Docker with basic examples

Posted on March 8, 2023  •  5 minutes  • 893 words

What is Docker ?

Docker :

• It is a open platform to deliver your application faster to production.
• It helps in developing, running, testing, shipping the same environment without worrying about the host infrastructure.
• Advantages:
  • Suitable for progressive deployment
  • Dynamic workload management and scaling
  • Cost efficient as compared to VMs(Virtual Machines)
  • Open platform to use and to contribute
  • Environment isolation
  • Easy to share

Components of Docker

Docker Daemon:

• Listens to all the Docker API requests coming in the form of commands.
• Manages Docker objects like images, containers, networks and volumnes.
• It also communicate with other docker-daemons to manage the docker-service.

Docker Client:

• It is the primary way with which user can interact with docker-daemon.
• Listens to the commands like docker run, docker build (via Docker APIs) and sends it to docker-daemon.
• This client can communicate to multiple docker-daemons at the same time.

Docker Registry:

• The place to store the docker images is called docker registry.
• Docker Hub is the public registry available for everyone to access.
• You can docker pull the docker-image and docker push your docker-image to the registry.

Docker Objects:

Images:

• It is a read-only template which defines the environment you want to set for your application.
• Created with the file called Dockerfile (name should be the same).
• Each instruction in Dockerfile is a layer. Other images can also be used as layers.
• When you update and rebuild an image, only the layer which changed gets rebuilt.

Containers:

• These are the runnable instance of docker-images.
• Controlled via commands like start, stop, create etc using docker-APIs or CLI.
• Containers can connect to one or more networks and can attach storage in the form of volumnes.
• All the containers are isolated from each other and its host machine too.

Two ways to create a container:

Note: Install the docker : https://docs.docker.com/get-docker/ before you go further.

Let’s say we want to create a container with python installed.

1. With pre-configured image from Docker-Registry:

• Get the docker-image which contains python pre-installed and pull it from Registry( Docker-Hub : https://hub.docker.com/_/python ). Use the command docker pull:

☺ docker pull python

Using default tag: latest
latest: Pulling from library/python
Digest: sha256:d3c16df33787f3d03b2e096037f6deb3c1c5fc92c57994a7d6f2de018de01a6b
Status: Image is up to date for python:latest
docker.io/library/python:latestdocker run -dt python

• Verify if the image is present locally by running docker images:

☺ docker images

REPOSITORY   TAG       IMAGE ID       CREATED      SIZE
python       latest    8cad8eec9ec5   4 days ago   875MB

• Create a container for this image using docker run [Options] Image:
  Options:
  -d, --detach=false Run container in background & print ID
  -t, --tty=false Allocate a pseudo-TTY

☺  docker run -dt python

0311adddf2e1b83d2a1e8f4b1bf7d1e189b61f22fab62ff74f32ab61ef8c6752

• Verify if the container is created with docker ps:

☺  docker ps

CONTAINER ID   IMAGE     COMMAND     CREATED         STATUS         PORTS     NAMES
0311adddf2e1   python    "python3"   6 seconds ago   Up 5 seconds

• Finally, you have a container running locally on your machine with the image pulled from registry.

• Let’s verify if it is actually having python inside. For that, let’s print hello world inside the container.

• Add the following in the file main.py:

print("hello world")

• Run the following commands to copy this file to the container and then run the python file from the container:
  docker cp [OPTIONS] SRC_PATH CONTAINER:DEST_PATH

  docker cp main.py c9e35fd2e994:main.py
  

  docker exec -it [CONTAINER] command python_file

  -i, --interactive Keep STDIN open even if not attached
  -t, --tty Allocate a pseudo-TTY

  docker exec -it c9e35fd2e994 python main.py
  

• Great ! That worked!! Hope it worked for you too.

• Lets stop and delete the container using docker stop [Container_ID](may take a few sec) and docker rm [Container_ID]:

☺  docker stop 9cbff115d0d9
9cbff115d0d9

☺  docker rm 9cbff115d0d9
9cbff115d0d9

☺  docker ps
CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

• Remove the image using docker rmi [Image_Name]/[Image_Id]:

☺  docker rmi python
Untagged: python:latest
Untagged: python@sha256:d3c16df33787f3d03b2e096037f6deb3c1c5fc92c57994a7d6f2de018de01a6b
Deleted: sha256:8cad8eec9ec53ea519f50fce862660e731b8a3b4ff5bda2af059d12495f5df08


☺  docker images
REPOSITORY   TAG       IMAGE ID   CREATED   SIZE

2. With Dockerfile :

• Create this docker file with the name Dockerfile and add the following inside this:

# Pull the latest python docker image layer in as a base.
FROM python:latest

#Copy this file to the container
COPY ./main.py .

#Run the python file
CMD ["python", "main.py"]

• Build the Dockerfile to create an image and a container :
  -t: tag -> Name to the image created

☺  docker build -t docker-first .


[+] Building 0.1s (5/5) FINISHED
 => [internal] load build definition from Dockerfile                                                                                                             0.0s
 => => transferring dockerfile: 129B                                                                                                                             0.0s
 => [internal] load .dockerignore                                                                                                                                0.0s
 => => transferring context: 2B                                                                                                                                  0.0s
 => [internal] load metadata for docker.io/library/python:latest                                                                                                 0.0s
 => CACHED [1/1] FROM docker.io/library/python:latest                                                                                                            0.0s
 => exporting to image                                                                                                                                           0.0s
 => => exporting layers                                                                                                                                          0.0s
 => => writing image sha256:f6690f886d3e9a06263105e9d638936ba1ecef702eefba0522ed3fc3a0ad383e                                                                     0.0s
 => => naming to docker.io/library/docker-first

• Verify the image:

☺  docker images
REPOSITORY     TAG       IMAGE ID       CREATED      SIZE
docker-first   latest    f6690f886d3e   4 days ago   875MB

• Run the python file which we copied inside by running:

☺  docker run docker-first
hello world

• You will see there is no container running, as it started, ran the python file and stopped:

☺  docker ps
CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

• But when you run this to see all the containers, you will see :
  docker ps [OPTIONS]
-a, --all Show all containers

☺  docker ps -a
CONTAINER ID   IMAGE          COMMAND            CREATED         STATUS                     PORTS     NAMES
d1820c86eecd   docker-first   "python main.py"   5 minutes ago   Exited (0) 5 minutes ago             ecstatic_engelbart

• Great! You can take a rest for now… We will get into more details soon. 😁