And Keep Your Hot Code Reloading I’ve seen a lot of articles lately suggesting how to use Docker for development. I haven’t seen one yet that does it correctly. Obviously, correctly, is subjective, but I’d like to compare the typical wisdom, vs. how I usually approach the problem. The Conventional Wisdom In many tutorials, the first thing introduced is the Dockerfile. At the foundation of any Dockerized application, you will find a — Dockerfile https://blog.codeship.com/using-docker-compose-for-nodejs-development/ It apparently, is the foundation. The first several results on Google all suggest the first thing you need is a Dockerfile, as well. After all, how can you have a Docker environment without creating a Dockerfile? I’m here to tell that while this is true for production, it’s the wrong approach for development. You do not need to create your own. A Dockerfile is a way to package your application. You don’t need to package your application for development, and honestly, you really shouldn’t. Development and production are not the same environment. When you develop on your MacBook you install different tools than you use running in production. Just because “it runs the same way everywhere” doesn’t mean it should. Your app runs differently in development. Packaging it in a time where it is meant to be flexible and malleable is why many engineers have come to the conclusion the Docker isn’t for development. You lose the flexibility of development by needing to build new containers when you have changes to dependencies change for example. Sure, you could exec into the container and perform some commands, install some libs, but is it really less effort at this point? Now, some of the above articles got this more right than others, but if you’re using a Dockerfile for development, you’ve probably already gone too far. There are situations where you will want one, but probably not in the manner you think. : If your Dockerfile contains an npm install you’ve gone too far. Hint The docker-compose builder pattern Let’s talk about what Docker for a moment. is Docker is a way to package your code. This is the typical for using Docker. context Docker is also a way to create an isolated environment which is capable of executing certain types of applications. Docker allows you to package environments that are capable of running your code. When you use Docker for production you are using the most specialized Docker containers you can make. They are customized and specifically built for application, packaged in the way built it. For this purpose, creating a Dockerfile makes sense. your you When you set up your computer for development, that’s not what you do. You instead install the tools that you need for development. You just need to create an environment which your code can run in. This means you can use a more generalized Dockerfile. Usually, these generalized Dockerfiles you need for development already exist. For example, when developing a Node.js application, you need installed on your machine. That’s it. node You don’t need alpine linux. You don’t need to package your into an immutable build. You don’t need little containers to exec into to make significant changes. You just need to be able to execute and . node_modules node npm Therefore, in a container, that’s all you need as well, meaning the official image on Docker Hub will do just fine. node Without further ado, my approach to development with Docker. In I showed how to use Parcel for development and production. Let’s keep that rolling, and build on top of that. my last article I think it’s a good example because Hot Module Reloading is essential for developing React apps efficiently. Step One First, we need a docker-compose file. In it, we need our development environment. Seems how we are making a node app, that means the officalnode image is probably a safe bet. Let’s add a file : docker-compose.yml version: '3' services: dev: image: node:11 Next, we need our code to be in the environment, but we don’t want it to be baked into the image. If we are using this for development, when our files change, the files in the container also need to change. To accomplish this we can use a volume. We will mount our current directory to in the container. We will also need to tell docker where our “working directory” is. Meaning — what directory did we put the code in? . /usr/src/service version: '3' services: dev: image: node:11 volumes: - .:/usr/src/service working_dir: /usr/src/service Now, every time we make a change on our local machine, the same file changes will be reflected in . /usr/src/service Next, we need to execute the command . This is easily accomplished with a . We also want to access it locally on port . npm run dev command 1234 Finally, hot module reloading with Parcel happens on a random port by default, which won’t work for us, as we need to map the HMR port as well. Modify the script in to include the option . dev package.json --hmr-port=1235 : , "dev" "npm run generate-imported-components && parcel app/index.html --hmr-port 1235" And with that in place, let’s update the Docker file to map the ports on our local machine to the same ports on our container. version: '3' services: dev: image: node:11 volumes: - .:/usr/src/service working_dir: /usr/src/service command: npm run dev ports: - 1234 :1234 - 1235 :1235 If you’ve done enough Node development, you’ll notice we have a problem. You can’t just run a node app without installing dependencies. Also, you can’t just install your node modules locally on Mac or Windows and expect them to work on the linux container. When you run a build in some cases libraries compile natively and the resulting artifact only works on the operating system it was built on! As a first attempt, you may be tempted to just chain and in a single command, and sure enough that would work, but it’s not quite what we want. This would require to run an install every time we started development mode with the container. npm install npm run dev Also, some services beyond needing an install, also might need a build step. In our case, this isn’t needed for developing the client because parcel or nodemon handle it, but not all apps were built in the past week with the latest tech. For educational purposes the way to chain commands is using or to execute the command. If you try bash ash : npm && npm run dev command install You will learn that doesn’t work. Instead you can could use. : - command bash c "npm install && npm run dev" This would in fact work, but is not the optimal solution we are looking for. Which brings us to Step Two. Step Two Let’s create another docker-compose file, this time named . docker-compose.builder.yml We will need to use this time to make use of a feature in that isn’t available in the version 3 specification. version: 2 docker-compose Version 3 is more suited towards use in production than version 2, which has more development friendly features. UPDATE: V3 also supports this now in a slightly different syntax — would love a PR to get it updated :) Here’s the docs: https://docs.docker.com/compose/compose-file/#extension-fields The first thing we want to define in is a base image. docker-compose.builder.yml version: '2' services: base: image: node:11 volumes: - .:/usr/src/service working_dir: /usr/src/service This should look pretty familiar. It’s the same base we use in our file. docker-compose.yml Now, we can extend the base to execute a whole bunch of different commands. For example: version: '2' services: base: image: node:11 volumes: - .:/usr/src/service/ working_dir: /usr/src/service/ install: extends: service: base command: npm i build: extends: service: base command: npm run build create-bundles: extends: service: base command: npm run create-bundles Now, to install dependencies using a image which matches our development service in we can run: node:11 docker-compose.yml docker-compose -f docker-compose run --rm install .builder .yml To install the versions of binaries needed. Admittedly, docker-compose -f docker-compose.builder.yml run — rm install, doesn’t really “roll off the tongue”, does it? I usually put this in a Makefile so can just run make install, etc. Pro Tip: After running the install, docker-compose up will bring up our development environment, which works exactly the same as it would on your local machine. ➜  docker-compose up
Creating stream-all-the-things_dev_1 ... done
Attaching to stream-all-the-things_dev_1
dev_1  | | -all-the-things@ dev /usr/src/service
dev_1  | run generate-imported-components && parcel app/index.html
dev_1  | | | -all-the-things@ generate-imported-components /usr/src/service
dev_1  | -components app app/imported.js
dev_1  | | app imports...
dev_1  | imports found, saving to app/imported.js
dev_1  | running http://localhost: dev_1 > stream 1.0 .0 > npm dev_1 dev_1 > stream 1.0 .0 > imported dev_1 scanning for 1 Server at 1234 And when we make a change, hot code reloading works as expected! All with no Dockerfile! Bonus I just wanted to quickly add an example Makefile that will the commands easier to remember and use. make Create a file called : Makefile install: docker-compose -f docker-compose.builder.yml run --rm install dev: docker-compose up Makefiles use tabs. Makefile’s will not work with spaces. 😢 👋 😬 Now you can run and . make install make dev The End? Not quite. It appears I’ve caused some confusion by sharing a local volume between two containers. Many were quick to point out that you can use a volume and suggested something like the following: version: services:
  dev:
    image: node: volumes:
      - .: src/service  
      - : src/service/node_modules
    working_dir: src/service
    command: npm run dev
    ports:
      - : - : '3' 11 /usr/ /usr/ /usr/ 1234 1234 1235 1235 Which will allow within the container to live on it’s own, isolated completely from local. node_modules While sound in theory, this will break the process we’ve just defined for sharing between the builder and the running container. node_modules Not doing it, on the other hand, causes problems if you are moving between local and docker development, as would need to be deleted between each switch. node_modules A happy medium is to use an “external volume” instead of the local volume. First, let’s update our to take care of that as well, with a script that simply calls the command. Makefile setup docker volume create setup:
  docker volume create nodemodules (Again, tabs not spaces) With the volume created, we can now reference it from the bottom of each of our two docker-compose files. Add the following to the docker-compose files: docker-compose.yml version: services:
  dev:
    image: node: volumes:
      - nodemodules: src/service/node_modules
      - .: src/service
    environment:
      - NODE_ENV=development
    working_dir: src/service
    command: npm run dev
    ports:
      - : - : volumes:
  nodemodules:
    external: '3' 11 /usr/ /usr/ /usr/ 1234 1234 1235 1235 true docker-compose.builder.yml # ...
  base:
    image: node: volumes:
      - nodemodules: src/service/node_modules
      - .: src/service/
    working_dir: src/service/
volumes:
  nodemodules:
    external: 11 /usr/ /usr/ /usr/ true This changes our startup process slightly as well, as on the first run we need to make sure the volume exists with make setup. Conclusion You don’t always need to make a Dockerfile to use Docker! Oftentimes, for development, you can just use someone else’s! I hope I’ve been able to show you an easy way to get up and running quickly with Docker, and docker-compose for development. To learn about how to create a multi-stage build for production, in CI pipelines, or how to use docker-compose to run staging tests, check out my article: I have a confession to make… I commit to master. In the next article, I show you , a critical step before we finish up with a production ready multi-stage Dockerfile to package our code. how to enforce code quality using Linting, Formatting, and Unit Testing with Code Coverage Best, Patrick Lee Scott Check out the other articles in this series! This was Part 2. Part 1: Move over Next.js and Webpack Part 3: Enforcing Code Quality for Node.js Part 4: The 100% Code Coverage Myth Part 5: A Tale of Two (Docker Multi-Stage Build) Layers

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