Introduction to Containers¶
Containers are an important common currency for app development, web services, scientific computing, and more. Containers allow you to package an application along with all of its dependencies, isolate it from other applications and services, and deploy it consistently and reproducibly and platform-agnostically. In this introductory module, we will learn about containers and their uses, in particular the containerization platform Docker.
After going through this module, students should be able to:
Describe what a container is
Use essential docker commands
Find and pull existing containers from Docker Hub
Run containers interactively and non-interactively
What is a Container?¶
A container is a standard unit of software that packages up code and all its dependencies so the application runs quickly and reliably from one computing environment to another.
Containers allow a developer to package up an application with all of the parts it needs, such as libraries and other dependencies, and ship it all out as one package.
Multiple containers can run on the same machine and share the OS kernel with other containers, each running as isolated processes in user space, hence are lightweight and have low overhead.
Containers ensure portability and reproducibility by isolating the application from environment.
How is a Container Different from a VM?¶
Virtual machines enable application and resource isolation, run on top of a hypervisor (high overhead). Multiple VMs can run on the same physical infrastructure - from a few to dozens depending on resources. VMs take up more disk space and have long start up times (~minutes).
Applications isolated by VMs.¶
Containers enable application and resource isolation, run on top of the host operating system. Many containers can run on the same physical infrastructure - up to 1,000s depending on resources. Containers take up less disk space than VMs and have very short start up times (~100s of ms).
Applications isolated by containers.¶
Docker¶
Docker is a containerization platform that uses OS-level virtualization to package software and dependencies in deliverable units called containers. It is by far the most common containerization platform today, and most other container platforms are compatible with Docker. (E.g. Singularity and Shifter are two containerization platforms you’ll find in HPC environments).
We can find existing containers at:
Some Quick Definitions¶
CONTAINER
A container is a standard unit of software that packages up code and all its dependencies so the application runs quickly and reliably from one computing environment to another. Containers includes everything from the operating system, user-added files, metadata.
IMAGE
A Docker image is a read-only file used to produce Docker containers. It is comprised of layers of other images, and any changes made to an image can only be saved and propagated on by adding new layers. The “base image” is the bottom-most layer that does not depend on any other layer and typically defines, e.g., the operating system for the container. Running a Docker image creates an instance of a Docker container.
DOCKERFILE
The Dockerfile is a recipe for creating a Docker image. They are simple, usually short plain text files that contain a sequential set of commands (a recipe) for installing and configuring your application and all of its dependencies. The Docker command line interface is used to “build” an image from a Dockerfile.
IMAGE REGISTRY
The Docker images you build can be stored in online image registries, such as Docker Hub. (It is similar to the way we store Git repositories on GitHub.) Image registries support the notion of tags on images to identify specific versions of images. It is mostly public, and many “official” images can be found.
Summing it Up¶
If you are developing an app or web service, you will almost certainly want to work with containers. First you must either build an image from a Dockerfile, or pull an image from a public registry. Then, you run (or deploy) an instance of your image into a container. The container represents your app or web service, running in the wild, isolated from other apps and services.
Simple Docker workflow.¶
Getting Started With Docker¶
Much like the git command line tools, the docker command line tools
follow the syntax: docker <verb> <parameters>. Discover all the verbs
available by typing docker --help, and discover help for each verb by typing
docker <verb> --help. Open up your favorite terminal, log in to the class
server, and try running the following:
[isp02]$ docker version
Client: Docker Engine - Community
Version: 20.10.3
API version: 1.41
Go version: go1.13.15
Git commit: 48d30b5
Built: Fri Jan 29 14:34:14 2021
OS/Arch: linux/amd64
Context: default
Experimental: true
Server: Docker Engine - Community
Engine:
Version: 20.10.3
API version: 1.41 (minimum version 1.12)
Go version: go1.13.15
Git commit: 46229ca
Built: Fri Jan 29 14:32:37 2021
OS/Arch: linux/amd64
Experimental: false
containerd:
Version: 1.4.3
GitCommit: 269548fa27e0089a8b8278fc4fc781d7f65a939b
runc:
Version: 1.0.0-rc92
GitCommit: ff819c7e9184c13b7c2607fe6c30ae19403a7aff
docker-init:
Version: 0.19.0
GitCommit: de40ad0
Warning
Please let the instructors know if you get any errors on issuing the above command.
EXERCISE
Take a few minutes to run docker --help and a few examples of
docker <verb> --help to make sure you can find and read the help text.
Working with Images from Docker Hub¶
To introduce ourselves to some of the most essential Docker commands, we will go through the process of listing images that are currently available on the ISP server, we will pull a ‘hello-world’ image from Docker Hub, then we will run the ‘hello-world’ image to see what it says.
List images on the ISP server with the docker images command. This peeks
into the Docker daemon, which is shared by all users on this system, to see
which images are available, when they were created, and how large they are:
[isp02]$ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
final_web latest dc4cda1aa2f1 8 months ago 749MB
final_worker latest dc4cda1aa2f1 8 months ago 749MB
flask latest 58cdeed93a41 9 months ago 448MB
master-web latest 58cdeed93a41 9 months ago 448MB
creatures 3 503cd4631565 9 months ago 446MB
my_image manisha 503cd4631565 9 months ago 446MB
homework5 latest f08364452cbd 9 months ago 490MB
phuong latest f08364452cbd 9 months ago 490MB
web_web latest a7d00df8fa6a 9 months ago 444MB
web_worker latest a7d00df8fa6a 9 months ago 444MB
redis latest 4cdbec704e47 10 months ago 98.2MB
ubuntu latest 4e5021d210f6 10 months ago 64.2MB
harmish/gnuplot latest b67698a87ae1 2 years ago 392MB
Pull an image from Docker hub with the docker pull command. This looks
through the Docker Hub registry and downloads the ‘latest’ version of that
image:
[isp02]$ docker pull hello-world
Using default tag: latest
latest: Pulling from library/hello-world
0e03bdcc26d7: Pull complete
Digest: sha256:31b9c7d48790f0d8c50ab433d9c3b7e17666d6993084c002c2ff1ca09b96391d
Status: Downloaded newer image for hello-world:latest
docker.io/library/hello-world:latest
Run the image we just pulled with the docker run command. In this case,
running the container will execute a simple shell script inside the container
that has been configured as the ‘default command’ when the image was built:
[isp02]$ docker run hello-world
Hello from Docker!
This message shows that your installation appears to be working correctly.
To generate this message, Docker took the following steps:
1. The Docker client contacted the Docker daemon.
2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
(amd64)
3. The Docker daemon created a new container from that image which runs the
executable that produces the output you are currently reading.
4. The Docker daemon streamed that output to the Docker client, which sent it
to your terminal.
To try something more ambitious, you can run an Ubuntu container with:
$ docker run -it ubuntu bash
Share images, automate workflows, and more with a free Docker ID:
https://hub.docker.com/
For more examples and ideas, visit:
https://docs.docker.com/get-started/
Check to see if any containers are still running using docker ps:
[isp02]$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
EXERCISE
The command docker ps shows only currently running containers. Pull up the
help text for that command and figure out how to show all containers, not just
currently running containers.
Pull Another Image¶
Navigate to the repositories of user wallen on Docker Hub
here.
Scroll down to find an image called wallen/bsd, then click on that image.
Pull the image using the suggested command, then check to make sure it is available locally:
[isp02]$ docker pull wallen/bsd:1.0
...
[isp02]$ docker images
...
[isp02]$ docker inspect wallen/bsd:1.0
...
Tip
Use docker inspect to find some metadata available for each image.
Start an Interactive Shell Inside a Container¶
Using an interactive shell is a great way to poke around inside a container and see what is in there. Imagine you are ssh-ing to a different Linux server, have root access, and can see what files, commands, environment, etc., are available.
Before starting an interactive shell inside the container, execute the following commands on the ISP server (we will see why in a minute):
[isp02]$ whoami
wallen
[isp02]$ pwd
/home/wallen
[isp02]$ cat /etc/os-release
NAME="CentOS Linux"
VERSION="7 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="7"
PRETTY_NAME="CentOS Linux 7 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:7"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"
CENTOS_MANTISBT_PROJECT="CentOS-7"
CENTOS_MANTISBT_PROJECT_VERSION="7"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="7"
[isp02]$ ls -l /usr/games/
total 0
Now start the interactive shell:
[isp02]$ docker run --rm -it wallen/bsd:1.0 /bin/bash
root@fc5b620c5a88:/#
Here is an explanation of the command options:
docker run # run a container
--rm # remove the container when we exit
-it # interactively attach terminal to inside of container
wallen/bsd:1.0 # image and tag on local machine
/bin/bash # shell to start inside container
Try the following commands and note what has changed:
root@fc5b620c5a88:/# whoami
root
root@fc5b620c5a88:/# pwd
/
root@fc5b620c5a88:/# cat /etc/os-release
NAME="Ubuntu"
VERSION="16.04.6 LTS (Xenial Xerus)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 16.04.6 LTS"
VERSION_ID="16.04"
HOME_URL="http://www.ubuntu.com/"
SUPPORT_URL="http://help.ubuntu.com/"
BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"
VERSION_CODENAME=xenial
UBUNTU_CODENAME=xenial
root@fc5b620c5a88:/# ls /usr/games/
adventure bcd countmail hack morse ppt robots teachgammon worms
arithmetic boggle cribbage hangman number primes rot13 tetris-bsd wtf
atc caesar dab hunt phantasia quiz sail trek wump
backgammon canfield go-fish mille pig rain snake wargames
battlestar cfscores gomoku monop pom random snscore worm
Now you are the root user on a different operating system inside a running
Linux container! You can type exit to escape the container.
EXERCISE
Before you exit the container, try running a few of the games (e.g. hangman).
Run a Command Inside a Container¶
Back out on the ISP server, we now know we have an image called
wallen/bsd:1.0 that has some terminal games inside it which would not
otherwise be available to us on the ISP server. They (and their dependencies)
are isolated from everything else. This image (wallen/bsd:1.0) is portable
and will run the exact same way on any OS that Docker supports.
In practice, though, we don’t want to start interactive shells each time we need to use a software application inside an image. Docker allows you to spin up an ad hoc container to run applications from outside. For example, try:
[isp02]$ docker run --rm wallen/bsd:1.0 whoami
root
[isp02]$ docker run --rm wallen/bsd:1.0 pwd
/
[isp02]$ docker run --rm wallen/bsd:1.0 cat /etc/os-release
NAME="Ubuntu"
VERSION="16.04.6 LTS (Xenial Xerus)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 16.04.6 LTS"
VERSION_ID="16.04"
HOME_URL="http://www.ubuntu.com/"
SUPPORT_URL="http://help.ubuntu.com/"
BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"
VERSION_CODENAME=xenial
UBUNTU_CODENAME=xenial
[isp02]$ docker run --rm wallen/bsd:1.0 ls /usr/games
adventure bcd countmail hack morse ppt robots teachgammon worms
arithmetic boggle cribbage hangman number primes rot13 tetris-bsd wtf
atc caesar dab hunt phantasia quiz sail trek wump
backgammon canfield go-fish mille pig rain snake wargames
battlestar cfscores gomoku monop pom random snscore worm
[isp02]$ docker run --rm -it wallen/bsd:1.0 hangman
The first four commands above omitted the -it flags because they did not
require an interactive terminal to run. On each of these commands, Docker finds
the image the command refers to, spins up a new container based on that image,
executes the given command inside, prints the result, and exits and removes the
container.
The last command, which executes the hangman game, requires an interactive
terminal so the -it flags were provided.
Essential Docker Command Summary¶
Command |
Usage |
|---|---|
docker login |
Authenticate to Docker Hub using username and password |
docker images |
List images on the local machine |
docker ps |
List containers on the local machine |
docker pull |
Download an image from Docker Hub |
docker run |
Run an instance of an image (a container) |
docker inspect |
Provide detailed information on Docker objects |
docker rmi |
Delete an image |
docker rm |
Delete a container |
docker stop |
Stop a container |
docker build |
Build a docker image from a Dockerfile in the current working directory |
docker tag |
Add a new tag to an image |
docker push |
Upload an image to Docker Hub |