just do a little more spying on the produced container itself & probably try to do some babby’s frist load testing things in there & spy on the container as a process without the box & all its processes within!

See you next time! Would love to hear from you. If I’ve missed the mark on anything with this pretty chewy piece of technology please let me know, or if there’s anything you’d like to see me cover leave a comment!

btw the title of this is a joke which was made during the Docker-fiddling open space I held at Open Source Bridge 2016 πŸ™‚ if you were there, thanks for coming! Super fun discussion.