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Containers Intro

Why Use Containers

The Traditional Setup

  • Host machine resources divided up into multiple virtual machines.
  • Each virtual machine running an application.
  • This has no ability to scale resources on the virtual machines:
    • At times of heavy load the machine resources are limited.
    • At times of low load the resources are under-utilised.


The New Way

  • Containers remove the need for virtual machines, by providing the isolation required between applications while retaining access to the essential low level operating system components on the main machine.
    • Instead of multiple separate operating systems running, a single operating system is required. Much more efficient.

Cool. What can I use them for?

  • Managing dependencies: isolating your code and dependencies from the host operating system allows you to have different versions of software installed in different containers.
  • Running software without an install: for example the utility rclone can be used from it's container image (a pre-packaged environment, with all required dependencies:, instead of installing directly on your machine. Clean.
  • Packaging your code for distribution or deployment: written a backend in Django? It can be packaged up into an image and deployed anywhere that has a container engine. Your laptop, local server, AWS, Azureā€¦ you name it.

Containers are now the de-facto way to distribute software. Software developers are required to know the basics of what they are, how to build an image, and how to use a container.


Note: 'Docker' is often used in place of the word 'Container', as this was the main project to popularise containers.

  • Container (Docker Image): essentially a frozen state of an operating system, including filesystem, built-in command line tools, and your application code. This is built from a series of build instructions, almost exactly how you would have deployed your application onto a virtual machine.
  • Container: a container image is used to create a running container. When you run a container, you may wish to specify a network to attach to, files to mount into the container, and other things.
  • Volume: when a container is shut down, the filesystem is normally lost - it is ephemeral. A volume allows you to keep data after the containers lifecycle.

Docker vs Kubernetes vs Other

  • In the graphic above, Docker would be the Container Engine. It is what actually executes the commands to run the container and keep it running.
  • Docker runs on your local machine with single containers. Docker Inc made a product called Docker Swarm, to allow for the management of containers across a fleet of servers. It essentially lost the battle to a Google-backed tool called Kubernetes.
  • Kubernetes is a container orchestration tool and now the standard for how businesses deploy their software, in a way that is resilient to server crashes, code logic errors, etc.
    • If a process fails on one server, e.g. a Django API server, it will automatically be replaced by an equivalent container on another server.

Show Me the Code

Running containers

  • Run a simple Ubuntu container, based off the Ubuntu Focal image:
docker run -it bash
  • The -it flag is to tell docker to open an interactive (i) terminal (t) for you to type commands into the container.
  • The command after the image name is simply bash, which runs a bash terminal (as opposed to a basic shell terminal: sh).
  • The other mode to run containers is detached (-d), but for this you need a process to run, instead of a terminal session, for example python /app/code/

Building Images

Two components are required here:

  • A Dockerfile (Containerfile). This contains the commands, in order, that install the dependencies from base image (e.g. Ubuntu), then add your application code into the image.
  • A build instruction. The command line instruction to build an image, giving it a name etc:

An example Dockerfile:

# Use a base image with pre-installed dependencies
FROM node:18-alpine
# Set the directory to run commands in
# Copy your code into the container image
COPY . .
# Install your node dependencies to run the app
RUN yarn install --production
# Command to execute at container start (i.e. run a server)
CMD ["node", "src/index.js"]

Additional References

Example build:

Lots of good tutorials can be found online, search for: Dockerfile build example / tutorial.