Dockerizing Applications - DevOps Playground #9

Requirements

1. Vagrant
2. Virtualbox

Summary

"Dockerizing" an application can be described as enabling an application to run in a container based environment such as Docker or Kubernetes. Applications can be purpose built to run in a Docker environment or existing applications can be retrofitted.

In this meetup, we will 'Dockerize' a simple Gitbook website. We will begin by using Docker to Serve the book and then we will improve this to separate out our Build steps from our Deployment steps.

0 - Setting up

To get started, install the requirements listed at the top. Once you have them, clone or download this repository, cd into it and type vagrant up. This might take a few minutes the first time around.

Once the VM is ready, type vagrant ssh into your terminal and you will be logged into the VM.

Throughout the project make sure you work on a file called "Dockerfile", this should be in /vagrant

1 - Our Dockerfile

Every Dockerized application needs at least one Dockerfile, this will describe the steps necessary to create our container image.

FROM node:latest
MAINTAINER Your Name <your@email.com>

ADD ./mybook /mybook

CMD ["/bin/bash"]

The above Dockerfile does the following:

1. Pulls the node:latest image
2. Copies the gitbook to the container
3. Tells the container to run bash on start

So next we need to build it:

• sudo docker build -t 'mybook' .

And then we can run it to check our files are there:

• sudo docker run --rm -it 'mybook'
• ls -la /mybook

When you're done, exit the container.
No need to remove it, the --rm in the run command did that for us! We also specified -it, meaning we're likely to interact with the container via terminal. This is a basic first container, however it doesn't really do much right now, all we do is put our files onto the container. Nonetheless, this is a good first step.

2 - Serving the book

The next step is to serve our Gitbook. The easiest way to do this is by using Gitbook's built in webserver, which will dynamically reload pages as they change. Handy for development!

FROM node:latest
MAINTAINER Your Name <your@email.com>

ADD ./mybook /mybook
RUN npm install -g gitbook
RUN npm install -g gitbook-cli
RUN gitbook install

WORKDIR /mybook
EXPOSE 4000

CMD ["/usr/local/bin/gitbook", "serve"]

So what's changed here? Firstly, we've added in a RUN directive to execute npm and install Gitbook. We've also specified a WORKDIR, this is where everything in the container takes place by default. To serve our book we also need to EXPOSE a port to allow incomming connections. Finally, we need to serve our book and have modified our CMD directive to reflect this.

Let's build our image again and then run it:

• sudo docker build -t 'mybook' .
• sudo docker run --rm -it -p 4000:4000 'mybook'
• Visit the book in your browser: http://localhost:40000

This is great progress, now that we have our book being served we can look to improve the Dockerfile, because right now it is attrocious!

3 - Improving our Dockerfile

Our last Dockerfile flaunted best practise, this not only makes our images less manageable but can also severely impact build time and the total size of the final image.

We could run it through a Linter such as FROM:LATEST or use one in our IDE, however this Dockerfile isn't really big enough for that.

So let's list everything wrong with this image:

1. FROM node:latest - The latest tag is awful and should never be used in a Dockerfile. How do we know what we're getting between builds?

2. ADD - As our source is not a URL or a tarball we should use COPY (see here for further reference)

3. RUN - We have multiple run commands that perform similar tasks. These should be grouped together to save on the number of layers (*see below) created.

4. CMD - There are very few situations where simply using this directive is the best choice. Instead we should be using ENTRYPOINT with a CMD directive to pass default parameters.

5. The ordering of our ADD and RUN statements means that changing the book's source will trigger the RUN statement's caches to be invalidated, and so they will run again. (You can observe this by creating a new file inside mybook and rebuilding the image.

6. We don't clean up after our npm install actions and so are wasting space.

7. gitbook serve is fine for development, however we'd be better off building the contents and serving them using something like nginx.

So let's take a look at our improved Dockerfile:

FROM node:argon-slim
MAINTAINER Your Name <your@email.com>

RUN npm install -g gitbook && npm install -g gitbook-cli && gitbook install \
&& rm -rf /tmp/*

COPY ./mybook /mybook
WORKDIR /mybook
EXPOSE 4000

ENTRYPOINT ["/usr/local/bin/gitbook"]
CMD ["serve"]

We've actioned most of the above complaints here. We've merged the RUN directives into one and tidied up the /tmp folder after. We also now make use of the ENTRYPOINT directive, which is generally best practise. Finally we've also made sure our statements are in a more sane order, going from least entropy (changes infrequently) to most entropy (changes often) from top to bottom.

* What's a layer? Think of it like a Git Commit, it contains a list of changes that can be used to build an image. Images can share these layers, like branches can share a common commit.

4 - Why serve when we can build?

As mentioned in the last section, currently this container is using a pretty slow and wasteful method of serving the book to us. Let's fix that by using gitbook to build our book for us into static HTML, which we can then serve in any HTTP server.

FROM node:argon-slim
MAINTAINER Your Name <your@email.com>

RUN npm install -g gitbook && npm install -g gitbook-cli && gitbook install \
&& rm -rf /tmp/* && mkdir /mybook

VOLUME /mybook
WORKDIR /mybook

ENTRYPOINT ["/usr/local/bin/gitbook"]
CMD ["build"]

As you might see, not a huge amount seems to have changed. That said, the effect is that this container will no longer serve the site, nor will it have a copy of the files inside it.We have added && mkdir /mybook to the end of the RUN block, just to ensure it exists.

We no longer COPY the source files in, now we have a VOLUME directive. This tells Docker that the contents of this folder is going to be stored outside of the container, meaning that data inside of it will persist between docker run commands. The purpose of this is so that we keep our source out of the container and do not need to rebuild this every time our code changes. It also means this container could be used to build multiple books from different projects!

(For more on this, please look here)

So, let's build the image:

• sudo docker build -t 'builders/gitbook' .

And then let's build our book:

• sudo docker run --rm -v $(pwd)/mybook:/mybook 'builders/gitbook'
• -v tells Docker to mount the specified directory into the container

And finally, let's inspect the output:

• ls -la ./mybook/_book

You should (hopefully) see the markdown has been compiled into static HTML! This is perfect, we can now put this inside an nginx container to serve our HTML for us.

5 - Let's nginx-ify this

Imagine thousands of users look at this book on a dailt basis, we probably want some robust load balancing. nginx is a good choice for hosting our site, and while load-balancing is out of scope for this meetup, we can certainly get nginx up and running in the most basic sense.

We have two options, we can either keep the code separate from the container as with our builder above, or we can bake it in. Which option we take really depends on how concerned we are with the portability of the container.

If we bake it in, the container can run anywhere easily but needs to be built every time we make a change (as well as compiling our HTML). If we keep the source out of the container, it needs to aleady be on whatever host we use to run the container.

Option 1 - Keep them separate

This is the easiest option to demo, all we need to do is run an nginx container, bind the HTTP port to the host and mount the source code inside the container.

Let's try:

• sudo docker run -d -p 80:80 -v /vagrant/mybook/_book:/usr/share/nginx/html 'nginx:latest'
• -d tells Docker to run this container as a daemon in the background
• Check the site out by visiting http://localhost:800

This method is useful if our source changes very regularly and is being ran from the same host, particularly in instances such as local development.

Before moving on, make sure to tidy up your old containers:

• sudo docker rm -f $(sudo docker ps -aq)
• Beware that this will remove all containers, else just remove the nginx container

Option 2 - Bake it baby!

The more 'Docker' method here is to bake the code in, we want our container to be built and ran anywhere without hassle, configuration or downloading multiple sources.

To do this, we need another Dockerfile. Call this one Dockerfile.nginx:

FROM nginx:1.11.8-alpine
MAINTAINER Your Name <your@name.com>

COPY mybook/_book/* /usr/share/nginx/html/

This is nice and simple, we don't need to do anything extra here as the original container already exposes ports and sets up an entrypoint. All we have to do is add in our code!

Let's build:

• sudo docker build -t 'mynginx/mybook' -f Dockerfile.nginx .
• This will save the source code inside it, so we don't need to provide it elsewhere

Let's run:

• sudo docker run -d -p 80:80 'mynginx/mybook'
• As before, let's check it out on http://localhost:800

Conclusion

It doesn't really take much to 'Dockerize' an application, the bigger effort is making sure that you make full use of the capabilities Docker provides you. Most applications should just need to have their binaries/source placed in a container with minimal heavy lifting needed. Otherwise, as ever Google is your friend - it's almost certain someone has already done what you're looking to do and probably has a container for that!