Hey, long time no see! Today I want to tell you something about a great tool that I'm extensively using on a daily basis for a few years now. It's called Docker, and I've already created couple posts about it, but none of them is dedicated to beginners. Time to fix it.
I've heard about Docker, but actually what it is?
Docker is a tool for creating and running "lightweight virtual-machines". They are distributed in packages usually containing a small distribution of Linux, all required dependencies for your application to run and your application code. We call them "docker images" and you can think about them as "executable black boxes". This image can be used to create an arbitrary number of "containers", isolated processes with access to all prepared dependencies. One important thing is that each container is an exact copy of an image, even if started on a different operating system. No matter what technology or language is required to run your application, if you have image and docker installed, you can run it.
I want to keep this article short, so I'll move to advantages that Docker gives you. If you want to learn more, check official documentation.
Why should I give Docker a chance?
Remember, you don't have to go "all in".
My advice is that you should add Docker support progressively to your application. Start with development. Tell others in your team that they can use docker, show how to use it. Wait for reactions, let them become familiar with it. Wait until they ask "Why the hell we discovered it so late?". And then think about production integration, I'll write about it soon.
In development, Docker allows keeping the whole infrastructure as a code and sharing it with others in your team. Do you know dialogues like this?
**you**: "Hey Chris, my development application won't start. Have you changed something?"
chris: "Oh yes, I forgot to tell. We have new database engine now. Also, you need to install redis, and drivers for it. It's very easy, take a look into docs and you'll do it in few moments."
Of course, it always takes more time. If you work in a bigger team, such situations can happen more often and are harder to spot. With Docker, to sync with changes you need to just run
docker-compose up --build, and after few moments new version of the development stack will be running, no matter what changed. Moreover, introducing new developer to the project will look exactly the same! Imagine how much time it can save?
Personally, I'm working with many different technologies at the same time, using different versions of common dependencies (e.g databases). Docker allows me to isolate them. Also, no matter what technology, I can start working in exactly the same way -
docker-compose up and rock.
There is a place called Docker Hub. It's an official store with a tremendous amount of ready to use images, that you can use in your stack. You need redis? Postgres? Mysql, apache, php, nginx, python, ruby, nodejs? All of them are just one click away!
Ok, it looks promising. How to start?
Second: add proper
Dockerfile to your application.
Dockerfile contains list of instructions that need to be run to prepare environment for running your application. For example, here you copy your project files, install all system and application dependencies, set all required settings etc. Let's take a look at a Dockerfile that I'm using for my Python projects:
FROM python:3.6 ENV PROJECT_ROOT=/srv WORKDIR $PROJECT_ROOT COPY requirements.txt . RUN pip install -r requirements.txt COPY . . CMD ["python", "manage.py", "runserver"]
Usually, you don't need anything fancier. To find instructions how to create Dockerfile for your language - just google it, there are lots of great quality tutorials.
Docker-compose is a brilliant tool allowing us to painlessly manage stack of containers at the same time. This special file tells docker-compose which containers should be started.
docker-compose.yml that I'm using:
version: '2' services: redis: image: redis database: image: postgres app: build: . environment: DEBUG: 1 ports: - 8000:8000 volumes: - .:/srv command: python manage.py runserver
This file tells that we want Redis and Postgres created using official images, and our application built from local Dockerfile, available from outside docker network on port 8000, with all local files mounted into the container for easy development. All options can be found here (I'm using version
3 adds Docker Swarm support, isn't needed for development).
Now, when we type
docker-compose up, following things will happen:
- Docker will try to download / create all required Images. If we used
imageoption, it will be downloaded from the internet, by default from
buildwill instruct Docker to build image using
Dockerfilefound in a specific directory, in our case
.(current dir). If image already exists, it won't be automatically rechecked (to do so, we must add flag
- When images are available,
docker-composewill check if configuration changed since the last run. If not, previously used containers are started, in other case new containers are created. Settings like command, ports, volumes, variables, networks etc are taken from
- If not stated otherwise, a special network is created, spanning all created containers. It allows them to reference others using names instead of ip addresses, for example for our application database will be available at
- All containers are started at the same time, and we see logs from all of them.
That's it! Now the whole stack starts automatically. Of course, you should do things like automatically creating a database if it doesn't exist, or reload application code on a change to make development really pleasant. What's important, you just need to prepare this one time, and the whole team will benefit from it immediately.
Note: During development, you should probably always use
volumes option in your
docker-compose.yml file, mounting your local files into the container. Without this option, you would need to rebuild container each time some changes occur.
Here are the most common commands that I'm using on a daily basis.
docker-compose up or
docker-compose up --rebuild if you know that some dependencies have changed
Running one-off command: It depends if we need other containers available, like the database. If yes, start the stack with a previous command, and in another terminal window run
docker-compose exec $SERVICE_NAME $COMMAND. If no, just use
docker-compose run --rm $SERVICE_NAME $COMMAND (I recommend
--rm flag to automatically clean after command)
Removing everything created by docker-compose:
docker-compose down. It will destroy all containers, images, and networks
Removing one container with all related data:
docker-compose rm -v $SERVICE_NAME (very useful if you want to hard-reset database)
That's all for now!
Hope my post will help you to start with Docker. If you like this post and want more, don't hesitate to share it on social media, it really helps to keep motivation. Thank you, and have a great day :)