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Docker Part 3 – HOWTO Create a Simple Python Web App in Docker

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If you’ve been following this series (last part here), we now have docker installed, but what do we do next? Create our first containers of course!

I think we need to make it a bit more interesting though as just creating containers is a bit meaningless, in real life we’re actually going to do something with them. The scenario is that we want a few copies of our simple python web application. To achieve this we need to use a few simple docker commands:

  • Create a new container
  • Install an application inside of it
  • Store it as an image
  • Duplicate it more than once and make these available to other clients
  • Test each instance to ensure they are unique and accessible

The good thing here is that all of the above steps are repeatable with whatever application you wish to install inside your containers. This is just a simple way to help get your head around the concepts and commands.

We start by creating our first empty Ubuntu container. The –i connects us to the shell of the container (interactive).

$ sudo docker run -i -t --name="firstcontainer" ubuntu:14.04 /bin/bash

Then in this case we need to install the python and web.py dependencies INSIDE of the container. This could be modified for any required dependencies or apps.

$ apt-get update
$ apt-get install -y python python-webpy

Within the container, create a new python script:

$ sudo mkdir /home/test1
$ sudo vi /home/test1/app.py

The contents of the script are:

import web,sys
urls = (
 '/', 'index'
app = web.application(urls, globals())
class index:
 def GET(self):
 argumentone = sys.argv[2]
 greeting = "Hello World, the test message is " + argumentone
 return greeting
if __name__ == '__main__' :
 app = web.application(urls, globals())

Exit the container, back to the Native OS:

$ exit

Confirm the name of your container (the last container run):

$ sudo docker ps –l
 f711ff0fd695 ubuntu:14.04 /bin/bash 32 minutes ago Exit 0 firstcontainer

Create a new image from your docker called testpython1

$ sudo docker commit firstcontainer testpython:0.1

Confirm you can see the image and get the image ID:

$ sudo docker images
 testpython 0.1 fcb365f7591b 2 minutes ago 247.8 MB

Finally, start up 3 instances of your web application:

$ sudo docker run -d -p 8081:8081 fcb365f7591b python /home/test1/app.py 8081 "instance1"
$ sudo docker run -d -p 8082:8082 fcb365f7591b python /home/test1/app.py 8082 "instance2"
$ sudo docker run -d -p 8083:8083 fcb365f7591b python /home/test1/app.py 8083 "instance3"

Open a browser on your network and connect to http://dockerserverip:8081
Try the same for the other two port numbers. Note we now have a system running 3 separate containers which could then be load balanced using a third party tool, or even run completely different content. Cool huh?

Next, how to mount a drive into your container…

Docker Part 2 – HOWTO Remove / Delete Docker Containers

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So you have been messing with docker for a few minutes or hours, and now you have a bunch of either running or stopped containers you no longer need. How do you get rid of them?

Removing Single Containers

To remove a single docker container, you simply start by listing all of the docker containers (started or stopped) to ensure you know which one to delete:

$ sudo docker ps –a

Then remove the chosen container:

$ sudo docker rm <container name>

If the container is currently running you can simply add –f to stop and remove the container in a single command:

$ docker rm -f <container name>

Unless it’s paused, then you will get an error something like the following:

Error response from daemon: Could not kill running container, cannot remove - Container e4f28eccb0cbcfbf4d78104bfe3e84039f62c5073f7301f8a39bb77a9598ae72 is paused. Unpause the container before stopping

This is easy to resolve. The “docker pause” command was added as of Docker 1.0, allowing for better resource utilisation if you have containers you don’t currently need to be wasting CPU cycles. As of Docker 1.1, running containers are also paused during commit activities, to ensure a consistent file system. Simply check the ID of the VM (with a ps command), unpause it, then remove:

sudo docker ps
sudo docker unpause <container id>
sudo docker rm -f <container id>


Removing Multiple Containers

Sometimes we have built up a number of containers and we just want to scrub the lot in one go. If you want to remove all containers (running or not), first you need to generate a list of all of the container IDs, then you pass that list to the docker rm command as follows:

sudo docker rm -f $(sudo docker ps -aq)

Alternatively if you wish to remove only the non-running containers:

sudo docker rm $(sudo docker ps -q)


That’ll do for now, but in the next post I will go into how to install your first app…

Docker Part 3 – HOWTO Create a Simple Python Web App in Docker

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