Docker

Docker Reference Page

Updated: 03 September 2023

Based on this Cognitive Class Course

Prerequisites

• Docker
• WSL2 for Windows (ideally) (if using Windows)

Run a Container

Introduction

Containers are a group of processes that run in isolation, these processes must all be able to run on a shared kernel

Virtual machines are heavy and include an entire operating system, whereas with a container they only contain what is necessary. Containers do not replace virtual machines

Docker is a toolset to manage containers and integrate into our CI/CD pipelines. This allows us to ensure that all our running environments are identical. Furthermore Docker provides a standard interface for developers to work with

Running a Container

To run a container on our local machine we use the Docker CLI

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docker container run -t ubuntu top

This command will look for the ubuntu image locally, which it will not find and will then check for the image online, after which it will run the ubuntu container with the top command. This can be seen with the following output

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Unable to find image 'ubuntu:latest' locally
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latest: Pulling from library/ubuntu
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473ede7ed136: Pull complete
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c46b5fa4d940: Pull complete
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93ae3df89c92: Pull complete
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6b1eed27cade: Pull complete
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Digest: sha256:29934af957c53004d7fb6340139880d23fb1952505a15d69a03af0d1418878cb
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Status: Downloaded newer image for ubuntu:latest
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top - 13:08:05 up  5:20,  0 users,  load average: 0.02, 0.03, 0.00
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Tasks:   1 total,   1 running,   0 sleeping,   0 stopped,   0 zombie
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%Cpu(s):  0.0 us,  0.0 sy,  0.0 ni, 99.8 id,  0.2 wa,  0.0 hi,  0.0 si,  0.0 st
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KiB Mem :  2027760 total,    93568 free,   410976 used,  1523216 buff/cache
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KiB Swap:  1048572 total,  1048564 free,        8 used.  1447916 avail Mem
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  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
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    1 root      20   0   36588   3148   2724 R   0.3  0.2   0:00.09 top

It is important to note that the container does not have its own kernel but instead runs on the host kernel, the Ubuntu image only provides the file system and tools

We can view our running containers with

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docker container ls

We can interact with the container by

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docker container exec -it be81304e2786 bash

Where

• -it states that we want to interact with the shell
• be81304e2786 is the container ID
• bash is the tool we want to use to inspect our container

Now that we are in the container we can view our running processes with

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ps -ef

To get out of our container and back to our host we run

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exit

Running Multiple Containers

Just run another container, basically

Ngix

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docker container run --detach --publish 8080:80 --name nginx nginx

Mongo

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docker container run --detach --publish 8081:27017 --name mongo mongo:3.4

If you run into the following error, simply restart Docker

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Error response from daemon: driver failed programming external connectivity on endpoint xenodochial_spence (d2836ffdcd649ba692d504e34af61c9aab57bf3a135587875db3c88ca0baa070): Error starting userland proxy: mkdir /port/tcp:0.0.0.0:8080:tcp:172.17.0.2:80: input/output error.

We can list running containers and inspect one that we chose with

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docker container ls
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docker container inspect <CONTAINER ID>

It is important to remember that each container includes all the dependencies that it needs to run

A list of available Docker images can be found here

Remove Containers

We can stop containers with

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docker container stop <CONTAINER IDs>
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docker container stop jn4 es3 fe3

Then remove all stopped containers with

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docker system prune

CI/CD with Docker Images

Introduction

A Docker image is an archive of a container that can be shared and containers can be created from them

Docker images can be shared via a central registry, the default store for Docker is Docker Hub

To create an image we use a Dockerfile which has instructions on how to build our image

Docker is made of layers, image layers are build on top of the layers before them, based on this we only need to update or rebuild layers that are changed or need to be updated, based on this we try to keep the area where we are making modifications to the bottom of our Dockerfile in order to prevent unnecessary layers from being rebuilt constantly

Create a Python App

Make a simple python app in a directory that you want your app to be in which contains the following

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from flask import Flask
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app = Flask(__name__)
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@app.route("/")
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def hello():
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    return "hello world!"
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if __name__ == "__main__":
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    app.run(host="0.0.0.0")

This app will simply use Flask to expose a web server on port 5000 (the default Flask port)

Note that the concepts used for this app can be used for any application in any language

Create and Build the Docker Image

Create a file named Dockerfile in the same directory with the following contents

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FROM python:3.6.1-alpine
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RUN pip install flask
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CMD ["python","app.py"]
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COPY app.py /app.py

So, what does this file do?

• FROM python:3.6.1-alpine is the starting point for our Dockerfile, each Dockerfile needs this to select the base layer we want for our application, we use the -alpine tag to ensure that changes to the parent dependency are controlled
• RUN pip install flask is executing a command that is necessary to set up our image for our application, in this case installing a package
• CMD ["python","app.py"] is what is run when our container is started, this is only run once for a container, we are using it here to run our app.py we can leave this here even though it will only be run once all the other lines are as this will not yield any changes to layers
• COPY app.py /app.py says that docker should copy the file in the local directory to our image, this is at the end as it is our source code which changes frequently and hence should affect as few layers as possible

From the directory of our application we can build our image

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docker image build -t python-hello-world .

If you run into the following error you may need to ensure that your encoding is UTF 8

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Error response from daemon: Dockerfile parse error line 1: unknown instruction: ��F R O M

We can then view our image in the list with

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docker image ls

We can run our image with

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docker run -p 5001:5000 -d python-hello-world

The -p option maps port 5001 on our host to port 5000 of our container

Navigating to http://localhost:5001 with our browser we should see

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hello world!

If we do not get a response from our application, and if our application is not shown under the list of running containers we can view our logs for information, we use the string that was output when we did docker run as this is the container ID we tried to run

We can view our container logs with

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docker container logs <CONTAINER ID>

Push to a Central Registry

We can push our docker images to Docker Hub by logging in, tagging our image with our username, and then pushing the image

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docker login
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docker tag python-hello-world <USERNAME>/python-hello-world
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docker push <USERNAME>/python-hello-world

Note that the <USERNAME>/python-hello-world refers to a repository to which we want to push our image

Thereafter we can log into Docker Hub via our browser and see the image

Deploy a Change

We can modify our app.py file and simply rebuild and push our update

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docker image build -t <USERNAME>/python-hello-world .
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docker push <USERNAME>/python-hello-world

We can view the history of our image with

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docker image history python-hello-world

Removing Containers

We can remove containers the same as before

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docker container stop <CONTAINER IDS>
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docker system prune

Container Orchestration with Swarm

Introduction

Orchestration addresses issues like scheduling and scaling, service discovery, server downtime, high availibility, A/B testing

Orchestration solutions work by us declaring our desired state and it maintaining that state

Create a Swarm

We will be using Play-With-Docker for this part

Click on Add a new instance to add three nodes

Thereafter initialize a swarm on Node 1 with

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docker swarm init --advertise-addr eth0

This will output something like the following

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docker swarm join --token SWMTKN-1-1m4p457lt447dgth3ovrnwajn7x66wghumgcw415rh7lj2nzfi-bsora9haiad4ikx6r9hap7rkm 192.168.0.28:2377

We can then add a manager to the swarm with

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docker swarm join-token manager

We can then run the docker swarm-join command from the other two nodes, then on node 1 we can view the swarm with

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docker node ls

Deploy a Service

On node 1 we can create an ngix service

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docker service create --detach=true --name nginx1 --publish 80:80  --mount source=/etc/hostname,target=/usr/share/nginx/html/index.html,type=bind,ro nginx:1.12

We can then list the services we have created with

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docker service ls

We can check the running container of a service with

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docker service ps <SERVICE ID>

Because of the way the swarm works, if we send a request for a specific service, it will automatically be routed to the container which has ngix running, we can test this from each node

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curl localhost:80

Scale the Service

If we want to replicate our service instances we can do so with

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docker service update --replicas=5 --detach=true nginx1

When we update our service replicas Docker Swarm recognises that we no longer match the service requirement and it therefore creates more instances of the service

We can view the running services

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docker service ps nginx1

We can send many requests to the node and we will see that the request is being handled by different nodes

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curl localhost:80

We can view our service logs with

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docker service logs ngix1

Rolling Updates

We can do a rolling update of a service with

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docker service update --image nginx:1.13 --detach=true nginx1

We can fine-tune our update process with

• --update-parallelism specifies the number of containers to update immediately
• --update-delay specifies the delay between finishing updating a set of containers before moving on to the next set

After a while we can view our ngix service instances to see that they have been updated

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docker service ps nginx1

Reconciliation

Docker Swarm will automatically manage the state we tell it to, for example if a node goes down it will automatically create a new one to replace it

How Many Nodes?

We typically aim to have between three and seven manager nodes, in order to correctly apply the consensus algorithm, which requires more than half our nodes to be in agreement of state, the following is advised

• Three manager nodes tolerate one node failure
• Five manager nodes tolerate two node failures
• Seven manager nodes tolerate three node failures

It is possible to have an even number of manager nodes but this adds no additional value in terms of consensus

However we can have as many worker nodes as we like, this is inconsequential