Using Docker for your Rails Development Environment

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Using Docker for your Rails Development Environment 2019/04/03 Tokyo Rubyist Meetup at PIXTA, Inc.

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Hi there ● Gentaro Terada (@hibariya) ● Works at ESM, Inc. ● A college student (engineering) ● Likes Ruby, Internet, and Programming ● https://hibariya.org

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Today we will talk about: ● Why/How to use Docker ● Problems/questions I faced

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Why To Use Docker

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What is Docker With Docker, we can: ● Create/run containers ● Create/share container images (kind of snapshots)

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Container is lighter Container vs. Virtual Machine https://www.docker.com/resources/what-container

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Why To Use Docker Docker makes it much easier: ● Build once, run anywhere ● Less “It works on my machine” ○ The same condition (Runtimes, Libraries, Server Applications, and so on) ● Re-creating environments easily/quickly ● Various environment can live together on the same host machine

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How To Use Docker

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The First Step A development environment for a small Rails application: 1. Create a Dockerfile to build containers 2. Create a docker-compose.yml to organize containers

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Dockerfile for a Rails app

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Creating a new Rails app On the host: $ docker build -f Dockerfile -t rails-docker-example . $ docker container run -it --rm \ --volume=$(pwd):/work --workdir=/work \ rails-docker-example bash On the container: # gem install rails --no-document # rails new example --database=postgresql --skip-bundle …

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docker-compose.yml to Organize

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Invoke the app $ docker-compose run --rm rails bin/setup $ docker-compose up

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Running a command inside a container Use a running container: $ docker-compose exec rails bin/rails c Create and run a new container: $ docker-compose run --rm rails bin/rails c

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For more details https://github.com/esminc/rails-docker-examples ● Example Rails app + PostgreSQL + Webpacker ● Configurations for Docker Compose ● Configurations for Kubernetes (w/ GKE)

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Problems I Faced

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Problems I Faced ● Testing with browsers ● Dockerfile doesn’t ensure the same image ● Too large images ● Generated files are owned by root ● Long / complicated commands ● Could not we use K8s for development?

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Testing with Browsers

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How to run browser testing? ● Plan A: Run a Selenium container and use it remotely (I recommend) ○ Easy debugging; Can view/inspect running browser via VNC ○ Does not add another dependency to the Rails container ● Plan B: Install a browser on the Rails container and use it in headless mode ○ Less custom settings (probably) ○ Needs browser installation on the Rails container

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docker-compose.yml test/application_system_test_case.rb

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Interactive Debugging with VNC Stop w/ a breakpoint, connect to localhost:5900 w/ VNC

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Dockerfile Doesn’t Ensure the Same Image

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Dockerfile doesn't ensure the same image ● Updates on base image ● Repository changes for package management system ○ e.g., EPEL-5 repository EOL (March 2017) ○ So the following Dockerfile does not work anymore

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Share the image

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Too Large Images

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Rails application images tend to be large We could: ● Use smaller base image ● Install only mandatory packages ● Remove caches at the same time ● Use multi-stage builds for assets compilation ● Avoid using Git repositories as gem sources ○ as long as you can

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The More You Layer, the More It Gets Bigger Deleting existing files does not decrease the whole size Exploring the layers of an image with github.com/wagoodman/dive

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Drop build-time deps w/ multi-stage build Node.js, NPM and node_modules can be removed in the same way https://docs.docker.com/develop/develop-images/multistage-build/

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Avoid Git source if possible gem ‘rails’, github: ‘rails/rails’ loads...

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Generated Files Are Owned By root

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Generated files are owned by root

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Plan A: Add a user Add a user with the same UID as the user on the host

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Plan B: userns-remap Or, adjust the UID seen from the host with userns-remap

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Long/Complex Commands

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A kind of typing game? More than twice as long as the original command! $ docker-compose exec rails bin/rails g model User email:string $ docker-compose exec rails bin/rails db:migrate $ docker-compose exec rails bin/rails c

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Binstubs? Ended up with creating dedicated binstubs for each project

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Exec vs. Run; Which should I use? It depends... ● docker-compose exec ○ Uses an already running container ○ Can’t publish additional ports ● docker-compose run ○ Creates and uses a dedicated container ○ Without `--use-aliases`, couldn’t resolve some of the host names in some cases ○ Without `--rm`, the created container will remains after exit

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Crane Cosy interface to develop ● Reads docker-compose.yml ● Grouping services ● There’s only `run` ○ Works nicely by the situation

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Could We Develop Apps w/ K8s?

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Is Kubernetes only for production?

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Why don’t you use the same tool https://speakerdeck.com/mtsmfm/development-env-in-the-microservices-age

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Is it possible to develop apps w/ K8s? Yup. ● Good news: We can use the same tool for development ● Bad news: We will have to manage a lot of YAML files ○ Because of the difference of the API ○ Because K8s projects tend to be large ○ For not only production, but also development

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Organize YAML files w/ Kustomize https://github.com/kubernetes-sigs/kustomize

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Et cetera...

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References ● Docker for Rails Developers ● gh.c/esminc/rails-docker-examples

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Example Repository https://github.com/esminc/rails-docker-examples ● Example Rails app + PostgreSQL + Webpacker ● Configurations for Docker Compose ● Configurations for Kubernetes (w/ GKE)