Docker

 

Q: What is Docker?

Ans:

Docker is a tool designed to make it easier to create, deploy, and run applications by using containers. Containers allow a developer to package up an application with all of the parts it needs, such as libraries and other dependencies, and deploy it as one package. By doing so, thanks to the container, the developer can rest assured that the application will run on any other Linux machine regardless of any customized settings that machine might have that could differ from the machine used for writing and testing the code.

Q: What are useful Docker commands ?

Ans :

To list docker version

docker --version

To list available images

docker images

To list running containers

docker ps

To list all stopped/terminated/Exited containers

docker ps -a

To pull image

docker pull "image_name" or docker pull "image_name" :"tag name"

tagname is kind of version for that particular image

To run/create a container in interactive mode

docker run -itd image_name --name=name of your container   process(bash)

ex: docker run -itd --name="container_name" "image_name" bash

To remove all stopped containers from the list 

docker rm $(docker ps -a -q)

To Stop all running container at once

docker stop $(docker ps -a -q)

To remove image

docker rmi imagename

For all images at once -> docker rmi $(docker images)

To go inside a running container

docker exec -it "container_id/container_name" bash

To save/convert changes from container to image

docker commit "container_id" image_name:<tagname>

To save the current image to ship it to another device or hardware

docker save "image name" > "my_image_name".tar

To load the above image back to container after shipping

docker load < "my_image_name".tar

To export the running container to another hardware or device with existing configuration

docker export "container_name" > "my_export_image".tar

To import the container

docker import  - "new_image_name" < my_export_image.tar

To run a webserver

docker run -itd --name=container_name -p 9999:80 image_name:tagname /usr/sbin/httpd -D FOREGROUND

To push docker image to your Docker Hub Account. Initially,

docker login

docker tag imagename yourdockerusername/imagename:tag

docker push yourdockerusername/imagename:tag

To list container file system from Host

docker exec "container_id" command

ex:- docker exec container_id ls /tmp/data0

or

docker exec container_id cat /etc/passwd

To see docker network

docker network ls

To see network information of the containers

docker network inspect bridge

To create out own network

docker network create --driver=bridge --subnet=10.10.0.0/24 "my_network"

To check our network

docker network inspect drivername

ex:- docker network inspect "your_network_name" (network name or network id)

To launch container in our network

docker run -itd --network=my_network --name=myweb -p 9999:80  httpd_image:aamirs /usr/sbin/httpd -D FOREGROUND

or

docker run -itd --network=my_network --name=mytest_conatiner centos

To delete a network

docker network rm "network_id or network_name"

Create a container with desired resources

docker run -itd --memory="250m" --cpus="0.02" --network=my_network --name=mywebserver -v /tmp/myefs/docker_volume/:/var/www/html -p 9999:80  mywebserver:1.0 /usr/sbin/httpd -D FOREGROUND

Docker Volume

To mount volume on containers -v source:destination

docker run -itd --name=container_name -v /tmp/webapp:/var/www/html image_name

How to create docker volume

docker volume create "volume_name"

by default it will be created under /var/lib/docker/volumes

To find history of docker images

docker history image_name

To find all the information about a container

docker inspect container_id

Docker Daemon logs

Below command will log docker logs only in RHEL based VM's , Other OS may have different location to log the docker logs.

cat /var/log/messages | grep docker

Docker container logs

cat /var/lib/docker/containers/<container_id>/<container_id>-json.log

Container config info files

cat /var/lib/docker/containers/container_id/config.v2.json

cat /var/lib/docker/containers/container_id/hostconfig.json

Q: What is Unix command to start Docker?

Ans:  systemctl start docker

          systemctl enable docker

          systemctl status docker

Q: What is multi stage build in Docker

Ans:

With multi-stage builds, you use multiple FROM statements in your Dockerfile. Each FROM instruction can use a different base, 

and each of them begins a new stage of the build. You can selectively copy artifacts from one stage to another, leaving behind everything you don’t want in the final image. 

To show how this works, let’s adapt the Dockerfile from the previous section to use multi-stage builds.

# syntax=docker/dockerfile:1

FROM golang:1.16

WORKDIR /go/src/github.com/alexellis/href-counter/

RUN go get -d -v golang.org/x/net/html  

COPY app.go ./

RUN CGO_ENABLED=0 go build -a -installsuffix cgo -o app .

FROM alpine:latest  

RUN apk --no-cache add ca-certificates

WORKDIR /root/

COPY --from=0 /go/src/github.com/alexellis/href-counter/app ./

CMD ["./app"]

You only need the single Dockerfile. You don’t need a separate build script, either. Just run docker build.

 docker build -t alexellis2/href-counter:latest .

The end result is the same tiny production image as before, with a significant reduction in complexity. You don’t need to create any intermediate images,

 and you don’t need to extract any artifacts to your local system at all.

How does it work? The second FROM instruction starts a new build stage with the alpine:latest image as its base. The COPY --from=0 line copies just the built 

artifact from the previous stage into this new stage. The Go SDK and any intermediate artifacts are left behind, and not saved in the final image.

Q:Name and explain the various Docker components.

Ans:

The three main Docker components are:

Docker Client. Performs Docker build pull and run operations to open up communication with the Docker Host. The Docker command then employs Docker API to call any queries to run.

Docker Host. Contains Docker daemon, containers, and associated images. The Docker daemon establishes a connection with the Registry. The stored images are the type of metadata dedicated to containerized applications.

Registry. This is where Docker images are stored. There are two of them, a public registry and a private one. Docker Hub and Docker Cloud are two public registries 

available for use by anyone.

Q: What is a container?

Ans:

Containers are deployed applications bundled with all necessary dependencies and configuration files. 

All of the elements share the same OS kernel. Since the container isn’t tied to any one IT infrastructure, it can run on a different system or the cloud.

Q:What’s the difference between virtualization and containerization?

Ans:

Virtualization is an abstract version of a physical machine, while containerization is the abstract version of an application.

Q:Describe a Docker container’s lifecycle.

Ans:

Although there are several different ways of describing the steps in a Docker container’s lifecycle, the following is the most common:

1. Create container
2. Run container
3. Pause container
4. Unpause container
5. Start container
6. Stop container
7. Restart container
8. Kill container
9. Destroy container

Q: How do you find stored Docker volumes?

Ans:

Use the command: /var/lib/docker/volumes

Q:How do you check the versions of Docker Client and Server?

Ans: $ docker --version

Q:how you would create a container from an image.

Ans:

To create a container, you pull an image from the Docker repository and run it using the following command: $ docker run -itd <image_name>

Q: How about a command to stop the container?

Ans:

Use the following command: $ sudo docker stop container name

Q: How would you list all of the containers currently running?

Use the command: $ docker ps

Q: What’s involved in scaling a Docker container?

Ans:

Docker containers have the potential to be scaled to any level needed. Thanks to the platform’s flexibility, you can have anything from a few hundred to a few thousand,

 to millions of containers, providing they all have continual, unconstrained access to the required memory and OS.

Q:List some of the more advanced Docker commands and what they do.

Some advanced commands include:

Docker info. Displays system-wide information regarding the Docker installation

Docker pull. Downloads an image

Docker stats. Provides you with container information

Docker images. Lists downloaded images

Q: Can you lose data stored in a container?

Any data stored in a container remains there unless you delete the container.

Q: What platforms can you run Docker on?

The Linux platforms are:

ArchLinux

CentOS 6+

CRUX 3.0+

Fedora 19/20+

Gentoo

openSUSE 12.3+

RHEL 6.5+

Ubuntu 12.04, 13.04 et al

Docker can also run on the following cloud-based platforms:

Amazon EC2

Amazon ECS

Google Compute Engine

Microsoft Azure

Rackspace

Q: Which is the best method for removing a container: the command “stop container” followed by the command “remove the container,” the rm command by itself?

Stop the container first, then remove it. Here’s how:

$ docker stop <coontainer_id>

$ docker rm -f <container_id>

Q: Can a container restart on its own?

Ans:

Since the default flag -reset is set to false, a container cannot restart by itself.

Q: How do Docker daemon and the Docker client communicate with each other?

You use a combination of Rest API, socket.IO, and TCP to facilitate communication.

Q: Can you implement continuous development (CD) and continuous integration (CI) in Docker?

Ans:

Yes, you can. You can run Jenkins on Docker and use Docker Compose to run integration tests.

Q: What is Docker image?

Ans:

The Docker image help to create Docker containers. You can create the Docker image with the build command. Due to this, it creates a container that

 starts when it begins to run. Every docker images are stored in the Docker registry.

Q: What is Docker Engine?

Ans:

Docker daemon or Docker engine represents the server. The docker daemon and the clients should be run on the same or remote host, which can communicate through command-line client binary and full RESTful API.

Q:What are the common instruction in Dockerfile?

Ans:

The common instruction in Dockerfile are: FROM, LABEL, RUN, and CMD.

Q:How can you run multiple containers using a single service?

Ans:

By using docker-compose, you can run multiple containers using a single service. All docker-compose files uses yaml language.

Q: How to pull any image from url?

Ans:

1. Log in to Artifactory:

docker login artifactory.example.com

2. Pull the Docker image:

docker pull artifactory.example.com/docker-repo/my-app:latest

3  Once the pull is complete, you can verify the image is available locally by running:

docker images