This has been updated as of 2023-04-22.
Disclaimer: I am not an expert. I'm quite the opposite. I usually learn just enough to accomplish what I need, and I move on. Do not take what I write here as best practice! This is only intended to expose the potential of technologies. I always recommend reading the official documentation or tutorials, and ideally, translating them if necessary :D
Today, I'd like to tell you about Docker. We're going to create some mock services and automate running them in containerized, source-controlled infrastructure using Docker. What does this mean?
First, a word about a preceding alternative. Most people are familiar with virtual machines: you setup an OS the way you want, create an image of it, use some hypervisor to host virtual machines, and deploy your images at will. For many years, this has been an effective method to diversify the usage of physical compute resources. However, virtualization has its downsides. Entire operating systems have a pretty large amount of overhead in terms of resource usage. Boot times can be painful. Virtual disk images are large, making it a pain to move things around, and they're a pain to create. (This has been mitigated by Vagrant, but that's also a recent development.) This works, and it has worked for many years. However, are there alternatives?
Enter Docker. Docker uses what are called "containers" instead of virtual machines. These utilize Linux cgroups (unfortunately, inside a virtual machine on Mac and Windows) to create an isolated but low-overhead operating environment. Containers have noticeably less overhead than virtual machines, and startup is wicked fast. Docker specifically provides a wealth of tools to make working with containers easy and even pleasant. Similar to Vagrant, you can define a text file called a Dockerfile that defines how the container is configured. There's a collection of CLI tools for building Docker images from Dockerfiles, pushing and pulling images from Dockerhub or Docker registry (git -> Github : Docker -> Dockerhub), and managing containers. There are also a wealth of tools to orchestrate Docker containers. We'll cover what orchestration is and means later in this post.
Sounds great! How do we get started? Well, it depends on your machine.
Running Windows? Heads up that you'll need to install the Windows Subsystem for Linux first: https://learn.microsoft.com/en-us/windows/wsl/install. Once that's done, check out Docker for Windows here: https://docs.docker.com/desktop/install/windows-install/
Running on Mac? You've got it easy. Check out Docker for Mac here: https://docs.docker.com/desktop/install/mac-install/
Running on Linux? Here's a link for Ubuntu: https://docs.docker.com/engine/install/ubuntu/ There are links for other distributions in the navigation pane on the left.
Double-check to make sure that the docker service is started!
Now that you have things installed, let's build something. Let's build a self-contained environment with an HTTP server in one container, a client in another container, and configurations for running all of it.
Let's get started. First, we're going to build a super simple HTTP server. Create a new folder (I called mine server), and create these two files in it.
server.sh:
#!/bin/sh
trap exit EXIT INT TERM HUP
while true
do
# Why don't we use -k? hello.http is only piped in once
# We also send it to the background so that `wait` can catch the trap
ncat -l $(hostname -I) 8000 < hello.http &
wait
done
hello.http:
HTTP/1.0 200 OK
<html>
<body>
<h1>Hello, world!</h1>
</body>
</html>
Sloppy, I know, but it works. This script uses nmap's ncat command to serve up some text written out following HTTP to simulate a web server. Since ncat stops listening after fulfilling a request, we just loop it forever. If you weren't sure, don't use ncat for a legitimate HTTP server. This is just a tech demo for another tool, so I think The Right Way™ gods will forgive me.
If you have a POSIX shell available, this will run an HTTP server on port 8000. While its running, you can visit localhost:8000 and see a tiny page with "Hello, world!". This will work for our example.
Now, let's containerize it. Create a new file called Dockerfile also in this folder. No extension! It should look like this:
FROM alpine
RUN apk add --no-cache nmap-ncat
ADD ./ ./
EXPOSE 8000
CMD ["./server.sh"]
This is Dockerfile syntax. You can see the full docs here: https://docs.docker.com/engine/reference/builder/
A Dockerfile defines a Docker image. This can be compared to a virtual machine image. Again, a Dockerfile is much like a Vagrantfile if you're familiar with Vagrant.
FROM indicates that we're going to inherit from another Dockerfile. Yes, Docker supports inheritance. It's lovely. Basically, this image will start from the mentioned image, and we'll build on top of it. Where does this other Dockerfile/image come from? Docker Hub! https://hub.docker.com/ In this case, we're going to base our image on Alpine.
RUN is simply a way to run a command to help setup the image. In this case, we're installing ncat since we'll need it for the demo. I added the chmod command just in case we didn't have execute permissions or in case we're on Windows.
ADD is how you get files from your filesystem into the image. Here, we're just copying everything in our directory to the image's working directory. I'm lazy.
EXPOSE tells the container which ports will need network access. This is important if you're expected inbound network requests.
CMD is the command that will run when the container is started.
Viola! We have our first Dockerfile setup. Let's run it:
docker build -t demo_http_server .
This will search the current directory for a Dockerfile, create an image from it, and tag it as demo_http_server:
Now, we can run the container:
docker run -d -p 8000:8000 demo_http_server
The -d flag tells Docker to detach from the container, meaning it will run in the background. The -p 8000:8000 tells docker to bind the container's port 8000 to the host's port 8000, meaning our server will be accessible via the host machine at port 8000. Lastly, we tell it the image ID or tag or the image we want to create a container from. Now, you can hit your web server from the browser.
Congratulations! You have your first docker container running!
However, let's clean up after ourselves. Let's see what containers are currently running:
docker ps
You can see the container, its ID, the image it's based on, the currently running command, its name, and other info. Docker will generate names for containers if you don't provide one.
Let's make sure to stop the container, since we won't use this specific one anymore. Since my container's name is suspicious_jackson, I can simply do
docker stop suspicious_jackson
Using the container's ID works, too. If you run docker ps again after this, you'll see no containers running.
Fancy, huh? Let's get more fancy.
Make a separate folder called client, and drop this shell script in there. Don't forget to add execute permissions!
#!/bin/sh
# This script should take in 2 arguments: the host to curl and its port
url=$1
port=$2
trap exit EXIT INT TERM HUP
while true
do
curl -s $url:$port
sleep 2
done
Now, create a Dockerfile in that folder:
FROM alpine
RUN apk add --no-cache curl
ADD client.sh client.sh
ARG server
ARG port=8000
ENV server $server
ENV port $port
CMD ["sh", "-c", "./client.sh $server $port"]
You'll notice this file is slightly different.
ARG allows our container to take arguments, meaning we can configure some things at image build time.
ENV sets environment variables within the container.
In this case, we're taking arguments to the container, making them available to the shell environment, then passing them as inputs to our shell script. This script just makes an HTTP request to the given server at the given port repeatedly with a 2-second delay. Let's see them work together.
Now, there is a way to get these two containers talking to each other running manually, but that's dumb. It's a lot of effort. There's an easier way.
Instead, we can create a docker-compose file, which sets up multiple containers. I have this in a docker-compose.yml file in the folder above my server and client folders.
version: '3'
services:
server:
build:
context: ./server
ports:
- "8000:8000"
client:
build:
context: ./client
args:
server: 'server'
port: 8000
links:
- "server"
This file will allow us to use the docker-compose command to build and run multiple containers that are networked together to talk to each other.
version refers to the version of docker-compose we're using.
services is where we define each of the services we'll run in containers.
args corresponds to the arguments we defined in our Dockerfile.
links establishes links between services, where the linked service has a hostname entry with the name of the service. So from the client, we can reference the server container as server since we defined the link and service as server.
I won't cover the rest, but if things are unclear to you, ask in a comment! No judgement here :)
From here, we can simply run the below:
docker-compose up
You'll see the server and client both startup, and you'll see the client request from the server every second infinitely. To stop them, just ctrl + c.
We could do more! There are prebuilt images for all sorts of things, like databases, message queues, and web servers. You can setup an entire application stack for your local development environment. I did this at work (and impressed my coworkers).
In production, you could use Docker Swarm, Kubernetes, or Mesos to orchestrate and manage containers for your various services. Easy, observable, fault tolerant.
We did a lot today!
- Installing Docker
- Creating a Dockerfile
- Building a Docker image
- Running a Docker container
- Defining services using
docker-compose - Running multiple services in a network
Great job!
For now, this blog entry is long enough. :) Thank you for your time! I hope you found the material helpful. Please comment with any questions, comments, or suggestions. Also let me know if you have any requests for technologies you'd like me to cover!
Huge thanks to Jiyoung with Django Girls Seoul for translating! You should definitely check them out if Python and/or Django are in your future. I also recommend my own meetup, Learn Teach Code Seoul. If you think this blog was good work, it's a testament to how well our two groups collaborate together, so you should check out both!
I welcome feedback! Don't worry, I won't be offended. I see feedback as a gift for helping me improve. I'm greedy. Give me gifts!
If you want need the full source, you can find it at https://github.com/TheBeege/blog-docker
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