Intro to Docker for (Rails) Developers

Intro to Docker for (Rails)
Developers
Chris Johnson :: @johnsonch :: Intro to Docker for (Rails) Developers

Who am I
Chris Johnson
— Dad and Husband
— Most places @johnsonch
— Ops Manager @ healthﬁnch
— Into IoT, Hockey, Racing, Food and Beer

What are we going to talk about today?
— Docker
— Some of my experiences
— Possibly Docker Compose

What are we not going to talk about today?
— Production deployments
— Cupcakes
— Things I don't want to talk about

Ground rules
— These are my experiences and opinions, your mileage
may vary
— I'm not here to argue, if you want to do that buy me a
beer afterwards
— Ask questions, but again I'm not looking for a ﬁght

What is Docker?

What is Docker?
Docker is used to run software packages called "containers".
Containers are isolated from each other and bundle their own
tools, libraries and configuration files; they can communicate
with each other through well-defined channels. All containers
are run by a single operating system kernel and are thus more
lightweight than virtual machines. Containers are created from
"images" that specify their precise contents. Images are often
created by combining and modifying standard images
downloaded from public repositories.

TL;DR;
Docker is a company that makes it easier to work with
containers

Think of it as a box for a single
product, one that makes
transporting and storing
consistent and universal
Well then smarty pants, what the h-e-double-hockey-sticks is a container
— An isolated execution environment
— Speciﬁcally an isolated process that doesn't know
about the rest of the system
— Different from VMs in that they don't require a
hypervisor and entire OS instead they piggy-back on a
single Kernel

What's an image?
— The blueprint for the container
— It's like a factory
— What we distribute

Container vs Image
You run a container and share an image

Installing docker
— From Docker: https://store.docker.com/search?
type=edition&offering=community

Demo Ge!ing Started

Verifying docker
$ docker version

By running the docker version command we'll get an output similar to the following.
Client:
Version: 18.06.1-ce
API version: 1.38
Go version: go1.10.3
Git commit: e68fc7a
Built: Tue Aug 21 17:21:31 2018
OS/Arch: darwin/amd64
Experimental: false
Server:
Engine:
Version: 18.06.1-ce
API version: 1.38 (minimum version 1.12)
Go version: go1.10.3
Git commit: e68fc7a
Built: Tue Aug 21 17:29:02 2018
OS/Arch: linux/amd64
Experimental: true

Our ﬁrst docker container
$ docker run bash:latest echo 'hello world'
Gives us the following
Unable to find image 'bash:latest' locally
latest: Pulling from library/bash
4fe2ade4980c: Pull complete
ec6d9ca5c66a: Pull complete
d8685fbd86ca: Pull complete
Digest: sha256:8634afcddefc8a10565b22d685df782058b096712a91bf45d75633f368dda729
Status: Downloaded newer image for bash:latest
hello world

Lets see another example
$ docker run ruby:2.4 ruby -e "puts :hello"
Here we can see that there is more to the Ruby image and it takes a little long to
pull down.
Unable to find image 'ruby:2.4' locally
2.4: Pulling from library/ruby
bc9ab73e5b14: Pull complete
193a6306c92a: Pull complete
e5c3f8c317dc: Pull complete
a587a86c9dcb: Pull complete
72744d0a318b: Pull complete
31d57ef7a684: Pull complete
c8c56b2e15ce: Pull complete
926f1d0d6e72: Pull complete
Digest: sha256:3c9ac52e86ebdb9cf0d42a459ccb016ebfba347c0a5d4621fa8d70a5ede72c7b
Status: Downloaded newer image for ruby:2.4
hello

That's because we already
downloaded the image so it's
cached locally. This is a
strength of Docker it makes
subsequent runs faster.
Let's run the example again:
It only shows the output!

We'll see later how complex
these can get!
Let's break apart the two commands we ran to see the
similarities between the two container runs
$ docker run bash:latest echo 'hello world'
Breaks down into
$ docker run [OPTIONS] <image>:<tag> <command>

So if there is caching what all is le! on my system?
First let's look at what is running on our system
$ docker ps

Now let's list all the containers including stopped ones
$ docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
ee0237d3741d ruby:2.4 "ruby -e 'puts :hell…" 9 hours ago Exited (0) 9 hours ago gracious_pasteur
b97d3f4758c3 ruby:2.4 "ruby -e 'puts :hell…" 9 hours ago Exited (0) 9 hours ago relaxed_swanson

There are other commands or
chains of commands out there,
this is unix afterall, that clean up
all images without you having to
type out a list but that's a bit more
advanced than what we're going
to get into.
We can clean up our containers by running
$ docker rm <container id> [<container id2> ...]

Demo generating a new Rails
app

Let's generate a new Rails app
$ mkdir new_rails_app
$ cd new_rails_app
$ docker run -it --rm -v ${PWD}:/usr/src/app ruby:2.4 bash
Inside the container move into the app directory and install rails gem install rails
$ rails new myapp --skip-test --skip-bundle
Let's see what was generated
$ ls -la

Ok now let's use what we know to run this "app"
$ docker run -it --rm -p 3000:3000 -v ${PWD}:/usr/src/
app ruby:2.4 bash -c "apt-get update -yqq && apt-get
install -yqq --no-install-recommends node js && cd /usr/
src/app && gem install bundler && bundle install && bin/
rails s -b 0.0.0.0"

That is a terrible command and
it takes for ever for the app to
boot!

Bring on the Dockerﬁle

Here we have a small example
of a Dockerfile. Each line has
an action or actvity. Those are
in all caps FROM, RUN and
COPY are examples.
Dockerﬁle
Inside of our Rails app we'll create a ﬁle called
Dockerfile with the contents
FROM ruby:2.4
RUN apt-get update -yqq
RUN apt-get install -yqq --no-install-recommends nodejs
COPY . /usr/src/app/
WORKDIR /usr/src/app
RUN bundle install

Let's use this Dockerﬁle to build an image!
$ docker build .

Shows us the images we have
locally and we can run using a
specific one by specifying the
image id
Then verify it
$ docker images

Much shorter but still a ton to
remember. Let's make it a little
less confusing by adding
some tagging
Now we can run our app again
$ docker run -p 3000:3000 4ea99c4f5025 bin/rails s -b
0.0.0.0

Tags
— They are references to a speciﬁc point in time
— Can be used again
— There are no 'magic' tags - latest isn't allways the
latest

Tags
Let's tag our image
$ docker tag <image id> mkecc

Tags
You can have multiple tags that point to an image
$ docker tag mkecc mkecc:1.0

Ok that's cool, let's get some house
keeping out of the way. Because each
layer is a cache of the state of the
image, we're going to want to make
sure we keep it lean and mean. We can
use a .dockerignore file to ignore
some things from being cached.
Tags
Then we can run our command specifying a tag
$ docker run -p 3000:3000 mkecc:1.0 bin/rails s -b
0.0.0.0

Caching
— Each line in the Dockerfile is a new layer
— It stores information about the state of the image at
that point in time, ﬁles and all

Caching
We can avoid some of our app being cached by adding a .dockerignore ﬁle, which works much like a .gitignore ﬁle for Git.
This will take out some of the known Rails wasted spaces
.git
.gitignore
log/*
tmp/*
*.swp
*.swo
Then we can build again!
$ docker build -t mkecc .

Gotcha with cache
If the code in the Dockerfile doesn't change it's not
going to run again. So for something like the apt-get
update we'll want to chain it with our apt-get install.
This also means that if we change files in our Rails app
it's going to copy and re-bundle! Let's fix the apt-get and
then make the rails app not re-bundle unless there is a
change to the gemfile.

Gotcha with cache
RUN apt-get update -yqq && apt-get install -yqq --no-install-recommends nodejs
COPY Gemfile* /usr/src/app/
WORKDIR /usr/src/app
RUN bundle install
COPY . /usr/src/app/

Gotcha with cache
Let's build it again (maybe even a few times)
$ docker build -t mkecc .
Now we see that we don't end up bundling every time
we build!

One ﬁnal adjustment
Before we were starting up the Rails server by specifying
the comand when we ran the container with $ docker
run -p 3000:3000 mkecc:latest bin/rails s -b 0.0.0.0
let's add a default command to our Dockerfile
CMD ["bin/rails", "s", "-b", "0.0.0.0"]

One ﬁnal adjustment
Now we can get it down to the following command!
$ docker run -p 3000:3000 mkecc:latest

Wrap up
We learned:
* What Docker is
* The difference between a container and an Image
* How to make a Dockerfile
* How to run a Rails application from a container

https://www.apress.com/us/
book/9781484224144
^ https://pragprog.com/book/
ridocker/docker-for-rails-
developers
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