Debug School: Nilesh Kumar

Steps to create docker image

Nilesh Kumar — Tue, 15 Nov 2022 07:27:00 +0000

Here we are trying to create an image out of base ubuntu image.

======================================================================

How to create an image?

Method - 1 - using containers - 1 Layer a time == docker commit

Method - 2 - using Dockerfile - N Layer a time == docker build

Method - 1 - using containers - 1 Layer a time == docker commit

IMAGE= ubuntu + git + apache2 + java

237 docker run -itd --name raj1 ubuntu
238 docker ps
239 docker exec -it raj1 /bin/bash

INSIDE Container

apt-get update
apt-get install git
apt-get install apache2
apt-get install openjdk-11-jdk -y

240 docker ps
241 clear
242 docker history ubuntu
243 docker ps
244 docker commit -m"up-ub-git-ap-java" -a"Rajesh Kumar" up-ub-git-ap-java
245 docker history ubuntu
246 docker history up-ub-git-ap-java
247 docker images
248 docker run -itd --name newcont up-ub-git-ap-java
249 docker ps
250 dcoker exec git
251 docker exec git
252 docker exec java
253 docker exec which apache2

What is a Docker image.

Nilesh Kumar — Tue, 15 Nov 2022 06:48:16 +0000

A docker image consists of layers of filesystem. Base and most bulkiest in size is usually the root file system.
Lets understand with the image httpd.
docker pull httpd and then do
docker inspect httpd
Run following max debt command
du -h --max-dept=1
This command will give you the list of layers that is present for this docker image, it will look something like below
`

0 ./l
87M ./cbc6425cda491b250b098dc0c74af0235bea8e6ce4b67cb3bbc115ea821b6eb6
8.0K ./29566a9c43b68426cb16d9a20876debb439290dc2d685095518ccf2050ba66f5
5.2M ./75f57747a9aafda7b1b02c02656e60f295d3f9217cf2798c69fc86b9712ed893
59M ./d97fef69b83d86abd40b6e61f84e1b94324cc72370d2e1ed2e5aa65f0196bc83
12K ./6294788f871fe9be307c7cfb77544879ec3c422b2db88bfae3cf9265cceceef0
151M .
`
All these layers when merged together forms a httpd image. Merging happens when we create a container out of this image.
When we starts the container all these layers are merged, if there is conflict in any file then top layer will take precedence. The merged image will have two important directory, diff and merge.
Diff will hold all the changes that we are doing in image, for e.g. lets say you want to create a file in this docer image in /opt/example.txt. This example.txt will go into diff folder of container merged image. and using this it will go and create an container.

What is Docker Update

Nilesh Kumar — Tue, 15 Nov 2022 05:16:59 +0000

The docker update command dynamically updates container configuration. You can use this command to prevent containers from consuming too many resources such as CPU and memory from their Docker host. With a single command, you can place limits on a single container or on many.

Notes

With the exception of the –kernel-memory option, you can specify these options on a running or a stopped container. On kernel version older than 4.6, you can only update –kernel-memory on a stopped container or on a running container with kernel memory initialized.

The docker update command dynamically updates container configuration. You can use this command to prevent containers from consuming too many resources such as CPU and memory from their Docker host. With a single command, you can place limits on a single container or on many.

*Some Examples
*
Update a container’s cpu-shares
$ docker update --cpu-shares 512 abebf7571666

Update a container with cpu-shares and memory
$ docker update --cpu-shares 512 -m 300M abebf7571666 hopeful_morse

Update a new container’s kernel memory constraints
$ $ docker run -dit --name test --kernel-memory 50M ubuntu bash

Update kernel memory while the container is running:
$ docker update --kernel-memory 80M test

Started a container without kernel memory initialized:
docker run -dit --name test2 --memory 300M ubuntu bash

To update restart policy for one or more containers:
$ docker update --restart=on-failure:3 abebf7571666 hopeful_morse

Add a restart policy to a container that was already created
$ docker update --restart=always

Update the "RunningContainerNameOrId" to use 1g of memory and only use cpu core 1
$ docker update --memory "1g" --cpuset-cpu "1"

To up date all running containers to use core 1 and 1g of memory:
$ docker update --cpuset-cpus "1" --memory "1g" $(docker ps | awk 'NR>1 {print $1}')

Disable auto-restart on a container?
$ docker update --restart=no my-container

Docker Wait

Nilesh Kumar — Tue, 15 Nov 2022 05:12:39 +0000

When we run docker wait command from the command line with multiple containers( docker wait containerid1 containerid2 ), it holds or blocks the terminal and continuously checks for the container status and outputs the status code once the container stops. However, if we have specified multiple containers, then it checks one by one, which means it will only check the status of the first container then only go to the next one, even if the second container stops before the first container.

Example of Docker wait
Let’s understand the working of the ‘docker wait’ command with the below examples: –

Scenario: – Simple test the command on the terminal.

Create a few containers as below:

docker run -d --name con2 nginx
docker run -d --name con3 redis
docker run -d --name con1 ubuntu sleep 60
docker ps

Now, run the ‘docker wait’ command to understand how it works. Run the below command:

docker wait con3 con1 con2

Explanation: – In the above example, We have put the con3 in the first place because this container is going to stop automatically, and we get the status code and cursor start blinking again –

It will be like this until the other two containers stop working. So, open a new terminal and stop the remaining containers.

docker stop con1
docker stop con2

Now, if we go back and check the terminal where the ‘docker wait’ command was running, we can see that we get the prompt : –

docker wait con3 con1 con2

Difference between docker pause and docker unpause

Nilesh Kumar — Mon, 14 Nov 2022 10:18:48 +0000

If we want to pause the processes running inside the container, we can use the “docker pause” command.

$ docker pause

To un-pause the container, use “docker un-pause” command.
$ docker unpause

Difference between docker stop and docker kill?

Nilesh Kumar — Mon, 14 Nov 2022 09:53:35 +0000

Docker stop terminates the process gracefully, while docker kill will terminate the process abruptly. The docker stop commands issue the SIGTERM signal, whereas the docker kill commands sends the SIGKILL signal.
The docker stop command stops the container gracefully and provides a safe way out. If a docker stop command fails to terminate a process within the specified timeout, the Docker issues a kill command implicitly and immediately.

Docker by Nilesh

Nilesh Kumar — Mon, 14 Nov 2022 07:24:00 +0000

What is Docker?
Docker is name of a company, which provides host of product like (docker engine etc). Using these we can get OS level virtualization to build, deploy, run, update, and stop containers easily.
Why Docker?
Docker provides abstraction and it makes it easier, simpler, and safer to build, deploy and manage multiple containers of different kinds.
What is Container.
Container can be considered as a packages of software, that combines applications and all its dependencies. For e.g : (bootfs (OS) + java + tomcat + application). These containers are independent and reliable across diffent computing environments.

Docker workflow
Docker workflow are series of steps to build and publish and image. Docker Deamon running on docker host builds an image based on the configuration of user. This image then gets saved in the Docker registry. Then Docker daemon either creates a new image or pulls an existing image from the Docker registry. Finally, Daemon creates an instance of a Docker image. Using this image a containers can be run.

How container created?
A containers is created using : 1) Ubuntu / Centos os 2) Software required on top of it (Java, PHP, Dotnet etc) 3) Server etc required to run applications like Jboss, Tomcat etc 4) and then we add our applications into it.