Basics of Containers with Docker

Last updated on 2024-10-03 | Edit this page

Overview

Questions

  • What is a Docker image?
  • What is a Docker container?
  • How do you start and stop a container?
  • How do retrieve output from a container to a local machine?

Objectives

  • Explain the difference between a Docker image and a Docker container
  • Retrieve a Docker image from the cloud
  • Start a Docker container running on a local machine
  • Use the command line to check the status of the container
  • Clean the environment by stopping the container

Introduction

TODO Flavor text introducing containers. Why we use them. Include at least a couple use cases. If necessary, provide high level distinction from Virtual Machines.

Images versus containers

There are two big pieces of the container world: images and containers. They are related to one another, but they are not synonymous. Briefly, images provide the plans for making a container, and a container is similar to a virtual machine in that it is effectively another computer running on your computer. To use an analogy from architecture, images are the blueprints and containers are the actual building.

Callout

If you are a fan of philosophy, images are for Platonists and containers are for nominalists.

Considering the differences between images and containers…

Images are

  1. Read-only
  2. Contain instructions (in a file called a “Dockerfile” - we talk about Dockerfiles later in the lesson)
  3. They do not actually “do” anything

Containers are

  1. Modifiable (while running)
  2. Can include files and programs (like your computer!)
  3. Can run analyses or web applications (and more)

Challenge 1: Images versus containers

You instructor introduced one analogy for explaining the difference between a Docker image and a Docker container. What is another way to explain images and containers?

Several analogies exist, and here are a few:

  • An image is a recipe, say, for your favorite curry, while the container is the actual curry dish you can eat.
  • “Think of a container as a shipping container for software - it holds important content like files and programs so that an application can be delivered efficiently from producer to consumer. An image is more like a read-only manifest or schematic of what will be inside the container.” (from Jacob Schmitt)
  • If you are familiar with object-oriented programming, you can think of an image as a class, and a container an object of that class.

Working with containers

One thing to note right away is that a lot of the work of running containers happens through the command line interface. That is, we do not have a graphical user interface (GUI) with menus to work with. Instead, we type commands into a terminal for starting and stopping containers.

For the purposes of this lesson, we are going to use a relatively lightweight workflow of using a container. Briefly, the steps of using a container are:

  1. Retrieve the image we would like to use from an online repository.
  2. Start the container running (like turning on a computer).
  3. Interact with the container, if the container has such functionality (some containers are just programmed to run without additional interaction from users).
  4. Check the status of the container.
  5. Upon completion of whatever task we are using the container for, stop the container (like turning off the computer).

Steps 1, 2, 4, and 5 are all associated with a specific docker command:

  1. Retrieve image: docker pull
  2. Start container: docker run
  3. Check status: docker ps
  4. Stop container: docker stop

Retrieving images

The first step of using containers is to download a copy of the image you would like to use. For Docker images, there are multiple sites on the internet that serve as sources for Docker images. Two common repositories are DockerHub and GitHub’s Container Registry; for this lesson, we will be downloading from DockerHub. The nice thing is that we do not have to open a web browser and manually download a file - instead we can use the Docker commands to do this for us. For downloading images, the syntax is:

docker pull <image creator>/<image name>

Where we replace <image creator> with the username of the person or organization responsible for the image and <image name> with the name of the image. For this lesson, we are going to use an image that includes the OpenRefine software. OpenRefine is a powerful data-wrangling tool that runs in a web browser.

Callout

Want to learn more about OpenRefine? Check out the Library Carpentry Lesson on Open Refine.

TODO docker pull

$ docker pull felixlohmeier/openrefine

Starting an image

TODO docker start

$ docker run -p 3333:3333 felixlohmeier/openrefine

Status check

TODO docker ps

$ docker ps

Using the container

TODO Do something in OpenRefine

Stopping the container

TODO docker stop (after docker ps)

$ docker ps
$ docker stop <container ID>

Challenge 2: Checking the status of containers

We saw before that we could check the status of running containers by using the command docker ps. What happens when you run the same command now? What about when you run the same command with the -a flag?

  • docker ps will show the status of all running containers. If you have no containers running, and you probably do not at this point of the lesson, you should see an empty table, like:
$ docker ps
CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES
$
  • docker ps -a will show all containers that are running or have been run on the machine. This includes the container that we stopped earlier.
$ docker ps -a
CONTAINER ID   IMAGE                      COMMAND                  CREATED         STATUS                       PORTS     NAMES
906072ff88f6   felixlohmeier/openrefine   "/app/refine -i 0.0.…"   2 days ago      Exited (143) 2 days ago                determined_torvalds

$

Note the date information (in the CREATED and STATUS fields) and the container name (the NAMES field) will likely be different on your machine.

Challenge 3: Order of operations

Rearrange the following commands to (in the following order) (1) start the OpenRefine container, (2) find the container image ID of the running OpenRefine container, and (3) terminal the OpenRefine container.

docker stop <container ID>
docker run -p 3333:3333 felixlohmeier/openrefine
docker ps
docker run -p 3333:3333 felixlohmeier/openrefine
docker ps
docker stop <container ID>

Callout

TODO Add any notes that may be relevant, but not necessary for lesson?

Key Points

  • Containers are a way to provide a consistent environment for reproducible work.
  • Use docker pull to copy an image to your machine
  • Use docker start to start running a container
  • Use docker ps to check the status of running containers
  • Use docker stop to stop running a container