Intro to Docker

Upcase
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On this week's episode, Chris is joined by thoughtbot CTO Joe Ferris to discuss Docker, the open platform for building and running containerized applications. What is Docker? Docker is a platform built to support working with containers for isol...
This is a companion discussion topic for the original entry at https://thoughtbot.com/upcase/videos/intro-to-docker
2 Likes
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Thanks, that was great explanation of some Docker concepts Iā€™ve found tricky to understand. Itā€™s encouraged me to try containerize one of own apps.

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Certainly the biggest caveat in dev is performance.

It is perfectly fine when used on Linux.
But on a mac since it runs through boot2docker (a small vm) and since you have to share your projet repo with boot2docker and then boot2docker shares them with docker containersā€¦ It slows everything down

It works with small projects but when you begin to have a lot of assets it becomes really really slow.

Make sure to share your directories with boot2docker through nfs.
An even better solution is to create a custom vm with vagrant. Install docker on this vm and sync your directories with rsync (rsync is available as an option with vagrant and shared directories).

4

Thanks Chris, Joe, and Thom. This motivated me to noodle around a bit with docker. I did run into an issue with one of the commands from the show notes. I figured Iā€™d add some documentation here about my experience for anyone else following along and having trouble on OS X.

Environment details:

OS X with a Virtualbox guest provisioned with docker-machine

VirtualBox 5.0.4-102546

=> docker-machine --version
docker-machine version 0.4.1 (HEAD)

=> docker --version
Docker version 1.8.2, build 0a8c2e3

Starting the upcase rails server with the following command fails:

docker run --link=db -it upcase -p 3000 bin/rails server -b 0.0.0.0

Error response from daemon: Cannot start container e82e64ed06bb1dc476d5f0a2b6156afd56d458a96fb95daf6be23411649de663: [8] System error: exec: "-p": executable file not found in $PATH

This indicates that the container is trying to run the port option as a command within the container. Ok no problem, just move the -p 3000 option before the -it upcase option.

Now the container starts, but requests to localhost:3000 fail with the following error:

Failed to load resource: Could not connect to the server.

This indicates that the host OS canā€™t connect to the Virtualbox Docker VM running our container. So forward the VM ports through to the host.

  1. Open Virtualbox
  2. Open the Settings for the docker-machine VM
  3. Choose the Network tab
  4. Click the Port Forwarding button
  5. Add a rule with the following values:
  6. Name: Webrick
  7. Protocol: TCP
  8. Host IP: 127.0.0.1
  9. Host Port: 3000
  10. Guest IP: (intentionally blank)
  11. Guest Port: 3000
  12. Click OK
  13. Click OK

Now when we navigate to localhost:3000 from the host OS weā€™re faced with the following error:

Failed to load resource: The network connection was lost.

Simple enough, now we need the docker container to setup the host-guest port forwarding. So stop the upcase docker container and start it again with the following command ā€“ notice the changes to the -p 3000:3000 parameter:

=> docker run --link=db -p 3000:3000 -it upcase bin/rails server -b 0.0.0.0

Now visit localhost:3000 and you should see a response from the rails process running inside the upcase container.

Hope this helps someone else whoā€™s having a hard go of it on OS X.

1 Like
5

Any suggestions on using docker in development and still being able to do BDD / TDD. I like being able to run specs from my editor, and I like that spring has made running specs and getting feedback super fast, but I am not sure how to achieve that using docker.

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Iā€™ve been doing TDD with Docker. Iā€™m pretty swamped today, so I canā€™t give a cohesive answer, but here are some tips:

  • You can override the command for a service using docker-compose run, so you can run commands like docker-compose run web rspec <file>.
  • If youā€™re using vim-rspec, you can override the command to include docker-compose run. You can also create new binstubs which include the docker-compose commands.

Iā€™ll try to post a more comprehensive guide soon.

7

Huge help! Thanks for taking the time to write such a detailed explanation. (=

1 Like
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@jferris Iā€™m having issues with Docker and capybara-webkit. Didnā€™t find much help on google. Using the headless gem as suggested here didnā€™t help.

The error Iā€™m getting is:

Capybara::Webkit::ConnectionError:
       /usr/local/bundle/gems/capybara-webkit-1.6.0/bin/webkit_server failed to start.

My Dockerfile looks like:

FROM ruby:2.3.0
RUN apt-get update -qq \
  && apt-get install -y --no-install-recommends \
  dbus-1-dbg \
  libpq-dev \
  libqt5webkit5-dev \
  libqtwebkit-dev \
  nodejs \
  qt5-default \
  xvfb

RUN mkdir /app
WORKDIR /app
COPY Gemfile /app/
COPY Gemfile.lock /app/
RUN bundle install
COPY . /app/

Any ideas?

9

Have you tried using xvfb-run? Iā€™ve had good luck with that. Looks like you already have xvfb installed, so you can do xvfb-run -a rake, for example. Check out the man page for more info.

If you run webkit_server directly, it may provide more information about what went wrong. You can use something like: docker-compose run web /usr/local/bundle/gems/capybara-webkit-1.6.0/bin/webkit_server.

10

Out of interest, does anyone using Rails with Docker also have a private gem server they need to install Gems from? If so how do you manage that in your Dockerfile? By default bundler stores the credentials for a private repo in ~/.bundler/config on the host machine. A Docker build cannot see that file by design, so I canā€™t copy it into a Docker image so that private gems will be resolved.