Cloud Computing Basics 6

Docker here, Docker there, and no Docker build anywhere. That's not fair. How about we create the first docker? Docker image is built from Dockerfile. The syntax of Dockerfile is almost the same as Bash. This is an example of Dockerfile:

FROM debian:jessie
MAINTAINER Goran Mekić <meka@lugons.org>
RUN touch /some-file.txt

To build it run docker build -t username/repo . in a directory containing Dockerfile. It's advisable to have a HUB username, as registration is free and has autobuild capabilities. We'll deal with simple builds for now, and leave autobuild for some future post.

To upload your image issue this command:

docker push username/repo

Docker will ask you for username/password/email combination. Fill it up, wait for upload to finish and that's it. You have your first Docker image. Let's give it a spin.

docker run --rm -i -t username/repo /bin/bash

What it does is (simplified):

• --rm: remove container when it stops
• -i: this will be interactive container (read: someone will type commands in it)
• -t: give me a terminal emulation

Note that every command in Dockerfile will create additional layer. It means that Docker images are organized as multiple file system layers which have dependencies. Much like a git repo branch is pointer to commit which has its own dependencies, Docker image remembers which file system layer is on top. As every layer remembers which layer it depends on, you can have dependency line. In the example above, there will be at least 3 layers: FROM, MAINTAINER and RUN lines make them. This has consequences you have to be aware of. First, if the line in Dockerfile and dependent layers didn't change from last build, Docker will use last build's layer, not build it (read: cache). Second, EVERY line in Dockerfile creates layer. So, if you create 1GB file on one line, delete it on the other, you'll have a small layer (from deleting 1GB) dependent on a big layer (where you created 1GB), although lower layer is unusable, because upper layer effectively masked it.

The "trick" I use is to have a build script which will cause 2 lines in Dockerfile: one for ADD and one for RUN. I start with debian:jessie, add all build tools, build my app, remove build tools and do the cleanup. The build does last much longer, but you end up with MUCH smaller images. I managed to shrink One Love API image from 1GB to 298MB just using this. What I think would be the optimal solution are two images, one for building, one for using application. And guess what. There are. For example, you have python:latest and python:onbuild images.

I leave it up to you how to build your applications, these are just some ideas. Idealy, you can base your application on busybox environment, and use images as small as 5MB. If not, lurk around for your perfect solution.

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